Introduction to Linux
A Hands on Guide
Machtelt Garrels
Garrels.be
                        <tille wants no spam _at_ garrels dot be>
                   Â
Copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008 Machtelt Garrels
20080606
-------------------------------------------------------------------------------
Table of Contents
Introduction
1._Why_this_guide?
2._Who_should_read_this_book?
3._New_versions_and_availability
4._Revision_History
5._Contributions
6._Feedback
7._Copyright_information
8._What_do_you_need?
9._Conventions_used_in_this_document
10._Organization_of_this_document
1._What_is_Linux?
1._History
1.1._UNIX
1.2._Linus_and_Linux
1.3._Current_application_of_Linux_systems
2._The_user_interface
2.1._Is_Linux_difficult?
2.2._Linux_for_non-experienced_users
3._Does_Linux_have_a_future?
3.1._Open_Source
3.2._Ten_years_of_experience_at_your_service
4._Properties_of_Linux
4.1._Linux_Pros
4.2._Linux_Cons
5._Linux_Flavors
5.1._Linux_and_GNU
5.2._GNU/Linux
5.3._Which_distribution_should_I_install?
6._Summary
7._Exercises
2._Quickstart
1._Logging_in,_activating_the_user_interface_and_logging_out
1.1._Introduction
1.2._Graphical_mode
1.3._Text_mode
2._Absolute_basics
2.1._The_commands
2.2._General_remarks
2.3._Using_Bash_features
3._Getting_help
3.1._Be_warned
3.2._The_man_pages
3.3._More_info
4._Summary
5._Exercises
5.1._Connecting_and_disconnecting
5.2._Passwords
5.3._Directories
5.4._Files
5.5._Getting_help
3._About_files_and_the_file_system
1._General_overview_of_the_Linux_file_system
1.1._Files
1.2._About_partitioning
1.3._More_file_system_layout
2._Orientation_in_the_file_system
2.1._The_path
2.2._Absolute_and_relative_paths
2.3._The_most_important_files_and_directories
2.4._The_most_important_configuration_files
2.5._The_most_common_devices
2.6._The_most_common_variable_files
3._Manipulating_files
3.1._Viewing_file_properties
3.2._Creating_and_deleting_files_and_directories
3.3._Finding_files
3.4._More_ways_to_view_file_content
3.5._Linking_files
4._File_security
4.1._Access_rights:_Linux's_first_line_of_defense
4.2._The_tools
5._Summary
6._Exercises
6.1._Partitions
6.2._Paths
6.3._Tour_of_the_system
6.4._Manipulating_files
6.5._File_permissions
4._Processes
1._Processes_inside_out
1.1._Multi-user_and_multi-tasking
1.2._Process_types
1.3._Process_attributes
1.4._Displaying_process_information
1.5._Life_and_death_of_a_process
1.6._SUID_and_SGID
2._Boot_process,_Init_and_shutdown
2.1._Introduction
2.2._The_boot_process
2.3._GRUB_features
2.4._Init
2.5._Init_run_levels
2.6._Shutdown
3._Managing_processes
3.1._Work_for_the_system_admin
3.2._How_long_does_it_take?
3.3._Performance
3.4._Load
3.5._Can_I_do_anything_as_a_user?
4._Scheduling_processes
4.1._Use_that_idle_time!
4.2._The_sleep_command
4.3._The_at_command
4.4._Cron_and_crontab
5._Summary
6._Exercises
6.1._General
6.2._Booting,_init_etc.
6.3._Scheduling
5._I/O_redirection
1._Simple_redirections
1.1._What_are_standard_input_and_standard_output?
1.2._The_redirection_operators
2._Advanced_redirection_features
2.1._Use_of_file_descriptors
2.2._Examples
3._Filters
3.1._More_about_grep
3.2._Filtering_output
4._Summary
5._Exercises
6._Text_editors
1._Text_editors
1.1._Why_should_I_use_an_editor?
1.2._Which_editor_should_I_use?
2._Using_the_Vim_editor
2.1._Two_modes
2.2._Basic_commands
2.3._The_easy_way
3._Linux_in_the_office
3.1._History
3.2._Suites_and_programs
3.3._Remarks
4._Summary
5._Exercises
7._Home_sweet_/home
1._General_good_housekeeping
1.1._Introduction
1.2._Make_space
2._Your_text_environment
2.1._Environment_variables
2.2._Shell_setup_files
2.3._A_typical_set_of_setup_files
2.4._The_Bash_prompt
2.5._Shell_scripts
3._The_graphical_environment
3.1._Introduction
3.2._The_X_Window_System
3.3._X_server_configuration
4._Region_specific_settings
4.1._Keyboard_setup
4.2._Fonts
4.3._Date_and_time_zone
4.4._Language
4.5._Country-specific_Information
5._Installing_new_software
5.1._General
5.2._Package_formats
5.3._Automating_package_management_and_updates
5.4._Upgrading_your_kernel
5.5._Installing_extra_packages_from_the_installation_CDs
6._Summary
7._Exercises
7.1._Shell_environment
7.2._Graphical_environment
8._Printers_and_printing
1._Printing_files
1.1._Command_line_printing
1.2._Formatting
2._The_server_side
2.1._General
2.2._Graphical_printer_configuration
2.3._Buying_a_printer_for_Linux
3._Print_problems
3.1._Wrong_file
3.2._My_print_hasn't_come_out
4._Summary
5._Exercises
9._Fundamental_Backup_Techniques
1._Introduction
1.1._Preparing_your_data
2._Moving_your_data_to_a_backup_device
2.1._Making_a_copy_on_a_floppy_disk
2.2._Making_a_copy_with_a_CD-writer
2.3._Backups_on/from_jazz_drives,_USB_devices_and_other
removables
2.4._Backing_up_data_using_a_tape_device
2.5._Tools_from_your_distribution
3._Using_rsync
3.1._Introduction
3.2._An_example:_rsync_to_a_USB_storage_device
4._Encryption
4.1._General_remarks
4.2._Generate_a_key
4.3._About_your_key
4.4._Encrypt_data
4.5._Decrypting_files
5._Summary
6._Exercises
10._Networking
1._Networking_Overview
1.1._The_OSI_Model
1.2._Some_popular_networking_protocols
2._Network_configuration_and_information
2.1._Configuration_of_network_interfaces
2.2._Network_configuration_files
2.3._Network_configuration_commands
2.4._Network_interface_names
2.5._Checking_the_host_configuration_with_netstat
2.6._Other_hosts
3._Internet/Intranet_applications
3.1._Server_types
3.2._Mail
3.3._Web
3.4._File_Transfer_Protocol
3.5._Chatting_and_conferencing
3.6._News_services
3.7._The_Domain_Name_System
3.8._DHCP
3.9._Authentication_services
4._Remote_execution_of_applications
4.1._Introduction
4.2._Rsh,_rlogin_and_telnet
4.3._The_X_Window_System
4.4._The_SSH_suite
4.5._VNC
4.6._The_rdesktop_protocol
4.7._Cygwin
5._Security
5.1._Introduction
5.2._Services
5.3._Update_regularly
5.4._Firewalls_and_access_policies
5.5._Intrusion_detection
5.6._More_tips
5.7._Have_I_been_hacked?
5.8._Recovering_from_intrusion
6._Summary
7._Exercises
7.1._General_networking
7.2._Remote_connections
7.3._Security
11._Sound_and_Video
1._Audio_Basics
1.1._Installation
1.2._Drivers_and_Architecture
2._Sound_and_video_playing
2.1._CD_playing_and_copying
2.2._Playing_music_files
2.3._Recording
3._Video_playing,_streams_and_television_watching
4._Internet_Telephony
4.1._What_is_it?
4.2._What_do_you_need?
5._Summary
6._Exercises
A._Where_to_go_from_here?
1._Useful_Books
1.1._General_Linux
1.2._Editors
1.3._Shells
1.4._X_Window
1.5._Networking
2._Useful_sites
2.1._General_information
2.2._Architecture_Specific_References
2.3._Distributions
2.4._Software
B._DOS_versus_Linux_commands
C._Shell_Features
1._Common_features
2._Differing_features
Glossary
Index
List of Figures
1. Introduction_to_Linux_front_cover
1.1. OpenOffice_MS-compatible_Spreadsheet
2.1. Terminal_window
2.2. Konqueror_as_help_browser
3.1. Linux_file_system_layout
3.2. Hard_and_soft_link_mechanism
4.1. Fork-and-exec_mechanism
4.2. Can't_you_go_faster?
4.3. Gnome_System_Monitor
8.1. Printer_Status_through_web_interface
9.1. Floppy_formatter
10.1. Evolution_mail_and_news_reader
10.2. X-Chat
10.3. SSH_X11_forwarding
11.1. XMMS_mp3_player
List of Tables
1. Typographic_and_usage_conventions
2.1. Quickstart_commands
2.2. Key_combinations_in_Bash
2.3. New_commands_in_chapter_2:_Basics
3.1. File_types_in_a_long_list
3.2. Subdirectories_of_the_root_directory
3.3. Most_common_configuration_files
3.4. Common_devices
3.5. Color-ls_default_color_scheme
3.6. Default_suffix_scheme_for_ls
3.7. Access_mode_codes
3.8. User_group_codes
3.9. File_protection_with_chmod
3.10. New_commands_in_chapter_3:_Files_and_the_file_system
3.11. File_permissions
4.1. Controlling_processes
4.2. Common_signals
4.3. New_commands_in_chapter_4:_Processes
5.1. New_commands_in_chapter_5:_I/O_redirection
7.1. Common_environment_variables
7.2. New_commands_in_chapter_7:_Making_yourself_at_home
8.1. New_commands_in_chapter_8:_Printing
9.1. New_commands_in_chapter_9:_Backup
10.1. The_simplified_OSI_Model
10.2. New_commands_in_chapter_10:_Networking
11.1. New_commands_in_chapter_11:_Audio
B.1. Overview_of_DOS/Linux_commands
C.1. Common_Shell_Features
C.2. Differing_Shell_Features
Introduction
Table of Contents
1._Why_this_guide?
2._Who_should_read_this_book?
3._New_versions_and_availability
4._Revision_History
5._Contributions
6._Feedback
7._Copyright_information
8._What_do_you_need?
9._Conventions_used_in_this_document
10._Organization_of_this_document
1. Why this guide?
Many people still believe that learning Linux is difficult, or that only
experts can understand how a Linux system works. Though there is a lot of free
documentation available, the documentation is widely scattered on the Web, and
often confusing, since it is usually oriented toward experienced UNIX or Linux
users. Today, thanks to the advancements in development, Linux has grown in
popularity both at home and at work. The goal of this guide is to show people
of all ages that Linux can be simple and fun, and used for all kinds of
purposes.
2. Who should read this book?
This guide was created as an overview of the Linux Operating System, geared
toward new users as an exploration tour and getting started guide, with
exercises at the end of each chapter. For more advanced trainees it can be a
desktop reference, and a collection of the base knowledge needed to proceed
with system and network administration. This book contains many real life
examples derived from the author's experience as a Linux system and network
administrator, trainer and consultant. We hope these examples will help you to
get a better understanding of the Linux system and that you feel encouraged to
try out things on your own.
Everybody who wants to get a “CLUEâ€, a Command Line User Experience, with Linux
(and UNIX in general) will find this book useful.
3. New versions and availability
This document is published in the Guides section of the Linux Documentation
Project collection at http://www.tldp.org/guides.html; you can also download
PDF and PostScript formatted versions here.
The most recent edition is available athttp://tille.garrels.be/training/tldp/.
The second edition of this guide is available in print from Fultus.com_Books as
paperback Print On Demand (POD) book. Fultus distributes_this_document through
Ingram and Baker & Taylor to many bookstores, including Amazon.com,
Amazon.co.uk, BarnesAndNoble.com and Google's_Froogle global shopping portal
and Google_Book_Search.
Figure 1. Introduction to Linux front cover
Front cover of the Intro to Linux guide, featuring a baby penguin that has
just come out of the egg.
The guide has been translated_into_Hindi by:
* Alok Kumar
* Dhananjay Sharma
* Kapil
* Puneet Goel
* Ravikant Yuyutsu
Andrea Montagner translated_the_guide_into_Italian.
4. Revision History
Revision History
Revision 1.27 20080606 MG
updates.
Revision 1.26 20070919 MG
Comments from readers, license.
Revision 1.25 20070511 MG
Comments from readers, minor updates, E-mail etiquette, updated info about
availability (thanks Oleg).
Revision 1.24 2006-11-01 MG
added index terms, prepared for second printed edition, added gpg and proxy
info.
Revision 1.23 2006-07-25 MG and FK
Updates and corrections, removed app5 again, adapted license to enable
inclusion in Debian docs.
Revision 1.22 2006-04-06 MG
chap8 revised completely, chap10: clarified examples, added ifconfig and
cygwin info, revised network apps.
Revision 1.21 2006-03-14 MG
Added exercises in chap11, corrected newline errors, command overview
completed for chapter 9, minor corrections in chap10.
Revision 1.20 2006-01-06 MG
Split chap7: audio stuff is now in separate chapter, chap11.xml. Small
revisions, updates for commands like aptitude, more on USB storage, Internet
telephony, corrections from readers.
Revision 1.13 2004-04-27 MG
Last read-through before sending everything to Fultus for printout. Added
Fultus referrence in New Versions section, updated Conventions and
Organization sections. Minor changes in chapters 4, 5, 6 and 8, added
rdesktop info in chapter 10, updated glossary, replaced references to
fileutils with coreutils, thankyou to Hindi translators.
5. Contributions
Many thanks to all the people who shared their experiences. And especially to
the Belgian Linux users for hearing me out every day and always being generous
in their comments.
Also a special thought for Tabatha Marshall for doing a really thorough
revision, spell check and styling, and to Eugene Crosser for spotting the
errors that we two overlooked.
And thanks to all the readers who notified me about missing topics and who
helped to pick out the last errors, unclear definitions and typos by going
through the trouble of mailing me all their remarks. These are also the people
who help me keep this guide up to date, like Filipus Klutiero who did a
complete review in 2005 and 2006 and helps me getting the guide into the Debian
docs collection, and Alexey Eremenko who sent me the foundation for chapter 11.
In 2006, Suresh Rajashekara created a Debian package of this documentation.
Finally, a big thank you for the volunteers who are currently translating this
document in French, Swedish, German, Farsi, Hindi and more. It is a big work
that should not be underestimated; I admire your courage.
6. Feedback
Missing information, missing links, missing characters? Mail it to the
maintainer of this document:
                    <tille wants no spam _at_ garrels dot be>
               Â
Don't forget to check with the latest_version first!
7. Copyright information
* Copyright (c) 2002-2007, Machtelt Garrels
* All rights reserved.
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of the author, Machtelt Garrels, nor the
* names of its contributors may be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE AUTHOR AND CONTRIBUTORS BE LIABLE FOR ANY
* DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES;
* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
* ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
The logos, trademarks and symbols used in this book are the properties of their
respective owners.
8. What do you need?
You will require a computer and a medium containing a Linux distribution. Most
of this guide applies to all Linux distributions - and UNIX in general. Apart
from time, there are no further specific requirements.
The Installation_HOWTO contains helpful information on how to obtain Linux
software and install it on your computer. Hardware requirements and coexistence
with other operating systems are also discussed.
CD images can be downloaded from linux-iso.com and many other locations, see
Appendix A,_Where_to_go_from_here?.
An interesting alternative for those who don't dare to take the step of an
actual Linux installation on their machine are the Linux distributions that you
can run from a CD, such as the Knoppix distribution.
9. Conventions used in this document
The following typographic and usage conventions occur in this text:
Table 1. Typographic and usage conventions
___________________________________________________________________________________________________
|Text_type______________|Meaning____________________________________________________________________|
|“Quoted_textâ€______|Quotes_from_people,_quoted_computer_output.________________________________|
| |Literal computer input and output captured from the terminal, usually |
| terminal view |rendered with a light grey background. |
|_______________________|___________________________________________________________________________|
|command________________|Name_of_a_command_that_can_be_entered_on_the_command_line._________________|
|VARIABLE_______________|Name_of_a_variable_or_pointer_to_content_of_a_variable,_as_in_$VARNAME.____|
|option_________________|Option_to_a_command,_as_in_“the_-a_option_to_the_ls_commandâ€.__________|
|argument_______________|Argument_to_a_command,_as_in_“read_man_ls_â€.___________________________|
|prompt |User prompt, usually followed by a command that you type in a terminal |
|_______________________|window,_like_in_hilda@home>_ls_-l__________________________________________|
|command options |Command synopsis or general usage, on a separated line. |
|arguments______________|___________________________________________________________________________|
|filename |Name of a file or directory, for example “Change to the /usr/bin |
|_______________________|directory.â€______________________________________________________________|
|Key____________________|Keys_to_hit_on_the_keyboard,_such_as_“type_Q_to_quitâ€._________________|
|Button_________________|Graphical_button_to_click,_like_the_OK_button._____________________________|
|Menu → Choice |Choice to select from a graphical menu, for instance: “SelectHelp → Abo|t
|_______________________|Mozilla_in_your_browser.â€________________________________________________|
|Terminology____________|Important_term_or_concept:_“The_Linux_kernel_is_the_heart_of_the_system.â|
| |The backslash in a terminal view or command synopsis indicates an |
| \ |unfinished line. In other words, if you see a long command that is cut into|
|_______________________|multiple_lines,_\_means_“Don't_press_Enter_yet!â€_______________________|
|See Chapter 1,_What_is|link to related subject within this guide. |
|Linux?_________________|___________________________________________________________________________|
|The_author_____________|Clickable_link_to_an_external_web_resource.________________________________|
The following images are used:
This is a note
It contains additional information or remarks.
This is a caution
It means be careful.
This is a warning
Be very careful.
This is a tip
Tips and tricks.
10. Organization of this document
This guide is part of the Linux Documentation Project and aims to be the
foundation for all other materials that you can get from the Project. As such,
it provides you with the fundamental knowledge needed by anyone who wants to
start working with a Linux system, while at the same time it tries to
consciously avoid re-inventing the hot water. Thus, you can expect this book to
be incomplete and full of links to sources of additional information on your
system, on the Internet and in your system documentation.
The first chapter is an introduction to the subject on Linux; the next two
discuss absolute basic commands. Chapters 4 and 5 discuss some more advanced
but still basic topics. Chapter 6 is needed for continuing with the rest, since
it discusses editing files, an ability you need to pass from Linux newbie to
Linux user. The following chapters discuss somewhat more advanced topics that
you will have to deal with in everyday Linux use.
All chapters come with exercises that will test your preparedness for the next
chapter.
* Chapter 1,_What_is_Linux?: What is Linux, how did it come into existence,
advantages and disadvantages, what does the future hold for Linux, who should
use it, installing your computer.
* Chapter 2,_Quickstart: Getting started, connecting to the system, basic
commands, where to find help.
* Chapter 3,_About_files_and_the_file_system: The filesystem, important files
and directories, managing files and directories, protecting your data.
* Chapter 4,_Processes: Understanding and managing processes, boot and shutdown
procedures, postponing tasks, repetitive tasks.
* Chapter 5,_I/O_redirection: What are standard input, output and error and how
are these features used from the command line.
* Chapter 6,_Text_editors: Why you should learn to work with an editor,
discussion of the most common editors.
* Chapter 7,_Home_sweet_/home: Configuring your graphical, text and audio
environment, settings for the non-native English speaking Linux user, tips
for adding extra software.
* Chapter 8,_Printers_and_printing: Converting files to a printable format,
getting them out of the printer, hints for solving print problems.
* Chapter 9,_Fundamental_Backup_Techniques: Preparing data to be backed up,
discussion of various tools, remote backup.
* Chapter 10,_Networking: Overview of Linux networking tools and user
applications, with a short discussion of the underlying service daemon
programs and secure networking.
* Chapter 11,_Sound_and_Video: Sound and video, including Voice over IP and
sound recording is discussed in this chapter.
* Appendix A,_Where_to_go_from_here?: Which books to read and sites to visit
when you have finished reading this one.
* Appendix B,_DOS_versus_Linux_commands: A comparison.
* Appendix C,_Shell_Features: If you ever get stuck, these tables might be an
outcome. Also a good argument when your boss insists that YOU should use HIS
favorite shell.
Chapter 1. What is Linux?
Table of Contents
1._History
1.1._UNIX
1.2._Linus_and_Linux
1.3._Current_application_of_Linux_systems
2._The_user_interface
2.1._Is_Linux_difficult?
2.2._Linux_for_non-experienced_users
3._Does_Linux_have_a_future?
3.1._Open_Source
3.2._Ten_years_of_experience_at_your_service
4._Properties_of_Linux
4.1._Linux_Pros
4.2._Linux_Cons
5._Linux_Flavors
5.1._Linux_and_GNU
5.2._GNU/Linux
5.3._Which_distribution_should_I_install?
6._Summary
7._Exercises
Abstract
We will start with an overview of how Linux became the operating system it is
today. We will discuss past and future development and take a closer look at
the advantages and disadvantages of this system. We will talk about
distributions, about Open Source in general and try to explain a little
something about GNU.
This chapter answers questions like:
* What is Linux?
* Where and how did Linux start?
* Isn't Linux that system where everything is done in text mode?
* Does Linux have a future or is it just hype?
* What are the advantages of using Linux?
* What are the disadvantages?
* What kinds of Linux are there and how do I choose the one that fits me?
* What are the Open Source and GNU movements?
1. History
1.1. UNIX
In order to understand the popularity of Linux, we need to travel back in time,
about 30 years ago...
Imagine computers as big as houses, even stadiums. While the sizes of those
computers posed substantial problems, there was one thing that made this even
worse: every computer had a different operating system. Software was always
customized to serve a specific purpose, and software for one given system
didn't run on another system. Being able to work with one system didn't
automatically mean that you could work with another. It was difficult, both for
the users and the system administrators.
Computers were extremely expensive then, and sacrifices had to be made even
after the original purchase just to get the users to understand how they
worked. The total cost per unit of computing power was enormous.
Technologically the world was not quite that advanced, so they had to live with
the size for another decade. In 1969, a team of developers in the Bell Labs
laboratories started working on a solution for the software problem, to address
these compatibility issues. They developed a new operating system, which was
1. Simple and elegant.
2. Written in the C programming language instead of in assembly code.
3. Able to recycle code.
The Bell Labs developers named their project “UNIX.â€
The code recycling features were very important. Until then, all commercially
available computer systems were written in a code specifically developed for
one system. UNIX on the other hand needed only a small piece of that special
code, which is now commonly named the kernel. This kernel is the only piece of
code that needs to be adapted for every specific system and forms the base of
the UNIX system. The operating system and all other functions were built around
this kernel and written in a higher programming language, C. This language was
especially developed for creating the UNIX system. Using this new technique, it
was much easier to develop an operating system that could run on many different
types of hardware.
The software vendors were quick to adapt, since they could sell ten times more
software almost effortlessly. Weird new situations came in existence: imagine
for instance computers from different vendors communicating in the same
network, or users working on different systems without the need for extra
education to use another computer. UNIX did a great deal to help users become
compatible with different systems.
Throughout the next couple of decades the development of UNIX continued. More
things became possible to do and more hardware and software vendors added
support for UNIX to their products.
UNIX was initially found only in very large environments with mainframes and
minicomputers (note that a PC is a “micro†computer). You had to work at a
university, for the government or for large financial corporations in order to
get your hands on a UNIX system.
But smaller computers were being developed, and by the end of the 80's, many
people had home computers. By that time, there were several versions of UNIX
available for the PC architecture, but none of them were truly free and more
important: they were all terribly slow, so most people ran MS DOS or Windows
3.1 on their home PCs.
1.2. Linus and Linux
By the beginning of the 90s home PCs were finally powerful enough to run a full
blown UNIX. Linus Torvalds, a young man studying computer science at the
university of Helsinki, thought it would be a good idea to have some sort of
freely available academic version of UNIX, and promptly started to code.
He started to ask questions, looking for answers and solutions that would help
him get UNIX on his PC. Below is one of his first posts in comp.os.minix,
dating from 1991:
From: torvalds@klaava.Helsinki.FI (Linus Benedict Torvalds)
Newsgroups: comp.os.minix
Subject: Gcc-1.40 and a posix-question
Message-ID: <1991Jul3.100050.9886@klaava.Helsinki.FI>
Date: 3 Jul 91 10:00:50 GMT
Hello netlanders,
Due to a project I'm working on (in minix), I'm interested in the posix
standard definition. Could somebody please point me to a (preferably)
machine-readable format of the latest posix rules? Ftp-sites would be
nice.
From the start, it was Linus' goal to have a free system that was completely
compliant with the original UNIX. That is why he asked for POSIX standards,
POSIX still being the standard for UNIX.
In those days plug-and-play wasn't invented yet, but so many people were
interested in having a UNIX system of their own, that this was only a small
obstacle. New drivers became available for all kinds of new hardware, at a
continuously rising speed. Almost as soon as a new piece of hardware became
available, someone bought it and submitted it to the Linux test, as the system
was gradually being called, releasing more free code for an ever wider range of
hardware. These coders didn't stop at their PC's; every piece of hardware they
could find was useful for Linux.
Back then, those people were called “nerds†or “freaksâ€, but it didn't matter
to them, as long as the supported hardware list grew longer and longer. Thanks
to these people, Linux is now not only ideal to run on new PC's, but is also
the system of choice for old and exotic hardware that would be useless if Linux
didn't exist.
Two years after Linus' post, there were 12000 Linux users. The project, popular
with hobbyists, grew steadily, all the while staying within the bounds of the
POSIX standard. All the features of UNIX were added over the next couple of
years, resulting in the mature operating system Linux has become today. Linux
is a full UNIX clone, fit for use on workstations as well as on middle-range
and high-end servers. Today, a lot of the important players on the hard- and
software market each have their team of Linux developers; at your local
dealer's you can even buy pre-installed Linux systems with official support -
eventhough there is still a lot of hard- and software that is not supported,
too.
1.3. Current application of Linux systems
Today Linux has joined the desktop market. Linux developers concentrated on
networking and services in the beginning, and office applications have been the
last barrier to be taken down. We don't like to admit that Microsoft is ruling
this market, so plenty of alternatives have been started over the last couple
of years to make Linux an acceptable choice as a workstation, providing an easy
user interface and MS compatible office applications like word processors,
spreadsheets, presentations and the like.
On the server side, Linux is well-known as a stable and reliable platform,
providing database and trading services for companies like Amazon, the well-
known online bookshop, US Post Office, the German army and many others.
Especially Internet providers and Internet service providers have grown fond of
Linux as firewall, proxy- and web server, and you will find a Linux box within
reach of every UNIX system administrator who appreciates a comfortable
management station. Clusters of Linux machines are used in the creation of
movies such as “Titanicâ€, “Shrek†and others. In post offices, they are the
nerve centers that route mail and in large search engine, clusters are used to
perform internet searches.These are only a few of the thousands of heavy-duty
jobs that Linux is performing day-to-day across the world.
It is also worth to note that modern Linux not only runs on workstations, mid-
and high-end servers, but also on “gadgets†like PDA's, mobiles, a shipload of
embedded applications and even on experimental wristwatches. This makes Linux
the only operating system in the world covering such a wide range of hardware.
2. The user interface
2.1. Is Linux difficult?
Whether Linux is difficult to learn depends on the person you're asking.
Experienced UNIX users will say no, because Linux is an ideal operating system
for power-users and programmers, because it has been and is being developed by
such people.
Everything a good programmer can wish for is available: compilers, libraries,
development and debugging tools. These packages come with every standard Linux
distribution. The C-compiler is included for free - as opposed to many UNIX
distributions demanding licensing fees for this tool. All the documentation and
manuals are there, and examples are often included to help you get started in
no time. It feels like UNIX and switching between UNIX and Linux is a natural
thing.
In the early days of Linux, being an expert was kind of required to start using
the system. Those who mastered Linux felt better than the rest of the “lusersâ€
who hadn't seen the light yet. It was common practice to tell a beginning user
to “RTFM†(read the manuals). While the manuals were on every system, it was
difficult to find the documentation, and even if someone did, explanations were
in such technical terms that the new user became easily discouraged from
learning the system.
The Linux-using community started to realize that if Linux was ever to be an
important player on the operating system market, there had to be some serious
changes in the accessibility of the system.
2.2. Linux for non-experienced users
Companies such as RedHat, SuSE and Mandriva have sprung up, providing packaged
Linux distributions suitable for mass consumption. They integrated a great deal
of graphical user interfaces (GUIs), developed by the community, in order to
ease management of programs and services. As a Linux user today you have all
the means of getting to know your system inside out, but it is no longer
necessary to have that knowledge in order to make the system comply to your
requests.
Nowadays you can log in graphically and start all required applications without
even having to type a single character, while you still have the ability to
access the core of the system if needed. Because of its structure, Linux allows
a user to grow into the system: it equally fits new and experienced users. New
users are not forced to do difficult things, while experienced users are not
forced to work in the same way they did when they first started learning Linux.
While development in the service area continues, great things are being done
for desktop users, generally considered as the group least likely to know how a
system works. Developers of desktop applications are making incredible efforts
to make the most beautiful desktops you've ever seen, or to make your Linux
machine look just like your former MS Windows or an Apple workstation. The
latest developments also include 3D acceleration support and support for USB
devices, single-click updates of system and packages, and so on. Linux has
these, and tries to present all available services in a logical form that
ordinary people can understand. Below is a short list containing some great
examples; these sites have a lot of screenshots that will give you a glimpse of
what Linux on the desktop can be like:
* http://www.gnome.org
* http://kde.org/screenshots/
* http://www.openoffice.org
* http://www.mozilla.org
3. Does Linux have a future?
3.1. Open Source
The idea behind Open Source software is rather simple: when programmers can
read, distribute and change code, the code will mature. People can adapt it,
fix it, debug it, and they can do it at a speed that dwarfs the performance of
software developers at conventional companies. This software will be more
flexible and of a better quality than software that has been developed using
the conventional channels, because more people have tested it in more different
conditions than the closed software developer ever can.
The Open Source initiative started to make this clear to the commercial world,
and very slowly, commercial vendors are starting to see the point. While lots
of academics and technical people have already been convinced for 20 years now
that this is the way to go, commercial vendors needed applications like the
Internet to make them realize they can profit from Open Source. Now Linux has
grown past the stage where it was almost exclusively an academic system, useful
only to a handful of people with a technical background. Now Linux provides
more than the operating system: there is an entire infrastructure supporting
the chain of effort of creating an operating system, of making and testing
programs for it, of bringing everything to the users, of supplying maintenance,
updates and support and customizations, etcetera. Today, Linux is ready to
accept the challenge of a fast-changing world.
3.2. Ten years of experience at your service
While Linux is probably the most well-known Open Source initiative, there is
another project that contributed enormously to the popularity of the Linux
operating system. This project is called SAMBA, and its achievement is the
reverse engineering of the Server Message Block (SMB)/Common Internet File
System (CIFS) protocol used for file- and print-serving on PC-related machines,
natively supported by MS Windows NT and OS/2, and Linux. Packages are now
available for almost every system and provide interconnection solutions in
mixed environments using MS Windows protocols: Windows-compatible (up to and
includingWinXP) file- and print-servers.
Maybe even more successful than the SAMBA project is the Apache HTTP server
project. The server runs on UNIX, Windows NT and many other operating systems.
Originally known as “A PAtCHy serverâ€, based on existing code and a series of
“patch filesâ€, the name for the matured code deserves to be connoted with the
native American tribe of the Apache, well-known for their superior skills in
warfare strategy and inexhaustible endurance. Apache has been shown to be
substantially faster, more stable and more feature-full than many other web
servers. Apache is run on sites that get millions of visitors per day, and
while no official support is provided by the developers, the Apache user
community provides answers to all your questions. Commercial support is now
being provided by a number of third parties.
In the category of office applications, a choice of MS Office suite clones is
available, ranging from partial to full implementations of the applications
available on MS Windows workstations. These initiatives helped a great deal to
make Linux acceptable for the desktop market, because the users don't need
extra training to learn how to work with new systems. With the desktop comes
the praise of the common users, and not only their praise, but also their
specific requirements, which are growing more intricate and demanding by the
day.
The Open Source community, consisting largely of people who have been
contributing for over half a decade, assures Linux' position as an important
player on the desktop market as well as in general IT application. Paid
employees and volunteers alike are working diligently so that Linux can
maintain a position in the market. The more users, the more questions. The Open
Source community makes sure answers keep coming, and watches the quality of the
answers with a suspicious eye, resulting in ever more stability and
accessibility.
Listing all the available Linux software is beyond the scope of this guide, as
there are tens of thousands of packages. Throughout this course we will present
you with the most common packages, which are almost all freely available. In
order to take away some of the fear of the beginning user, here's a screenshot
of one of your most-wanted programs. You can see for yourself that no effort
has been spared to make users who are switching from Windows feel at home:
Figure 1.1. OpenOffice MS-compatible Spreadsheet
OpenOffice spreadsheet looks just like MS Office .xls file.
4. Properties of Linux
4.1. Linux Pros
A lot of the advantages of Linux are a consequence of Linux' origins, deeply
rooted in UNIX, except for the first advantage, of course:
* Linux is free:
As in free beer, they say. If you want to spend absolutely nothing, you don't
even have to pay the price of a CD. Linux can be downloaded in its entirety
from the Internet completely for free. No registration fees, no costs per
user, free updates, and freely available source code in case you want to
change the behavior of your system.
Most of all, Linux is free as in free speech:
The license commonly used is the GNU Public License (GPL). The license says
that anybody who may want to do so, has the right to change Linux and
eventually to redistribute a changed version, on the one condition that the
code is still available after redistribution. In practice, you are free to
grab a kernel image, for instance to add support for teletransportation
machines or time travel and sell your new code, as long as your customers can
still have a copy of that code.
* Linux is portable to any hardware platform:
A vendor who wants to sell a new type of computer and who doesn't know what
kind of OS his new machine will run (say the CPU in your car or washing
machine), can take a Linux kernel and make it work on his hardware, because
documentation related to this activity is freely available.
* Linux was made to keep on running:
As with UNIX, a Linux system expects to run without rebooting all the time.
That is why a lot of tasks are being executed at night or scheduled
automatically for other calm moments, resulting in higher availability during
busier periods and a more balanced use of the hardware. This property allows
for Linux to be applicable also in environments where people don't have the
time or the possibility to control their systems night and day.
* Linux is secure and versatile:
The security model used in Linux is based on the UNIX idea of security, which
is known to be robust and of proven quality. But Linux is not only fit for
use as a fort against enemy attacks from the Internet: it will adapt equally
to other situations, utilizing the same high standards for security. Your
development machine or control station will be as secure as your firewall.
* Linux is scalable:
From a Palmtop with 2 MB of memory to a petabyte storage cluster with
hundreds of nodes: add or remove the appropriate packages and Linux fits all.
You don't need a supercomputer anymore, because you can use Linux to do big
things using the building blocks provided with the system. If you want to do
little things, such as making an operating system for an embedded processor
or just recycling your old 486, Linux will do that as well.
* The Linux OS and most Linux applications have very short debug-times:
Because Linux has been developed and tested by thousands of people, both
errors and people to fix them are usually found rather quickly. It sometimes
happens that there are only a couple of hours between discovery and fixing of
a bug.
4.2. Linux Cons
* There are far too many different distributions:
“Quot capites, tot rationesâ€, as the Romans already said: the more people,
the more opinions. At first glance, the amount of Linux distributions can be
frightening, or ridiculous, depending on your point of view. But it also
means that everyone will find what he or she needs. You don't need to be an
expert to find a suitable release.
When asked, generally every Linux user will say that the best distribution is
the specific version he is using. So which one should you choose? Don't worry
too much about that: all releases contain more or less the same set of basic
packages. On top of the basics, special third party software is added making,
for example, TurboLinux more suitable for the small and medium enterprise,
RedHat for servers and SuSE for workstations. However, the differences are
likely to be very superficial. The best strategy is to test a couple of
distributions; unfortunately not everybody has the time for this. Luckily,
there is plenty of advice on the subject of choosing your Linux. A quick
search on Google, using the keywords “choosing your distribution†brings up
tens of links to good advise. The Installation_HOWTO also discusses choosing
your distribution.
* Linux is not very user friendly and confusing for beginners:
It must be said that Linux, at least the core system, is less userfriendly to
use than MS Windows and certainly more difficult than MacOS, but... In light
of its popularity, considerable effort has been made to make Linux even
easier to use, especially for new users. More information is being released
daily, such as this guide, to help fill the gap for documentation available
to users at all levels.
* Is an Open Source product trustworthy?
How can something that is free also be reliable? Linux users have the choice
whether to use Linux or not, which gives them an enormous advantage compared
to users of proprietary software, who don't have that kind of freedom. After
long periods of testing, most Linux users come to the conclusion that Linux
is not only as good, but in many cases better and faster that the traditional
solutions. If Linux were not trustworthy, it would have been long gone, never
knowing the popularity it has now, with millions of users. Now users can
influence their systems and share their remarks with the community, so the
system gets better and better every day. It is a project that is never
finished, that is true, but in an ever changing environment, Linux is also a
project that continues to strive for perfection.
5. Linux Flavors
5.1. Linux and GNU
Although there are a large number of Linux implementations, you will find a lot
of similarities in the different distributions, if only because every Linux
machine is a box with building blocks that you may put together following your
own needs and views. Installing the system is only the beginning of a longterm
relationship. Just when you think you have a nice running system, Linux will
stimulate your imagination and creativeness, and the more you realize what
power the system can give you, the more you will try to redefine its limits.
Linux may appear different depending on the distribution, your hardware and
personal taste, but the fundamentals on which all graphical and other
interfaces are built, remain the same. The Linux system is based on GNU tools
(Gnu's Not UNIX), which provide a set of standard ways to handle and use the
system. All GNU tools are open source, so they can be installed on any system.
Most distributions offer pre-compiled packages of most common tools, such as
RPM packages on RedHat and Debian packages (also called deb or dpkg) on Debian,
so you needn't be a programmer to install a package on your system. However, if
you are and like doing things yourself, you will enjoy Linux all the better,
since most distributions come with a complete set of development tools,
allowing installation of new software purely from source code. This setup also
allows you to install software even if it does not exist in a pre-packaged form
suitable for your system.
A list of common GNU software:
* Bash: The GNU shell
* GCC: The GNU C Compiler
* GDB: The GNU Debugger
* Coreutils: a set of basic UNIX-style utilities, such as ls, cat and chmod
* Findutils: to search and find files
* Fontutils: to convert fonts from one format to another or make new fonts
* The Gimp: GNU Image Manipulation Program
* Gnome: the GNU desktop environment
* Emacs: a very powerful editor
* Ghostscript and Ghostview: interpreter and graphical frontend for PostScript
files.
* GNU Photo: software for interaction with digital cameras
* Octave: a programming language, primarily intended to perform numerical
computations and image processing.
* GNU SQL: relational database system
* Radius: a remote authentication and accounting server
* ...
Many commercial applications are available for Linux, and for more information
about these packages we refer to their specific documentation. Throughout this
guide we will only discuss freely available software, which comes (in most
cases) with a GNU license.
To install missing or new packages, you will need some form of software
management. The most common implementations include RPM and dpkg. RPM is the
RedHat Package Manager, which is used on a variety of Linux systems, eventhough
the name does not suggest this. Dpkg is the Debian package management system,
which uses an interface called apt-get, that can manage RPM packages as well.
Novell Ximian Red Carpet is a third party implementation of RPM with a
graphical front-end. Other third party software vendors may have their own
installation procedures, sometimes resembling the InstallShield and such, as
known on MS Windows and other platforms. As you advance into Linux, you will
likely get in touch with one or more of these programs.
5.2. GNU/Linux
The Linux kernel (the bones of your system, see Section 2.3.1,_“The_kernelâ€) is
not part of the GNU project but uses the same license as GNU software. A great
majority of utilities and development tools (the meat of your system), which
are not Linux-specific, are taken from the GNU project. Because any usable
system must contain both the kernel and at least a minimal set of utilities,
some people argue that such a system should be called a GNU/Linux system.
In order to obtain the highest possible degree of independence between
distributions, this is the sort of Linux that we will discuss throughout this
course. If we are not talking about a GNU/Linux system, the specific
distribution, version or program name will be mentioned.
5.3. Which distribution should I install?
Prior to installation, the most important factor is your hardware. Since every
Linux distribution contains the basic packages and can be built to meet almost
any requirement (because they all use the Linux kernel), you only need to
consider if the distribution will run on your hardware. LinuxPPC for example
has been made to run on Apple and other PowerPCs and does not run on an
ordinary x86 based PC. LinuxPPC does run on the new Macs, but you can't use it
for some of the older ones with ancient bus technology. Another tricky case is
Sun hardware, which could be an old SPARC CPU or a newer UltraSparc, both
requiring different versions of Linux.
Some Linux distributions are optimized for certain processors, such as Athlon
CPUs, while they will at the same time run decent enough on the standard 486,
586 and 686 Intel processors. Sometimes distributions for special CPUs are not
as reliable, since they are tested by fewer people.
Most Linux distributions offer a set of programs for generic PCs with special
packages containing optimized kernels for the x86 Intel based CPUs. These
distributions are well-tested and maintained on a regular basis, focusing on
reliant server implementation and easy installation and update procedures.
Examples are Debian, Ubuntu, Fedora, SuSE and Mandriva, which are by far the
most popular Linux systems and generally considered easy to handle for the
beginning user, while not blocking professionals from getting the most out of
their Linux machines. Linux also runs decently on laptops and middle-range
servers. Drivers for new hardware are included only after extensive testing,
which adds to the stability of a system.
While the standard desktop might be Gnome on one system, another might offer
KDE by default. Generally, both Gnome and KDE are available for all major Linux
distributions. Other window and desktop managers are available for more
advanced users.
The standard installation process allows users to choose between different
basic setups, such as a workstation, where all packages needed for everyday use
and development are installed, or a server installation, where different
network services can be selected. Expert users can install every combination of
packages they want during the initial installation process.
The goal of this guide is to apply to all Linux distributions. For your own
convenience, however, it is strongly advised that beginners stick to a
mainstream distribution, supporting all common hardware and applications by
default. The following are very good choices for novices:
* Fedora_Core
* Debian
* SuSE_Linux
* Mandriva_(former_MandrakeSoft)
* Knoppix: an operating system that runs from your CD-ROM, you don't need to
install anything.
Downloadable ISO-images can be obtained from LinuxISO.org. The main
distributions can be purchased in any decent computer shop.
6. Summary
In this chapter, we learned that:
* Linux is an implementation of UNIX.
* The Linux operating system is written in the C programming language.
* “De gustibus et coloribus non disputandum estâ€: there's a Linux for everyone.
* Linux uses GNU tools, a set of freely available standard tools for handling
the operating system.
7. Exercises
A practical exercise for starters: install Linux on your PC. Read the
installation manual for your distribution and/or the Installation HOWTO and do
it.
Read the docs!
Most errors stem from not reading the information provided during the install.
Reading the installation messages carefully is the first step on the road to
success.
Things you must know BEFORE starting a Linux installation:
* Will this distribution run on my hardware?
Check with http://www.tldp.org/HOWTO/Hardware-HOWTO/index.html when in doubt
about compatibility of your hardware.
* What kind of keyboard do I have (number of keys, layout)? What kind of mouse
(serial/parallel, number of buttons)? How many MB of RAM?
* Will I install a basic workstation or a server, or will I need to select
specific packages myself?
* Will I install from my hard disk, from a CD-ROM, or using the network? Should
I adapt the BIOS for any of this? Does the installation method require a boot
disk?
* Will Linux be the only system on this computer, or will it be a dual boot
installation? Should I make a large partition in order to install virtual
systems later on, or is this a virtual installation itself?
* Is this computer in a network? What is its hostname, IP address? Are there
any gateway servers or other important networked machines my box should
communicate with?
Linux expects to be networked
Not using the network or configuring it incorrectly may result in slow
startup.
* Is this computer a gateway/router/firewall? (If you have to think about this
question, it probably isn't.)
* Partitioning: let the installation program do it for you this time, we will
discuss partitions in detail in Chapter 3,_About_files_and_the_file_system.
There is system-specific documentation available if you want to know
everything about it. If your Linux distribution does not offer default
partitioning, that probably means it is not suited for beginners.
* Will this machine start up in text mode or in graphical mode?
* Think of a good password for the administrator of this machine (root). Create
a non-root user account (non-privileged access to the system).
* Do I need a rescue disk? (recommended)
* Which languages do I want?
The full checklist can be found at http://www.tldp.org/HOWTO/Installation-
HOWTO/index.html.
In the following chapters we will find out if the installation has been
successful.
Chapter 2. Quickstart
Table of Contents
1._Logging_in,_activating_the_user_interface_and_logging_out
1.1._Introduction
1.2._Graphical_mode
1.3._Text_mode
2._Absolute_basics
2.1._The_commands
2.2._General_remarks
2.3._Using_Bash_features
3._Getting_help
3.1._Be_warned
3.2._The_man_pages
3.3._More_info
4._Summary
5._Exercises
5.1._Connecting_and_disconnecting
5.2._Passwords
5.3._Directories
5.4._Files
5.5._Getting_help
Abstract
In order to get the most out of this guide, we will immediately start with a
practical chapter on connecting to the Linux system and doing some basic
things.
We will discuss:
* Connecting to the system
* Disconnecting from the system
* Text and graphic mode
* Changing your password
* Navigating through the file system
* Determining file type
* Looking at text files
* Finding help
1. Logging in, activating the user interface and logging out
1.1. Introduction
In order to work on a Linux system directly, you will need to provide a user
name and password. You always need to authenticate to the system. As we already
mentioned in the exercise from Chapter 1,_What_is_Linux?, most PC-based Linux
systems have two basic modes for a system to run in: either quick and sober in
text console mode, which looks like DOS with mouse, multitasking and multi-user
features, or in graphical mode, which looks better but eats more system
resources.
1.2. Graphical mode
This is the default nowadays on most desktop computers. You know you will
connect to the system using graphical mode when you are first asked for your
user name, and then, in a new window, to type your password.
To log in, make sure the mouse pointer is in the login window, provide your
user name and password to the system and click OK or press Enter.
Careful with that root account!
It is generally considered a bad idea to connect (graphically) using the root
user name, the system adminstrator's account, since the use of graphics
includes running a lot of extra programs, in root's case with a lot of extra
permissions. To keep all risks as low as possible, use a normal user account to
connect graphically. But there are enough risks to keep this in mind as a
general advice, for all use of the root account: only log in as root when extra
privileges are required.
After entering your user name/password combination, it can take a little while
before the graphical environment is started, depending on the CPU speed of your
computer, on the software you use and on your personal settings.
To continue, you will need to open a
terminal window
or xterm for short (X being the name for the underlying software supporting
the graphical environment). This program can be found in the Applications →
Utilities, System Tools or Internet menu, depending on what window manager you
are using. There might be icons that you can use as a shortcut to get an xterm
window as well, and clicking the right mouse button on the desktop background
will usually present you with a menu containing a terminal window application.
While browsing the menus, you will notice that a lot of things can be done
without entering commands via the keyboard. For most users, the good old point-
'n'-click method of dealing with the computer will do. But this guide is for
future network and system administrators, who will need to meddle with the
heart of the system. They need a stronger tool than a mouse to handle all the
tasks they will face. This tool is the shell, and when in graphical mode, we
activate our shell by opening a terminal window.
The terminal window is your control panel for the system. Almost everything
that follows is done using this simple but powerful text tool. A terminal
window should always show a command prompt when you open one. This terminal
shows a standard prompt, which displays the user's login name, and the current
working directory, represented by the twiddle (~):
Figure 2.1. Terminal window
X terminal window when just opened displays a prompt.
Another common form for a prompt is this one:
[user@host dir]
In the above example, user will be your login name, hosts the name of the
machine you are working on, and dir an indication of your current location in
the file system.
Later we will discuss prompts and their behavior in detail. For now, it
suffices to know that prompts can display all kinds of information, but that
they are not part of the commands you are giving to your system.
To disconnect from the system in graphical mode, you need to close all terminal
windows and other applications. After that, hit the logout icon or find Log Out
in the menu. Closing everything is not really necessary, and the system can do
this for you, but session management might put all currently open applications
back on your screen when you connect again, which takes longer and is not
always the desired effect. However, this behavior is configurable.
When you see the login screen again, asking to enter user name and password,
logout was successful.
Gnome or KDE?
We mentioned both the Gnome and KDE desktops already a couple of times. These
are the two most popular ways of managing your desktop, although there are
many, many others. Whatever desktop you chose to work with is fine - as long as
you know how to open a terminal window. However, we will continue to refer to
both Gnome and KDE for the most popular ways of achieving certain tasks.
1.3. Text mode
You know you're in text mode when the whole screen is black, showing (in most
cases white) characters. A text mode login screen typically shows some
information about the machine you are working on, the name of the machine and a
prompt waiting for you to log in:
RedHat Linux Release 8.0 (Psyche)
blast login: _
The login is different from a graphical login, in that you have to hit the
Enter key after providing your user name, because there are no buttons on the
screen that you can click with the mouse. Then you should type your password,
followed by another Enter. You won't see any indication that you are entering
something, not even an asterisk, and you won't see the cursor move. But this is
normal on Linux and is done for security reasons.
When the system has accepted you as a valid user, you may get some more
information, called the message of the day, which can be anything.
Additionally, it is popular on UNIX systems to display a fortune cookie, which
contains some general wise or unwise (this is up to you) thoughts. After that,
you will be given a shell, indicated with the same prompt that you would get in
graphical mode.
Don't log in as root
Also in text mode: log in as root only to do setup and configuration that
absolutely requires administrator privileges, such as adding users, installing
software packages, and performing network and other system configuration. Once
you are finished, immediately leave the special account and resume your work as
a non-privileged user. Alternatively, some systems, like Ubuntu, force you to
use
sudo
, so that you do not need direct access to the administrative account.
Logging out is done by entering the
logout
command, followed by Enter. You are successfully disconnected from the system
when you see the login screen again.
The power button
While Linux was not meant to be shut off without application of the proper
procedures for halting the system, hitting the power button is equivalent to
starting those procedures on newer systems. However, powering off an old system
without going through the halting process might cause severe damage! If you
want to be sure, always use the Shut down option when you log out from the
graphical interface, or, when on the login screen (where you have to give your
user name and password) look around for a shutdown button.
Now that we know how to connect to and disconnect from the system, we're ready
for our first commands.
2. Absolute basics
2.1. The commands
These are the quickies, which we need to get started; we will discuss them
later in more detail.
Table 2.1. Quickstart commands
_________________________________________________________________________
|Command_______|Meaning___________________________________________________|
|ls |Displays a list of files in the current working directory,|
|______________|like_the_dir_command_in_DOS_______________________________|
|cd_directory__|change_directories________________________________________|
|passwd________|change_the_password_for_the_current_user__________________|
|file_filename_|display_file_type_of_file_with_name_filename______________|
|cat_textfile__|throws_content_of_textfile_on_the_screen__________________|
|pwd___________|display_present_working_directory_________________________|
|exit_or_logout|leave_this_session________________________________________|
|man_command___|read_man_pages_on_command_________________________________|
|info_command__|read_Info_pages_on_command________________________________|
|apropos_string|search_the_whatis_database_for_strings____________________|
2.2. General remarks
You type these commands after the prompt, in a terminal window in graphical
mode or in text mode, followed by Enter.
Commands can be issued by themselves, such as
ls
. A command behaves different when you specify an
option
, usually preceded with a dash (-), as in ls -a. The same option character may
have a different meaning for another command. GNU programs take long options,
preceded by two dashes (--), like ls --all. Some commands have no options.
The argument(s) to a command are specifications for the object(s) on which you
want the command to take effect. An example is ls /etc, where the directory /
etc is the argument to the ls command. This indicates that you want to see the
content of that directory, instead of the default, which would be the content
of the current directory, obtained by just typing ls followed by Enter. Some
commands require arguments, sometimes arguments are optional.
You can find out whether a command takes options and arguments, and which ones
are valid, by checking the online help for that command, see Section 3,
“Getting_helpâ€.
In Linux, like in UNIX, directories are separated using forward slashes, like
the ones used in web addresses (URLs). We will discuss directory structure in-
depth later.
The symbols . and .. have special meaning when directories are concerned. We
will try to find out about those during the exercises, and more in the next
chapter.
Try to avoid logging in with or using the system administrator's account, root.
Besides doing your normal work, most tasks, including checking the system,
collecting information etc., can be executed using a normal user account with
no special permissions at all. If needed, for instance when creating a new user
or installing new software, the preferred way of obtaining root access is by
switching user IDs, see Section 2.1,_“The_path†for an example.
Almost all commands in this book can be executed without system administrator
privileges. In most cases, when issuing a command or starting a program as a
non-privileged user, the system will warn you or prompt you for the root
password when root access is required. Once you're done, leave the application
or session that gives you root privileges immediately.
Reading documentation should become your second nature. Especially in the
beginning, it is important to read system documentation, manuals for basic
commands, HOWTOs and so on. Since the amount of documentation is so enormous,
it is impossible to include all related documentation. This book will try to
guide you to the most appropriate documentation on every subject discussed, in
order to stimulate the habit of reading the man pages.
2.3. Using Bash features
Several special key combinations allow you to do things easier and faster with
the GNU shell, Bash, which is the default on almost any Linux system, see
Section 2.3.2,_“The_shellâ€. Below is a list of the most commonly used features;
you are strongly suggested to make a habit out of using them, so as to get the
most out of your Linux experience from the very beginning.
Table 2.2. Key combinations in Bash
_____________________________________________________________________________
|Key or key |Function |
|combination_____|____________________________________________________________|
|Ctrl+A__________|Move_cursor_to_the_beginning_of_the_command_line.___________|
|Ctrl+C |End a running program and return the prompt, see Chapter 4,|
|________________|Processes.__________________________________________________|
|Ctrl+D |Log out of the current shell session, equal to typing exit |
|________________|or_logout.__________________________________________________|
|Ctrl+E__________|Move_cursor_to_the_end_of_the_command_line._________________|
|Ctrl+H__________|Generate_backspace_character._______________________________|
|Ctrl+L__________|Clear_this_terminal.________________________________________|
|Ctrl+R |Search command history, see Section 3.3.4,_“The_grep |
|________________|commandâ€._________________________________________________|
|Ctrl+Z__________|Suspend_a_program,_see_Chapter 4,_Processes._______________|
|ArrowLeft and |Move the cursor one place to the left or right on the |
|ArrowRight |command line, so that you can insert characters at other |
|________________|places_than_just_at_the_beginning_and_the_end.______________|
|ArrowUp and |Browse history. Go to the line that you want to repeat, edit|
|ArrowDown_______|details_if_necessary,_and_press_Enter_to_save_time._________|
|Shift+PageUp and|Browse terminal buffer (to see text that has "scrolled off" |
|Shift+PageDown__|the_screen).________________________________________________|
| |Command or filename completion; when multiple choices are |
|Tab |possible, the system will either signal with an audio or |
| |visual bell, or, if too many choices are possible, ask you |
|________________|if_you_want_to_see_them_all.________________________________|
|Tab_Tab_________|Shows_file_or_command_completion_possibilities._____________|
The last two items in the above table may need some extra explanations. For
instance, if you want to change into the directory
directory_with_a_very_long_name, you are not going to type that very long name,
no. You just type on the command line cd dir, then you press Tab and the shell
completes the name for you, if no other files are starting with the same three
characters. Of course, if there are no other items starting with “dâ€, then you
might just as wel type cd d and then Tab. If more than one file starts with the
same characters, the shell will signal this to you, upon which you can hit Tab
twice with short interval, and the shell presents the choices you have:
your_prompt> cd st
starthere stuff stuffit
In the above example, if you type “a†after the first two characters and hit
Tab again, no other possibilities are left, and the shell completes the
directory name, without you having to type the string “rthereâ€:
your_prompt> cd starthere
Of course, you'll still have to hit Enter to accept this choice.
In the same example, if you type “uâ€, and then hit Tab, the shell will add the
“ff†for you, but then it protests again, because multiple choices are
possible. If you type Tab Tab again, you'll see the choices; if you type one or
more characters that make the choice unambiguous to the system, and Tab again,
or Enter when you've reach the end of the file name that you want to choose,
the shell completes the file name and changes you into that directory - if
indeed it is a directory name.
This works for all file names that are arguments to commands.
The same goes for command name completion. Typing ls and then hitting the Tab
key twice, lists all the commands in your PATH (see Section 2.1,_“The_pathâ€)
that start with these two characters:
your_prompt> ls
ls lsdev lspci lsraid lsw
lsattr lsmod lspgpot lss16toppm
lsb_release lsof lspnp lsusb
3. Getting help
3.1. Be warned
GNU/Linux is all about becoming more self-reliant. And as usual with this
system, there are several ways to achieve the goal. A common way of getting
help is finding someone who knows, and however patient and peace-loving the
Linux-using community will be, almost everybody will expect you to have tried
one or more of the methods in this section before asking them, and the ways in
which this viewpoint is expressed may be rather harsh if you prove not to have
followed this basic rule.
3.2. The man pages
A lot of beginning users fear the man (manual) pages, because they are an
overwhelming source of documentation. They are, however, very structured, as
you will see from the example below on: man man.
Reading man pages is usually done in a terminal window when in graphical mode,
or just in text mode if you prefer it. Type the command like this at the
prompt, followed by Enter:
yourname@yourcomp ~> man man
The documentation for man will be displayed on your screen after you press
Enter:
man(1) man(1)
NAME
man - format and display the on-line manual pages
manpath - determine user's search path for man pages
SYNOPSIS
man [-acdfFhkKtwW] [--path] [-m system] [-p string] [-C config_file]
[-M pathlist] [-P pager] [-S section_list] [section] name ...
DESCRIPTION
man formats and displays the on-line manual pages. If you specify
section, man only looks in that section of the manual.
name is normally the name of the manual page, which is typically the
name of a command, function, or file. However, if name contains a
slash (/) then man interprets it as a file specification, so that you
can do man ./foo.5 or even man /cd/foo/bar.1.gz.
See below for a description of where man looks for the manual
page files.
OPTIONS
-C config_file
lines 1-27
Browse to the next page using the space bar. You can go back to the previous
page using the b-key. When you reach the end, man will usually quit and you get
the prompt back. Type q if you want to leave the man page before reaching the
end, or if the viewer does not quit automatically at the end of the page.
Pagers
The available key combinations for manipulating the man pages depend on the
pager used in your distribution. Most distributions use
less
to view the man pages and to scroll around. See Section 3.4.2,_““less_is
moreâ€â€ for more info on pagers.
Each man page usually contains a couple of standard sections, as we can see
from the man man example:
* The first line contains the name of the command you are reading about, and
the id of the section in which this man page is located. The man pages are
ordered in chapters. Commands are likely to have multiple man pages, for
example the man page from the user section, the man page from the system
admin section, and the man page from the programmer section.
* The name of the command and a short description are given, which is used for
building an index of the man pages. You can look for any given search string
in this index using the
apropos
command.
* The synopsis of the command provides a technical notation of all the options
and/or arguments this command can take. You can think of an option as a way
of executing the command. The argument is what you execute it on. Some
commands have no options or no arguments. Optional options and arguments are
put in between “[†and “]†to indicate that they can be left out.
* A longer description of the command is given.
* Options with their descriptions are listed. Options can usually be combined.
If not so, this section will tell you about it.
* Environment describes the shell variables that influence the behavior of this
command (not all commands have this).
* Sometimes sections specific to this command are provided.
* A reference to other man pages is given in the “SEE ALSO†section. In between
parentheses is the number of the man page section in which to find this
command. Experienced users often switch to the “SEE ALSO†part using the /
command followed by the search string SEE and press Enter.
* Usually there is also information about known bugs (anomalies) and where to
report new bugs you may find.
* There might also be author and copyright information.
Some commands have multiple man pages. For instance, the passwd command has a
man page in section 1 and another in section 5. By default, the man page with
the lowest number is shown. If you want to see another section than the
default, specify it after the man command:
man 5 passwd
If you want to see all man pages about a command, one after the other, use the
-a to man:
man -a passwd
This way, when you reach the end of the first man page and press SPACE again,
the man page from the next section will be displayed.
3.3. More info
3.3.1. The Info pages
In addition to the man pages, you can read the Info pages about a command,
using the
info
command. These usually contain more recent information and are somewhat easier
to use. The man pages for some commands refer to the Info pages.
Get started by typing info info in a terminal window:
File: info.info, Node: Top, Next: Getting Started, Up: (dir)
Info: An Introduction
*********************
Info is a program, which you are using now, for reading
documentation of computer programs. The GNU Project distributes most
of its on-line manuals in the Info format, so you need a program called
"Info reader" to read the manuals. One of such programs you are using
now.
If you are new to Info and want to learn how to use it, type the
command `h' now. It brings you to a programmed instruction sequence.
To learn advanced Info commands, type `n' twice. This brings you to
`Info for Experts', skipping over the `Getting Started' chapter.
* Menu:
* Getting Started:: Getting started using an Info reader.
* Advanced Info:: Advanced commands within Info.
* Creating an Info File:: How to make your own Info file.
--zz-Info: (info.info.gz)Top, 24 lines --Top-------------------------------
Welcome to Info version 4.2. Type C-h for help, m for menu item.
Use the arrow keys to browse through the text and move the cursor on a line
starting with an asterisk, containing the keyword about which you want info,
then hit Enter. Use the P and N keys to go to the previous or next subject. The
space bar will move you one page further, no matter whether this starts a new
subject or an Info page for another command. Use Q to quit. The info program
has more information.
3.3.2. The whatis and apropos commands
A short index of explanations for commands is available using the
whatis
command, like in the examples below:
[your_prompt] whatis ls
ls (1) - list directory contents
This displays short information about a command, and the first section in the
collection of man pages that contains an appropriate page.
If you don't know where to get started and which man page to read,
apropos
gives more information. Say that you don't know how to start a browser, then
you could enter the following command:
another prompt> apropos browser
Galeon [galeon](1) - gecko-based GNOME web browser
lynx (1) - a general purpose distributed information browser
for the World Wide Web
ncftp (1) - Browser program for the File Transfer Protocol
opera (1) - a graphical web browser
pilot (1) - simple file system browser in the style of the
Pine Composer
pinfo (1) - curses based lynx-style info browser
pinfo [pman] (1) - curses based lynx-style info browser
viewres (1x) - graphical class browser for Xt
After pressing Enter you will see that a lot of browser related stuff is on
your machine: not only web browsers, but also file and FTP browsers, and
browsers for documentation. If you have development packages installed, you may
also have the accompanying man pages dealing with writing programs having to do
with browsers. Generally, a command with a man page in section one, so one
marked with “(1)â€, is suitable for trying out as a user. The user who issued
the above apropos might consequently try to start the commands galeon, lynx or
opera, since these clearly have to do with browsing the world wide web.
3.3.3. The --help option
Most GNU commands support the --help, which gives a short explanation about how
to use the command and a list of available options. Below is the output of this
option with the cat command:
userprompt@host: cat --help
Usage: cat [OPTION] [FILE]...
Concatenate FILE(s), or standard input, to standard output.
-A, --show-all equivalent to -vET
-b, --number-nonblank number nonblank output lines
-e equivalent to -vE
-E, --show-ends display $ at end of each line
-n, --number number all output lines
-s, --squeeze-blank never more than one single blank line
-t equivalent to -vT
-T, --show-tabs display TAB characters as ^I
-u (ignored)
-v, --show-nonprinting use ^ and M- notation,
except for LFD and TAB
--help display this help and exit
--version output version information and exit
With no FILE, or when FILE is -, read standard input.
Report bugs to <bug-textutils@gnu.org>.
3.3.4. Graphical help
Don't despair if you prefer a graphical user interface. Konqueror, the default
KDE file manager, provides painless and colourful access to the man and Info
pages. You may want to try “info:info†in the Location address bar, and you
will get a browsable Info page about the info command. Similarly, “man:ls†will
present you with the man page for the ls command. You even get command name
completion: you will see the man pages for all the commands starting with “lsâ€
in a scroll-down menu. Entering “info:/dir†in the address location toolbar
displays all the Info pages, arranged in utility categories. Excellent Help
content, including the Konqueror Handbook. Start up from the menu or by typing
the command konqueror in a terminal window, followed by Enter; see the
screenshot below.
Figure 2.2. Konqueror as help browser
Type "man:cat" in the location toolbar of Konqueror to display the cat man
page.
The Gnome Help Browser is very user friendly as well. You can start it
selecting Applications → Help from the Gnome menu, by clicking the lifeguard
icon on your desktop or by entering the command gnome-help in a terminal
window. The system documentation and man pages are easily browsable with a
plain interface.
The
nautilus
file manager provides a searchable index of the man and Info pages, they are
easily browsable and interlinked. Nautilus is started from the command line, or
clicking your home directory icon, or from the Gnome menu.
The big advantage of GUIs for system documentation is that all information is
completely interlinked, so you can click through in the “SEE ALSO†sections and
wherever links to other man pages appear, and thus browse and acquire knowledge
without interruption for hours at the time.
3.3.5. Exceptions
Some commands don't have separate documentation, because they are part of
another command. cd, exit, logout and pwd are such exceptions. They are part of
your shell program and are called shell built-in commands. For information
about these, refer to the man or info page of your shell. Most beginning Linux
users have a Bash shell. See Section 2.3.2,_“The_shell†for more about shells.
If you have been changing your original system configuration, it might also be
possible that man pages are still there, but not visible because your shell
environment has changed. In that case, you will need to check the MANPATH
variable. How to do this is explained in Section 2.1.2,_“Exporting_variablesâ€.
Some programs or packages only have a set of instructions or references in the
directory
/usr/share/doc
. See Section 3.4,_“More_ways_to_view_file_content†to display.
In the worst case, you may have removed the documentation from your system by
accident (hopefully by accident, because it is a very bad idea to do this on
purpose). In that case, first try to make sure that there is really nothing
appropriate left using a search tool, read on in Section 3.3,_“Finding_filesâ€.
If so, you may have to re-install the package that contains the command to
which the documentation applied, see Section 5,_“Installing_new_softwareâ€.
4. Summary
Linux traditionally operates in text mode or in graphical mode. Since CPU power
and RAM are not the cost anymore these days, every Linux user can afford to
work in graphical mode and will usually do so. This does not mean that you
don't have to know about text mode: we will work in the text environment
throughout this course, using a terminal window.
Linux encourages its users to acquire knowledge and to become independent.
Inevitably, you will have to read a lot of documentation to achieve that goal;
that is why, as you will notice, we refer to extra documentation for almost
every command, tool and problem listed in this book. The more docs you read,
the easier it will become and the faster you will leaf through manuals. Make
reading documentation a habit as soon as possible. When you don't know the
answer to a problem, refering to the documentation should become a second
nature.
We already learned some commands:
Table 2.3. New commands in chapter 2: Basics
______________________________________________________
|Command|Meaning_______________________________________|
|apropos|Search_information_about_a_command_or_subject.|
|cat____|Show_content_of_one_or_more_files.____________|
|cd_____|Change_into_another_directory.________________|
|exit___|Leave_a_shell_session.________________________|
|file___|Get_information_about_the_content_of_a_file.__|
|info___|Read_Info_pages_about_a_command.______________|
|logout_|Leave_a_shell_session.________________________|
|ls_____|List_directory_content._______________________|
|man____|Read_manual_pages_of_a_command._______________|
|passwd_|Change_your_password._________________________|
|pwd____|Display_the_current_working_directory.________|
5. Exercises
Most of what we learn is by making mistakes and by seeing how things can go
wrong. These exercises are made to get you to read some error messages. The
order in which you do these exercises is important.
Don't forget to use the Bash features on the command line: try to do the
exercises typing as few characters as possible!
5.1. Connecting and disconnecting
* Determine whether you are working in text or in graphical mode.
I am working in text/graphical mode. (cross out what's not applicable)
* Log in with the user name and password you made for yourself during the
installation.
* Log out.
* Log in again, using a non-existent user name
-> What happens?
5.2. Passwords
Log in again with your user name and password.
* Change your password into P6p3.aa! and hit the Enter key.
-> What happens?
* Try again, this time enter a password that is ridiculously easy, like 123 or
aaa.
-> What happens?
* Try again, this time don't enter a password but just hit the Enter key.
-> What happens?
* Try the command psswd instead of passwd
-> What happens?
New password
Unless you change your password back again to what it was before this exercise,
it will be “P6p3.aa!â€. Change your password after this exercise!
Note that some systems might not allow to recycle passwords, i.e. restore the
original one within a certain amount of time or a certain amount of password
changes, or both.
5.3. Directories
These are some exercises to help you get the feel.
* Enter the command cd blah
-> What happens?
* Enter the command cd ..
Mind the space between "cd" and ".."! Use the pwd command.
-> What happens?
* List the directory contents with the ls command.
-> What do you see?
-> What do you think these are?
-> Check using the pwd command.
* Enter the cd command.
-> What happens?
* Repeat step 2 two times.
-> What happens?
* Display the content of this directory.
* Try the command cd root
-> What happens?
-> To which directories do you have access?
* Repeat step 4.
Do you know another possibility to get where you are now?
5.4. Files
* Change directory to / and then to etc. Type ls; if the output is longer than
your screen, make the window longer, or try Shift+PageUp and Shift+PageDown.
The file inittab contains the answer to the first question in this list. Try
the file command on it.
-> The file type of my inittab is .....
* Use the command cat inittab and read the file.
-> What is the default mode of your computer?
* Return to your home directory using the cd command.
* Enter the command file .
-> Does this help to find the meaning of “.�
* Can you look at "." using the cat command?
* Display help for the cat program, using the --help option. Use the option for
numbering of output lines to count how many users are listed in the file /
etc/passwd.
5.5. Getting help
* Read man intro
* Read man ls
* Read info passwd
* Enter the apropos pwd command.
* Try man or info on cd.
-> How would you find out more about cd?
* Read ls --help and try it out.
Chapter 3. About files and the file system
Table of Contents
1._General_overview_of_the_Linux_file_system
1.1._Files
1.2._About_partitioning
1.3._More_file_system_layout
2._Orientation_in_the_file_system
2.1._The_path
2.2._Absolute_and_relative_paths
2.3._The_most_important_files_and_directories
2.4._The_most_important_configuration_files
2.5._The_most_common_devices
2.6._The_most_common_variable_files
3._Manipulating_files
3.1._Viewing_file_properties
3.2._Creating_and_deleting_files_and_directories
3.3._Finding_files
3.4._More_ways_to_view_file_content
3.5._Linking_files
4._File_security
4.1._Access_rights:_Linux's_first_line_of_defense
4.2._The_tools
5._Summary
6._Exercises
6.1._Partitions
6.2._Paths
6.3._Tour_of_the_system
6.4._Manipulating_files
6.5._File_permissions
Abstract
After the initial exploration in Chapter 2,_Quickstart, we are ready to discuss
the files and directories on a Linux system in more detail. Many users have
difficulties with Linux because they lack an overview of what kind of data is
kept in which locations. We will try to shine some light on the organization of
files in the file system.
We will also list the most important files and directories and use different
methods of viewing the content of those files, and learn how files and
directories can be created, moved and deleted.
After completion of the exercises in this chapter, you will be able to:
* Describe the layout of a Linux file system
* Display and set paths
* Describe the most important files, including kernel and shell
* Find lost and hidden files
* Create, move and delete files and directories
* Display contents of files
* Understand and use different link types
* Find out about file properties and change file permissions
1. General overview of the Linux file system
1.1. Files
1.1.1. General
A simple description of the UNIX system, also applicable to Linux, is this:
“On a UNIX system, everything is a file; if something is not a file, it is a
process.â€
This statement is true because there are special files that are more than just
files (named pipes and sockets, for instance), but to keep things simple,
saying that everything is a file is an acceptable generalization. A Linux
system, just like UNIX, makes no difference between a file and a directory,
since a directory is just a file containing names of other files. Programs,
services, texts, images, and so forth, are all files. Input and output devices,
and generally all devices, are considered to be files, according to the system.
In order to manage all those files in an orderly fashion, man likes to think of
them in an ordered tree-like structure on the hard disk, as we know from MS-DOS
(Disk Operating System) for instance. The large branches contain more branches,
and the branches at the end contain the tree's leaves or normal files. For now
we will use this image of the tree, but we will find out later why this is not
a fully accurate image.
1.1.2. Sorts of files
Most files are just files, called
regular
files; they contain normal data, for example text files, executable files or
programs, input for or output from a program and so on.
While it is reasonably safe to suppose that everything you encounter on a Linux
system is a file, there are some exceptions.
* Directories: files that are lists of other files.
* Special files: the mechanism used for input and output. Most special files
are in /dev, we will discuss them later.
* Links: a system to make a file or directory visible in multiple parts of the
system's file tree. We will talk about links in detail.
* (Domain) sockets: a special file type, similar to TCP/IP sockets, providing
inter-process networking protected by the file system's access control.
* Named pipes: act more or less like sockets and form a way for processes to
communicate with each other, without using network socket semantics.
The -l option to
ls
displays the file type, using the first character of each input line:
jaime:~/Documents> ls -l
total 80
-rw-rw-r-- 1 jaime jaime 31744 Feb 21 17:56 intro Linux.doc
-rw-rw-r-- 1 jaime jaime 41472 Feb 21 17:56 Linux.doc
drwxrwxr-x 2 jaime jaime 4096 Feb 25 11:50 course
This table gives an overview of the characters determining the file type:
Table 3.1. File types in a long list
___________________
|Symbol|Meaning_____|
|-_____|Regular_file|
|d_____|Directory___|
|l_____|Link________|
|c_____|Special_file|
|s_____|Socket______|
|p_____|Named_pipe__|
|b_____|Block_device|
In order not to always have to perform a long listing for seeing the file type,
a lot of systems by default don't issue just
ls
, but ls -F, which suffixes file names with one of the characters “/=*|@†to
indicate the file type. To make it extra easy on the beginning user, both the -
F and
--color
options are usually combined, see Section 3.1.1,_“More_about_lsâ€. We will use
ls -F throughout this document for better readability.
As a user, you only need to deal directly with plain files, executable files,
directories and links. The special file types are there for making your system
do what you demand from it and are dealt with by system administrators and
programmers.
Now, before we look at the important files and directories, we need to know
more about partitions.
1.2. About partitioning
1.2.1. Why partition?
Most people have a vague knowledge of what partitions are, since every
operating system has the ability to create or remove them. It may seem strange
that Linux uses more than one partition on the same disk, even when using the
standard installation procedure, so some explanation is called for.
One of the goals of having different partitions is to achieve higher data
security in case of disaster. By dividing the hard disk in partitions, data can
be grouped and separated. When an accident occurs, only the data in the
partition that got the hit will be damaged, while the data on the other
partitions will most likely survive.
This principle dates from the days when Linux didn't have journaled file
systems and power failures might have lead to disaster. The use of partitions
remains for security and robustness reasons, so a breach on one part of the
system doesn't automatically mean that the whole computer is in danger. This is
currently the most important reason for partitioning. A simple example: a user
creates a script, a program or a web application that starts filling up the
disk. If the disk contains only one big partition, the entire system will stop
functioning if the disk is full. If the user stores the data on a separate
partition, then only that (data) partition will be affected, while the system
partitions and possible other data partitions keep functioning.
Mind that having a journaled file system only provides data security in case of
power failure and sudden disconnection of storage devices. This does not
protect your data against bad blocks and logical errors in the file system. In
those cases, you should use a RAID (Redundant Array of Inexpensive Disks)
solution.
1.2.2. Partition layout and types
There are two kinds of major partitions on a Linux system:
* data partition
: normal Linux system data, including the root partition containing all the
data to start up and run the system; and
* swap partition
: expansion of the computer's physical memory, extra memory on hard disk.
Most systems contain a root partition, one or more data partitions and one or
more swap partitions. Systems in mixed environments may contain partitions for
other system data, such as a partition with a FAT or VFAT file system for MS
Windows data.
Most Linux systems use
fdisk
at installation time to set the partition type. As you may have noticed during
the exercise from Chapter 1, this usually happens automatically. On some
occasions, however, you may not be so lucky. In such cases, you will need to
select the partition type manually and even manually do the actual
partitioning. The standard Linux partitions have number 82 for swap and 83 for
data, which can be journaled (ext3) or normal (ext2, on older systems). The
fdisk utility has built-in help, should you forget these values.
Apart from these two, Linux supports a variety of other file system types, such
as the relatively new Reiser file system, JFS, NFS, FATxx and many other file
systems natively available on other (proprietary) operating systems.
The standard root partition (indicated with a single forward slash, /) is about
100-500 MB, and contains the system configuration files, most basic commands
and server programs, system libraries, some temporary space and the home
directory of the administrative user. A standard installation requires about
250 MB for the root partition.
Swap space (indicated with
swap
) is only accessible for the system itself, and is hidden from view during
normal operation. Swap is the system that ensures, like on normal UNIX systems,
that you can keep on working, whatever happens. On Linux, you will virtually
never see irritating messages like Out of memory, please close some
applications first and try again, because of this extra memory. The swap or
virtual memory procedure has long been adopted by operating systems outside the
UNIX world by now.
Using memory on a hard disk is naturally slower than using the real memory
chips of a computer, but having this little extra is a great comfort. We will
learn more about swap when we discuss processes in Chapter 4,_Processes.
Linux generally counts on having twice the amount of physical memory in the
form of swap space on the hard disk. When installing a system, you have to know
how you are going to do this. An example on a system with 512 MB of RAM:
* 1st possibility: one swap partition of 1 GB
* 2nd possibility: two swap partitions of 512 MB
* 3rd possibility: with two hard disks: 1 partition of 512 MB on each disk.
The last option will give the best results when a lot of I/O is to be expected.
Read the software documentation for specific guidelines. Some applications,
such as databases, might require more swap space. Others, such as some handheld
systems, might not have any swap at all by lack of a hard disk. Swap space may
also depend on your kernel version.
The kernel is on a separate partition as well in many distributions, because it
is the most important file of your system. If this is the case, you will find
that you also have a
/boot
partition, holding your kernel(s) and accompanying data files.
The rest of the hard disk(s) is generally divided in data partitions, although
it may be that all of the non-system critical data resides on one partition,
for example when you perform a standard workstation installation. When non-
critical data is separated on different partitions, it usually happens
following a set pattern:
* a partition for user programs (/usr)
* a partition containing the users' personal data (/home)
* a partition to store temporary data like print- and mail-queues (/var)
* a partition for third party and extra software (/opt)
Once the partitions are made, you can only add more. Changing sizes or
properties of existing partitions is possible but not advisable.
The division of hard disks into partitions is determined by the system
administrator. On larger systems, he or she may even spread one partition over
several hard disks, using the appropriate software. Most distributions allow
for standard setups optimized for workstations (average users) and for general
server purposes, but also accept customized partitions. During the installation
process you can define your own partition layout using either your distribution
specific tool, which is usually a straight forward graphical interface, or
fdisk
, a text-based tool for creating partitions and setting their properties.
A workstation or client installation is for use by mainly one and the same
person. The selected software for installation reflects this and the stress is
on common user packages, such as nice desktop themes, development tools, client
programs for E-mail, multimedia software, web and other services. Everything is
put together on one large partition, swap space twice the amount of RAM is
added and your generic workstation is complete, providing the largest amount of
disk space possible for personal use, but with the disadvantage of possible
data integrity loss during problem situations.
On a server, system data tends to be separate from user data. Programs that
offer services are kept in a different place than the data handled by this
service. Different partitions will be created on such systems:
* a partition with all data necessary to boot the machine
* a partition with configuration data and server programs
* one or more partitions containing the server data such as database tables,
user mails, an ftp archive etc.
* a partition with user programs and applications
* one or more partitions for the user specific files (home directories)
* one or more swap partitions (virtual memory)
Servers usually have more memory and thus more swap space. Certain server
processes, such as databases, may require more swap space than usual; see the
specific documentation for detailed information. For better performance, swap
is often divided into different swap partitions.
1.2.3. Mount points
All partitions are attached to the system via a mount point. The mount point
defines the place of a particular data set in the file system. Usually, all
partitions are connected through the root partition. On this partition, which
is indicated with the slash (/), directories are created. These empty
directories will be the starting point of the partitions that are attached to
them. An example: given a partition that holds the following directories:
videos/ cd-images/ pictures/
We want to attach this partition in the filesystem in a directory called /opt/
media. In order to do this, the system administrator has to make sure that the
directory /opt/media exists on the system. Preferably, it should be an empty
directory. How this is done is explained later in this chapter. Then, using the
mount command, the administrator can attach the partition to the system. When
you look at the content of the formerly empty directory /opt/media, it will
contain the files and directories that are on the mounted medium (hard disk or
partition of a hard disk, CD, DVD, flash card, USB or other storage device).
During system startup, all the partitions are thus mounted, as described in the
file
/etc/fstab
. Some partitions are not mounted by default, for instance if they are not
constantly connected to the system, such like the storage used by your digital
camera. If well configured, the device will be mounted as soon as the system
notices that it is connected, or it can be user-mountable, i.e. you don't need
to be system administrator to attach and detach the device to and from the
system. There is an example in Section 3,_“Using_rsyncâ€.
On a running system, information about the partitions and their mount points
can be displayed using the
df
command (which stands for disk full or disk free). In Linux, df is the GNU
version, and supports the -h or human readable option which greatly improves
readability. Note that commercial UNIX machines commonly have their own
versions of df and many other commands. Their behavior is usually the same,
though GNU versions of common tools often have more and better features.
The df command only displays information about active non-swap partitions.
These can include partitions from other networked systems, like in the example
below where the home directories are mounted from a file server on the network,
a situation often encountered in corporate environments.
freddy:~> df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hda8 496M 183M 288M 39% /
/dev/hda1 124M 8.4M 109M 8% /boot
/dev/hda5 19G 15G 2.7G 85% /opt
/dev/hda6 7.0G 5.4G 1.2G 81% /usr
/dev/hda7 3.7G 2.7G 867M 77% /var
fs1:/home 8.9G 3.7G 4.7G 44% /.automount/fs1/root/home
1.3. More file system layout
1.3.1. Visual
For convenience, the Linux file system is usually thought of in a tree
structure. On a standard Linux system you will find the layout generally
follows the scheme presented below.
Figure 3.1. Linux file system layout
Graphical overview of Linux top level directories and most important
subdirectories.
This is a layout from a RedHat system. Depending on the system admin, the
operating system and the mission of the UNIX machine, the structure may vary,
and directories may be left out or added at will. The names are not even
required; they are only a convention.
The tree of the file system starts at the trunk or
slash
, indicated by a forward slash (/). This directory, containing all underlying
directories and files, is also called the
root directory
or “the root†of the file system.
Directories that are only one level below the root directory are often preceded
by a slash, to indicate their position and prevent confusion with other
directories that could have the same name. When starting with a new system, it
is always a good idea to take a look in the root directory. Let's see what you
could run into:
emmy:~> cd /
emmy:/> ls
bin/ dev/ home/ lib/ misc/ opt/ root/ tmp/ var/
boot/ etc/ initrd/ lost+found/ mnt/ proc/ sbin/ usr/
Table 3.2. Subdirectories of the root directory
_____________________________________________________________________________
|Directory__|Content__________________________________________________________|
|/bin |Common programs, shared by the system, the system administrator |
|___________|and_the_users.___________________________________________________|
| |The startup files and the kernel, vmlinuz. In some recent |
|/boot |distributions also grub data. Grub is the GRand Unified Boot |
| |loader and is an attempt to get rid of the many different boot- |
|___________|loaders_we_know_today.___________________________________________|
|/dev |Contains references to all the CPU peripheral hardware, which are|
|___________|represented_as_files_with_special_properties.____________________|
| |Most important system configuration files are in /etc, this |
|/etc |directory contains data similar to those in the Control Panel in |
|___________|Windows__________________________________________________________|
|/home______|Home_directories_of_the_common_users.____________________________|
|/initrd____|(on_some_distributions)_Information_for_booting._Do_not_remove!__|
|/lib |Library files, includes files for all kinds of programs needed by|
|___________|the_system_and_the_users.________________________________________|
|/lost+found|Every partition has a lost+found in its upper directory. Files |
|___________|that_were_saved_during_failures_are_here.________________________|
|/misc______|For_miscellaneous_purposes.______________________________________|
|/mnt |Standard mount point for external file systems, e.g. a CD-ROM or |
|___________|a_digital_camera.________________________________________________|
|/net_______|Standard_mount_point_for_entire_remote_file_systems______________|
|/opt_______|Typically_contains_extra_and_third_party_software._______________|
| |A virtual file system containing information about system |
| |resources. More information about the meaning of the files in |
|/proc |proc is obtained by entering the command man proc in a terminal |
| |window. The file proc.txt discusses the virtual file system in |
|___________|detail.__________________________________________________________|
| |The administrative user's home directory. Mind the difference |
|/root |between /, the root directory and /root, the home directory of |
|___________|the_root_user.___________________________________________________|
|/sbin______|Programs_for_use_by_the_system_and_the_system_administrator._____|
|/tmp |Temporary space for use by the system, cleaned upon reboot, so |
|___________|don't_use_this_for_saving_any_work!______________________________|
|/usr |Programs, libraries, documentation etc. for all user-related |
|___________|programs.________________________________________________________|
| |Storage for all variable files and temporary files created by |
|/var |users, such as log files, the mail queue, the print spooler area,|
| |space for temporary storage of files downloaded from the |
|___________|Internet,_or_to_keep_an_image_of_a_CD_before_burning_it._________|
How can you find out which partition a directory is on? Using the df command
with a dot (.) as an option shows the partition the current directory belongs
to, and informs about the amount of space used on this partition:
sandra:/lib> df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/hda7 980M 163M 767M 18% /
As a general rule, every directory under the root directory is on the root
partition, unless it has a separate entry in the full listing from df (or df -
h with no other options).
Read more in
man
hier
.
1.3.2. The file system in reality
For most users and for most common system administration tasks, it is enough to
accept that files and directories are ordered in a tree-like structure. The
computer, however, doesn't understand a thing about trees or tree-structures.
Every partition has its own file system. By imagining all those file systems
together, we can form an idea of the tree-structure of the entire system, but
it is not as simple as that. In a file system, a file is represented by an
inode
, a kind of serial number containing information about the actual data that
makes up the file: to whom this file belongs, and where is it located on the
hard disk.
Every partition has its own set of inodes; throughout a system with multiple
partitions, files with the same inode number can exist.
Each inode describes a data structure on the hard disk, storing the properties
of a file, including the physical location of the file data. When a hard disk
is initialized to accept data storage, usually during the initial system
installation process or when adding extra disks to an existing system, a fixed
number of inodes per partition is created. This number will be the maximum
amount of files, of all types (including directories, special files, links
etc.) that can exist at the same time on the partition. We typically count on
having 1 inode per 2 to 8 kilobytes of storage.
At the time a new file is created, it gets a free inode. In that inode is the
following information:
* Owner and group owner of the file.
* File type (regular, directory, ...)
* Permissions on the file Section 4.1,_“Access_rights:_Linux's_first_line_of
defenseâ€
* Date and time of creation, last read and change.
* Date and time this information has been changed in the inode.
* Number of links to this file (see later in this chapter).
* File size
* An address defining the actual location of the file data.
The only information not included in an inode, is the file name and directory.
These are stored in the special directory files. By comparing file names and
inode numbers, the system can make up a tree-structure that the user
understands. Users can display inode numbers using the -i option to ls. The
inodes have their own separate space on the disk.
2. Orientation in the file system
2.1. The path
When you want the system to execute a command, you almost never have to give
the full path to that command. For example, we know that the ls command is in
the /bin directory (check with which -a ls), yet we don't have to enter the
command /bin/ls for the computer to list the content of the current directory.
The
PATH
environment variable takes care of this. This variable lists those directories
in the system where executable files can be found, and thus saves the user a
lot of typing and memorizing locations of commands. So the path naturally
contains a lot of directories containing bin somewhere in their names, as the
user below demonstrates. The
echo
command is used to display the content (“$â€) of the variable
PATH
:
rogier:> echo $PATH
/opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin
In this example, the directories /opt/local/bin, /usr/X11R6/bin, /usr/bin, /
usr/sbin and /bin are subsequently searched for the required program. As soon
as a match is found, the search is stopped, even if not every directory in the
path has been searched. This can lead to strange situations. In the first
example below, the user knows there is a program called sendsms to send an SMS
message, and another user on the same system can use it, but she can't. The
difference is in the configuration of the PATH variable:
[jenny@blob jenny]$ sendsms
bash: sendsms: command not found
[jenny@blob jenny]$ echo $PATH
/bin:/usr/bin:/usr/bin/X11:/usr/X11R6/bin:/home/jenny/bin
[jenny@blob jenny]$ su - tony
Password:
tony:~>which sendsms
sendsms is /usr/local/bin/sendsms
tony:~>echo $PATH
/home/tony/bin.Linux:/home/tony/bin:/usr/local/bin:/usr/local/sbin:\
/usr/X11R6/bin:/usr/bin:/usr/sbin:/bin:/sbin
Note the use of the
su
(switch user) facility, which allows you to run a shell in the environment of
another user, on the condition that you know the user's password.
A backslash indicates the continuation of a line on the next, without an Enter
separating one line from the other.
In the next example, a user wants to call on the
wc
(word count) command to check the number of lines in a file, but nothing
happens and he has to break off his action using the Ctrl+C combination:
jumper:~> wc -l test
(Ctrl-C)
jumper:~> which wc
wc is hashed (/home/jumper/bin/wc)
jumper:~> echo $PATH
/home/jumper/bin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin
The use of the
which
command shows us that this user has a bin-directory in his home directory,
containing a program that is also called wc. Since the program in his home
directory is found first when searching the paths upon a call for wc, this
“home-made†program is executed, with input it probably doesn't understand, so
we have to stop it. To resolve this problem there are several ways (there are
always several ways to solve a problem in UNIX/Linux): one answer could be to
rename the user's wc program, or the user can give the full path to the exact
command he wants, which can be found by using the -a option to the which
command.
If the user uses programs in the other directories more frequently, he can
change his path to look in his own directories last:
jumper:~> export PATH=/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin:/home/jumper/bin
Changes are not permanent!
Note that when using the export command in a shell, the changes are temporary
and only valid for this session (until you log out). Opening new sessions, even
while the current one is still running, will not result in a new path in the
new session. We will see in Section 2,_“Your_text_environment†how we can make
these kinds of changes to the environment permanent, adding these lines to the
shell configuration files.
2.2. Absolute and relative paths
A path, which is the way you need to follow in the tree structure to reach a
given file, can be described as starting from the trunk of the tree (the / or
root directory). In that case, the path starts with a slash and is called an
absolute path, since there can be no mistake: only one file on the system can
comply.
In the other case, the path doesn't start with a slash and confusion is
possible between ~/bin/wc (in the user's home directory) and bin/wc in /usr,
from the previous example. Paths that don't start with a slash are always
relative.
In relative paths we also use the . and .. indications for the current and the
parent directory. A couple of practical examples:
* When you want to compile source code, the installation documentation often
instructs you to run the command ./configure, which runs the configure
program located in the current directory (that came with the new code), as
opposed to running another configure program elsewhere on the system.
* In HTML files, relative paths are often used to make a set of pages easily
movable to another place:
<img alt="Garden with trees" src="../images/garden.jpg">
* Notice the difference one more time:
theo:~> ls /mp3
ls: /mp3: No such file or directory
theo:~>ls mp3/
oriental/ pop/ sixties/
2.3. The most important files and directories
2.3.1. The kernel
The kernel is the heart of the system. It manages the communication between the
underlying hardware and the peripherals. The kernel also makes sure that
processes and daemons (server processes) are started and stopped at the exact
right times. The kernel has a lot of other important tasks, so many that there
is a special kernel-development mailing list on this subject only, where huge
amounts of information are shared. It would lead us too far to discuss the
kernel in detail. For now it suffices to know that the kernel is the most
important file on the system.
2.3.2. The shell
2.3.2.1. What is a shell?
When I was looking for an appropriate explanation on the concept of a shell, it
gave me more trouble than I expected. All kinds of definitions are available,
ranging from the simple comparison that “the shell is the steering wheel of the
carâ€, to the vague definition in the Bash manual which says that “bash is an
sh-compatible command language interpreter,†or an even more obscure
expression, “a shell manages the interaction between the system and its usersâ€.
A shell is much more than that.
A shell can best be compared with a way of talking to the computer, a language.
Most users do know that other language, the point-and-click language of the
desktop. But in that language the computer is leading the conversation, while
the user has the passive role of picking tasks from the ones presented. It is
very difficult for a programmer to include all options and possible uses of a
command in the GUI-format. Thus, GUIs are almost always less capable than the
command or commands that form the backend.
The shell, on the other hand, is an advanced way of communicating with the
system, because it allows for two-way conversation and taking initiative. Both
partners in the communication are equal, so new ideas can be tested. The shell
allows the user to handle a system in a very flexible way. An additional asset
is that the shell allows for task automation.
2.3.2.2. Shell types
Just like people know different languages and dialects, the computer knows
different shell types:
* sh
or Bourne Shell: the original shell still used on UNIX systems and in UNIX
related environments. This is the basic shell, a small program with few
features. When in POSIX-compatible mode, bash will emulate this shell.
* bash
or Bourne Again SHell: the standard GNU shell, intuitive and flexible.
Probably most advisable for beginning users while being at the same time a
powerful tool for the advanced and professional user. On Linux, bash is the
standard shell for common users. This shell is a so-called superset of the
Bourne shell, a set of add-ons and plug-ins. This means that the Bourne Again
SHell is compatible with the Bourne shell: commands that work in sh, also
work in bash. However, the reverse is not always the case. All examples and
exercises in this book use bash.
* csh
or C Shell: the syntax of this shell resembles that of the C programming
language. Sometimes asked for by programmers.
* tcsh
or Turbo C Shell: a superset of the common C Shell, enhancing user-
friendliness and speed.
* ksh
or the Korn shell: sometimes appreciated by people with a UNIX background. A
superset of the Bourne shell; with standard configuration a nightmare for
beginning users.
The file
/etc/shells
gives an overview of known shells on a Linux system:
mia:~> cat /etc/shells
/bin/bash
/bin/sh
/bin/tcsh
/bin/csh
Fake Bourne shell
Note that /bin/sh is usually a link to Bash, which will execute in Bourne shell
compatible mode when called on this way.
Your default shell is set in the
/etc/passwd
file, like this line for user mia:
mia:L2NOfqdlPrHwE:504:504:Mia Maya:/home/mia:/bin/bash
To switch from one shell to another, just enter the name of the new shell in
the active terminal. The system finds the directory where the name occurs using
the PATH settings, and since a shell is an executable file (program), the
current shell activates it and it gets executed. A new prompt is usually shown,
because each shell has its typical appearance:
mia:~> tcsh
[mia@post21 ~]$
2.3.2.3. Which shell am I using?
If you don't know which shell you are using, either check the line for your
account in /etc/passwd or type the command
echo $SHELL
2.3.3. Your home directory
Your home directory is your default destination when connecting to the system.
In most cases it is a subdirectory of
/home
, though this may vary. Your home directory may be located on the hard disk of
a remote file server; in that case your home directory may be found in /
nethome/your_user_name. In another case the system administrator may have opted
for a less comprehensible layout and your home directory may be on /disk6/HU/
07/jgillard.
Whatever the path to your home directory, you don't have to worry too much
about it. The correct path to your home directory is stored in the
HOME
environment variable, in case some program needs it. With the
echo
command you can display the content of this variable:
orlando:~> echo $HOME
/nethome/orlando
You can do whatever you like in your home directory. You can put as many files
in as many directories as you want, although the total amount of data and files
is naturally limited because of the hardware and size of the partitions, and
sometimes because the system administrator has applied a quota system. Limiting
disk usage was common practice when hard disk space was still expensive.
Nowadays, limits are almost exclusively applied in large environments. You can
see for yourself if a limit is set using the
quota
command:
pierre@lamaison:/> quota -v
Diskquotas for user pierre (uid 501): none
In case quotas have been set, you get a list of the limited partitions and
their specific limitations. Exceeding the limits may be tolerated during a
grace period with fewer or no restrictions at all. Detailed information can be
found using the info quota or man quota commands.
No Quota?
If your system can not find the quota, then no limitation of file system usage
is being applied.
Your home directory is indicated by a tilde (~), shorthand for /path_to_home/
user_name. This same path is stored in the HOME variable, so you don't have to
do anything to activate it. A simple application: switch from /var/music/
albums/arno/2001 to images in your home directory using one elegant command:
rom:/var/music/albums/arno/2001> cd ~/images
rom:~/images> pwd
/home/rom/images
Later in this chapter we will talk about the commands for managing files and
directories in order to keep your home directory tidy.
2.4. The most important configuration files
As we mentioned before, most configuration files are stored in the
/etc
directory. Content can be viewed using the
cat
command, which sends text files to the standard output (usually your monitor).
The syntax is straight forward:
cat file1 file2 ... fileN
In this section we try to give an overview of the most common configuration
files. This is certainly not a complete list. Adding extra packages may also
add extra configuration files in /etc. When reading the configuration files,
you will find that they are usually quite well commented and self-explanatory.
Some files also have man pages which contain extra documentation, such as man
group.
Table 3.3. Most common configuration files
_____________________________________________________________________________
|File________________________|Information/service_____________________________|
| |Mail aliases file for use with the Sendmail and |
| |Postfix mail server. Running a mail server on |
| |each and every system has long been common use |
|aliases |in the UNIX world, and almost every Linux |
| |distribution still comes with a Sendmail |
| |package. In this file local user names are |
| |matched with real names as they occur in E-mail |
|____________________________|addresses,_or_with_other_local_addresses._______|
|apache______________________|Config_files_for_the_Apache_web_server._________|
| |The system-wide configuration file for the |
|bashrc |Bourne Again SHell. Defines functions and |
| |aliases for all users. Other shells may have |
|____________________________|their_own_system-wide_config_files,_like_cshrc._|
|crontab |Configuration of tasks that need to be executed |
| and the |periodically - backups, updates of the system |
|cron.* |databases, cleaning of the system, rotating logs|
|_directories________________|etc.____________________________________________|
|default |Default options for certain commands, such as |
|____________________________|useradd.________________________________________|
|filesystems_________________|Known_file_systems:_ext3,_vfat,_iso9660_etc.____|
|fstab_______________________|Lists_partitions_and_their_mount_points.________|
| |Configuration of the ftp-server: who can |
|ftp* |connect, what parts of the system are accessible|
|____________________________|etc.____________________________________________|
| |Configuration file for user groups. Use the |
|group |shadow utilities groupadd, groupmod and groupdel|
| |to edit this file. Edit manually only if you |
|____________________________|really_know_what_you_are_doing._________________|
| |A list of machines that can be contacted using |
| |the network, but without the need for a domain |
|hosts |name service. This has nothing to do with the |
| |system's network configuration, which is done in|
|____________________________|/etc/sysconfig._________________________________|
|inittab |Information for booting: mode, number of text |
|____________________________|consoles_etc.___________________________________|
|issue |Information about the distribution (release |
|____________________________|version_and/or_kernel_info).____________________|
|ld.so.conf__________________|Locations_of_library_files._____________________|
|lilo.conf |Boot information for the LInux LOader, the |
|, silo.conf, aboot.conf etc.|system for booting that is now gradually being |
|____________________________|replaced_with_GRUB._____________________________|
|logrotate.* |Rotation of the logs, a system preventing the |
|____________________________|collection_of_huge_amounts_of_log_files.________|
|mail |Directory containing instructions for the |
|____________________________|behavior_of_the_mail_server.____________________|
|modules.conf |Configuration of modules that enable special |
|____________________________|features_(drivers)._____________________________|
| |Message Of The Day: Shown to everyone who |
|motd |connects to the system (in text mode), may be |
| |used by the system admin to announce system |
|____________________________|services/maintenance_etc._______________________|
|mtab |Currently mounted file systems. It is advised to|
|____________________________|never_edit_this_file.___________________________|
|nsswitch.conf |Order in which to contact the name resolvers |
|____________________________|when_a_process_demands_resolving_of_a_host_name.|
|pam.d_______________________|Configuration_of_authentication_modules.________|
| |Lists local users. Use the shadow utilities |
|passwd |useradd, usermod and userdel to edit this file. |
| |Edit manually only when you really know what you|
|____________________________|are_doing.______________________________________|
| |Outdated but still frequently used printer |
|printcap |configuration file. Don't edit this manually |
|____________________________|unless_you_really_know_what_you_are_doing.______|
| |System wide configuration of the shell |
|profile |environment: variables, default properties of |
|____________________________|new_files,_limitation_of_resources_etc._________|
|rc* |Directories defining active services for each |
|____________________________|run_level.______________________________________|
|resolv.conf |Order in which to contact DNS servers (Domain |
|____________________________|Name_Servers_only)._____________________________|
|sendmail.cf_________________|Main_config_file_for_the_Sendmail_server._______|
|services |Connections accepted by this machine (open |
|____________________________|ports)._________________________________________|
|sndconfig |Configuration of the sound card and sound |
| or |events. |
|sound_______________________|________________________________________________|
|ssh |Directory containing the config files for secure|
|____________________________|shell_client_and_server.________________________|
| |Directory containing the system configuration |
|sysconfig |files: mouse, keyboard, network, desktop, system|
| |clock, power management etc. (specific to |
|____________________________|RedHat)_________________________________________|
| |Settings for the graphical server, X. RedHat |
| |uses XFree, which is reflected in the name of |
|X11 |the main configuration file, XFree86Config. Also|
| |contains the general directions for the window |
| |managers available on the system, for example |
|____________________________|gdm,_fvwm,_twm,_etc.____________________________|
|xinetd.* |Configuration files for Internet services that |
| or |are run from the system's (extended) Internet |
|inetd.conf |services daemon (servers that don't run an |
|____________________________|independent_daemon).____________________________|
Throughout this guide we will learn more about these files and study some of
them in detail.
2.5. The most common devices
Devices, generally every peripheral attachment of a PC that is not the CPU
itself, is presented to the system as an entry in the
/dev
directory. One of the advantages of this UNIX-way of handling devices is that
neither the user nor the system has to worry much about the specification of
devices.
Users that are new to Linux or UNIX in general are often overwhelmed by the
amount of new names and concepts they have to learn. That is why a list of
common devices is included in this introduction.
Table 3.4. Common devices
____________________________________________________________________________
|Name_______________________________|Device__________________________________|
|cdrom______________________________|CD_drive________________________________|
|console |Special entry for the currently used |
|___________________________________|console.________________________________|
|cua*_______________________________|Serial_ports____________________________|
|dsp*_______________________________|Devices_for_sampling_and_recording______|
| |Entries for most kinds of floppy drives,|
|fd* |the default is /dev/fd0, a floppy drive |
|___________________________________|for_1.44_MB_floppies.___________________|
|hd[a-t][1-16] |Standard support for IDE drives with |
|___________________________________|maximum_amount_of_partitions_each.______|
|ir*________________________________|Infrared_devices________________________|
|isdn*______________________________|Management_of_ISDN_connections__________|
|js*________________________________|Joystick(s)_____________________________|
|lp*________________________________|Printers________________________________|
|mem________________________________|Memory__________________________________|
|midi*______________________________|midi_player_____________________________|
|mixer* |Idealized model of a mixer (combines or |
| and |adds signals) |
|music______________________________|________________________________________|
|modem______________________________|Modem___________________________________|
|mouse | |
| (also msmouse, logimouse, psmouse,|All kinds of mouses |
|input/mice,_psaux)_________________|________________________________________|
|null_______________________________|Bottomless_garbage_can__________________|
|par*_______________________________|Entries_for_parallel_port_support_______|
|pty*_______________________________|Pseudo_terminals________________________|
|radio*_____________________________|For_Radio_Amateurs_(HAMs).______________|
|ram*_______________________________|boot_device_____________________________|
|sd*________________________________|SCSI_disks_with_their_partitions________|
| |For audio applications using the |
|sequencer |synthesizer features of the sound card |
|___________________________________|(MIDI-device_controller)________________|
|tty* |Virtual consoles simulating vt100 |
|___________________________________|terminals.______________________________|
|usb*_______________________________|USB_card_and_scanner____________________|
|video* |For use with a graphics card supporting |
|___________________________________|video.__________________________________|
2.6. The most common variable files
In the
/var
directory we find a set of directories for storing specific non-constant data
(as opposed to the ls program or the system configuration files, which change
relatively infrequently or never at all). All files that change frequently,
such as log files, mailboxes, lock files, spoolers etc. are kept in a
subdirectory of /var.
As a security measure these files are usually kept in separate parts from the
main system files, so we can keep a close eye on them and set stricter
permissions where necessary. A lot of these files also need more permissions
than usual, like
/var/tmp
, which needs to be writable for everyone. A lot of user activity might be
expected here, which might even be generated by anonymous Internet users
connected to your system. This is one reason why the /var directory, including
all its subdirectories, is usually on a separate partition. This way, there is
for instance no risk that a mail bomb, for instance, fills up the rest of the
file system, containing more important data such as your programs and
configuration files.
/var/tmp and /tmp
Files in
/tmp
can be deleted without notice, by regular system tasks or because of a system
reboot. On some (customized) systems, also /var/tmp might behave unpredictably.
Nevertheless, since this is not the case by default, we advise to use the /var/
tmp directory for saving temporary files. When in doubt, check with your system
administrator. If you manage your own system, you can be reasonably sure that
this is a safe place if you did not consciously change settings on /var/tmp (as
root, a normal user can not do this).
Whatever you do, try to stick to the privileges granted to a normal user -
don't go saving files directly under the root (/) of the file system, don't put
them in /usr or some subdirectory or in another reserved place. This pretty
much limits your access to safe file systems.
One of the main security systems on a UNIX system, which is naturally
implemented on every Linux machine as well, is the log-keeping facility, which
logs all user actions, processes, system events etc. The configuration file of
the so-called
syslogdaemon
determines which and how long logged information will be kept. The default
location of all logs is /var/log, containing different files for access log,
server logs, system messages etc.
In /var we typically find server data, which is kept here to separate it from
critical data such as the server program itself and its configuration files. A
typical example on Linux systems is /var/www, which contains the actual HTML
pages, scripts and images that a web server offers. The FTP-tree of an FTP
server (data that can be downloaded by a remote client) is also best kept in
one of /var's subdirectories. Because this data is publicly accessible and
often changeable by anonymous users, it is safer to keep it here, away from
partitions or directories with sensitive data.
On most workstation installations,
/var/spool
will at least contain an at and a cron directory, containing scheduled tasks.
In office environments this directory usually contains lpd as well, which holds
the print queue(s) and further printer configuration files, as well as the
printer log files.
On server systems we will generally find /var/spool/mail, containing incoming
mails for local users, sorted in one file per user, the user's “inboxâ€. A
related directory is mqueue, the spooler area for unsent mail messages. These
parts of the system can be very busy on mail servers with a lot of users. News
servers also use the /var/spool area because of the enormous amounts of
messages they have to process.
The /var/lib/rpm directory is specific to RPM-based (RedHat Package Manager)
distributions; it is where RPM package information is stored. Other package
managers generally also store their data somewhere in /var.
3. Manipulating files
3.1. Viewing file properties
3.1.1. More about ls
Besides the name of the file,
ls
can give a lot of other information, such as the file type, as we already
discussed. It can also show permissions on a file, file size, inode number,
creation date and time, owners and amount of links to the file. With the -
a option to ls, files that are normally hidden from view can be displayed as
well. These are files that have a name starting with a dot. A couple of typical
examples include the configuration files in your home directory. When you've
worked with a certain system for a while, you will notice that tens of files
and directories have been created that are not automatically listed in a
directory index. Next to that, every directory contains a file named just dot
(.) and one with two dots (..), which are used in combination with their inode
number to determine the directory's position in the file system's tree
structure.
You should really read the Info pages about ls, since it is a very common
command with a lot of useful options. Options can be combined, as is the case
with most UNIX commands and their options. A common combination is ls -al; it
shows a long list of files and their properties as well as the destinations
that any symbolic links point to. ls -latr displays the same files, only now in
reversed order of the last change, so that the file changed most recently
occurs at the bottom of the list. Here are a couple of examples:
krissie:~/mp3> ls
Albums/ Radio/ Singles/ gene/ index.html
krissie:~/mp3> ls -a
./ .thumbs Radio gene/
../ Albums/ Singles/ index.html
krissie:~/mp3> ls -l Radio/
total 8
drwxr-xr-x 2 krissie krissie 4096 Oct 30 1999 Carolina/
drwxr-xr-x 2 krissie krissie 4096 Sep 24 1999 Slashdot/
krissie:~/mp3> ls -ld Radio/
drwxr-xr-x 4 krissie krissie 4096 Oct 30 1999 Radio/
krissie:~/mp3> ls -ltr
total 20
drwxr-xr-x 4 krissie krissie 4096 Oct 30 1999 Radio/
-rw-r--r-- 1 krissie krissie 453 Jan 7 2001 index.html
drwxrwxr-x 30 krissie krissie 4096 Oct 20 17:32 Singles/
drwxr-xr-x 2 krissie krissie 4096 Dec 4 23:22 gene/
drwxrwxr-x 13 krissie krissie 4096 Dec 21 11:40 Albums/
On most Linux versions ls is aliased to color-ls by default. This feature
allows to see the file type without using any options to ls. To achieve this,
every file type has its own color. The standard scheme is in /etc/DIR_COLORS:
Table 3.5. Color-ls default color scheme
________________________________
|Color_______|File_type__________|
|blue________|directories________|
|red_________|compressed_archives|
|white_______|text_files_________|
|pink________|images_____________|
|cyan________|links______________|
|yellow______|devices____________|
|green_______|executables________|
|flashing_red|broken_links_______|
More information is in the man page. The same information was in earlier days
displayed using suffixes to every non-standard file name. For mono-color use
(like printing a directory listing) and for general readability, this scheme is
still in use:
Table 3.6. Default suffix scheme for ls
_________________________
|Character|File_type______|
|nothing__|regular_file___|
|/________|directory______|
|*________|executable_file|
|@________|link___________|
|=________|socket_________|
||________|named_pipe_____|
A description of the full functionality and features of the ls command can be
read with info coreutils ls.
3.1.2. More tools
To find out more about the kind of data we are dealing with, we use the
file
command. By applying certain tests that check properties of a file in the file
system, magic numbers and language tests,
file
tries to make an educated guess about the format of a file. Some examples:
mike:~> file Documents/
Documents/: directory
mike:~> file high-tech-stats.pdf
high-tech-stats.pdf: PDF document, version 1.2
mike:~> file Nari-288.rm
Nari-288.rm: RealMedia file
mike:~> file bijlage10.sdw
bijlage10.sdw: Microsoft Office Document
mike:~> file logo.xcf
logo.xcf: GIMP XCF image data, version 0, 150 x 38, RGB Color
mike:~> file cv.txt
cv.txt: ISO-8859 text
mike:~> file image.png
image.png: PNG image data, 616 x 862, 8-bit grayscale, non-interlaced
mike:~> file figure
figure: ASCII text
mike:~> file me+tux.jpg
me+tux.jpg: JPEG image data, JFIF standard 1.01, resolution (DPI),
"28 Jun 1999", 144 x 144
mike:~> file 42.zip.gz
42.zip.gz: gzip compressed data, deflated, original filename,
`42.zip', last modified: Thu Nov 1 23:45:39 2001, os: Unix
mike:~> file vi.gif
vi.gif: GIF image data, version 89a, 88 x 31
mike:~> file slide1
slide1: HTML document text
mike:~> file template.xls
template.xls: Microsoft Office Document
mike:~> file abook.ps
abook.ps: PostScript document text conforming at level 2.0
mike:~> file /dev/log
/dev/log: socket
mike:~> file /dev/hda
/dev/hda: block special (3/0)
The file command has a series of options, among others the -z option to look
into compressed files. See info file for a detailed description. Keep in mind
that the results of file are not absolute, it is only a guess. In other words,
file can be tricked.
Why all the fuss about file types and formats?
Shortly, we will discuss a couple of command-line tools for looking at plain
text files. These tools will not work when used on the wrong type of files. In
the worst case, they will crash your terminal and/or make a lot of beeping
noises. If this happens to you, just close the terminal session and start a new
one. But try to avoid it, because it is usually very disturbing for other
people.
3.2. Creating and deleting files and directories
3.2.1. Making a mess...
... Is not a difficult thing to do. Today almost every system is networked, so
naturally files get copied from one machine to another. And especially when
working in a graphical environment, creating new files is a piece of cake and
is often done without the approval of the user. To illustrate the problem,
here's the full content of a new user's directory, created on a standard RedHat
system:
[newuser@blob user]$ ls -al
total 32
drwx------ 3 user user 4096 Jan 16 13:32 .
drwxr-xr-x 6 root root 4096 Jan 16 13:32 ..
-rw-r--r-- 1 user user 24 Jan 16 13:32 .bash_logout
-rw-r--r-- 1 user user 191 Jan 16 13:32 .bash_profile
-rw-r--r-- 1 user user 124 Jan 16 13:32 .bashrc
drwxr-xr-x 3 user user 4096 Jan 16 13:32 .kde
-rw-r--r-- 1 user user 3511 Jan 16 13:32 .screenrc
-rw------- 1 user user 61 Jan 16 13:32 .xauthDqztLr
On first sight, the content of a “used†home directory doesn't look that bad
either:
olduser:~> ls
app-defaults/ crossover/ Fvwm@ mp3/ OpenOffice.org638/
articles/ Desktop/ GNUstep/ Nautilus/ staroffice6.0/
bin/ Desktop1/ images/ nqc/ training/
brol/ desktoptest/ Machines@ ns_imap/ webstart/
C/ Documents/ mail/ nsmail/ xml/
closed/ Emacs@ Mail/ office52/ Xrootenv.0
But when all the directories and files starting with a dot are included, there
are 185 items in this directory. This is because most applications have their
own directories and/or files, containing user-specific settings, in the home
directory of that user. Usually these files are created the first time you
start an application. In some cases you will be notified when a non-existent
directory needs to be created, but most of the time everything is done
automatically.
Furthermore, new files are created seemingly continuously because users want to
save files, keep different versions of their work, use Internet applications,
and download files and attachments to their local machine. It doesn't stop. It
is clear that one definitely needs a scheme to keep an overview on things.
In the next section, we will discuss our means of keeping order. We only
discuss text tools available to the shell, since the graphical tools are very
intuitive and have the same look and feel as the well known point-and-click MS
Windows-style file managers, including graphical help functions and other
features you expect from this kind of applications. The following list is an
overview of the most popular file managers for GNU/Linux. Most file managers
can be started from the menu of your desktop manager, or by clicking your home
directory icon, or from the command line, issuing these commands:
* nautilus
: The default file manager in Gnome, the GNU desktop. Excellent documentation
about working with this tool can be found at http://www.gnome.org.
* konqueror
: The file manager typically used on a KDE desktop. The handbook is at http:/
/docs.kde.org.
* mc
: Midnight Commander, the Unix file manager after the fashion of Norton
Commander. All documentation available from http://gnu.org/directory/ or a
mirror, such as http://www.ibiblio.org.
These applications are certainly worth giving a try and usually impress
newcomers to Linux, if only because there is such a wide variety: these are
only the most popular tools for managing directories and files, and many other
projects are being developed. Now let's find out about the internals and see
how these graphical tools use common UNIX commands.
3.2.2. The tools
3.2.2.1. Creating directories
A way of keeping things in place is to give certain files specific default
locations by creating directories and subdirectories (or folders and sub-
folders if you wish). This is done with the
mkdir
command:
richard:~> mkdir archive
richard:~> ls -ld archive
drwxrwxrwx 2 richard richard 4096 Jan 13 14:09 archive/
Creating directories and subdirectories in one step is done using the -
p option:
richard:~> cd archive
richard:~/archive> mkdir 1999 2000 2001
richard:~/archive> ls
1999/ 2000/ 2001/
richard:~/archive> mkdir 2001/reports/Restaurants-Michelin/
mkdir: cannot create directory `2001/reports/Restaurants-Michelin/':
No such file or directory
richard:~/archive> mkdir -p 2001/reports/Restaurants-Michelin/
richard:~/archive> ls 2001/reports/
Restaurants-Michelin/
If the new file needs other permissions than the default file creation
permissions, the new access rights can be set in one move, still using the
mkdir command, see the Info pages for more. We are going to discuss access
modes in the next section on file security.
The name of a directory has to comply with the same rules as those applied on
regular file names. One of the most important restrictions is that you can't
have two files with the same name in one directory (but keep in mind that Linux
is, like UNIX, a case sensitive operating system). There are virtually no
limits on the length of a file name, but it is usually kept shorter than 80
characters, so it can fit on one line of a terminal. You can use any character
you want in a file name, although it is advised to exclude characters that have
a special meaning to the shell. When in doubt, check with Appendix C,_Shell
Features.
3.2.2.2. Moving files
Now that we have properly structured our home directory, it is time to clean up
unclassified files using the
mv
command:
richard:~/archive> mv ../report[1-4].doc reports/Restaurants-Michelin/
This command is also applicable when renaming files:
richard:~> ls To_Do
-rw-rw-r-- 1 richard richard 2534 Jan 15 12:39 To_Do
richard:~> mv To_Do done
richard:~> ls -l done
-rw-rw-r-- 1 richard richard 2534 Jan 15 12:39 done
It is clear that only the name of the file changes. All other properties remain
the same.
Detailed information about the syntax and features of the mv command can be
found in the man or Info pages. The use of this documentation should always be
your first reflex when confronted with a problem. The answer to your problem is
likely to be in the system documentation. Even experienced users read man pages
every day, so beginning users should read them all the time. After a while, you
will get to know the most common options to the common commands, but you will
still need the documentation as a primary source of information. Note that the
information contained in the HOWTOs, FAQs, man pages and other sources is
slowly being merged into the Info pages, which are today the most up-to-date
source of online (as in readily available on the system) documentation.
3.2.2.3. Copying files
Copying files and directories is done with the
cp
command. A useful option is recursive copy (copy all underlying files and
subdirectories), using the -R option to cp. The general syntax is
cp [-R] fromfile tofile
As an example the case of user newguy, who wants the same Gnome desktop
settings user oldguy has. One way to solve the problem is to copy the settings
of oldguy to the home directory of newguy:
victor:~> cp -R ../oldguy/.gnome/ .
This gives some errors involving file permissions, but all the errors have to
do with private files that newguy doesn't need anyway. We will discuss in the
next part how to change these permissions in case they really are a problem.
3.2.2.4. Removing files
Use the
rm
command to remove single files,
rmdir
to remove empty directories. (Use ls -a to check whether a directory is empty
or not). The rm command also has options for removing non-empty directories
with all their subdirectories, read the Info pages for these rather dangerous
options.
How empty can a directory be?
It is normal that the directories . (dot) and .. (dot-dot) can't be removed,
since they are also necessary in an empty directory to determine the
directories ranking in the file system hierarchy.
On Linux, just like on UNIX, there is no garbage can - at least not for the
shell, although there are plenty of solutions for graphical use. So once
removed, a file is really gone, and there is generally no way to get it back
unless you have backups, or you are really fast and have a real good system
administrator. To protect the beginning user from this malice, the interactive
behavior of the
rm
,
cp
and
mv
commands can be activated using the -i option. In that case the system won't
immediately act upon request. Instead it will ask for confirmation, so it takes
an additional click on the Enter key to inflict the damage:
mary:~> rm -ri archive/
rm: descend into directory `archive'? y
rm: descend into directory `archive/reports'? y
rm: remove directory `archive/reports'? y
rm: descend into directory `archive/backup'? y
rm: remove `archive/backup/sysbup200112.tar'? y
rm: remove directory `archive/backup'? y
rm: remove directory `archive'? y
We will discuss how to make this option the default in Chapter 7,_Home_sweet_/
home, which discusses customizing your shell environment.
3.3. Finding files
3.3.1. Using shell features
In the example on moving files we already saw how the shell can manipulate
multiple files at once. In that example, the shell finds out automatically what
the user means by the requirements between the square braces “[†and “]â€. The
shell can substitute ranges of numbers and upper or lower case characters
alike. It also substitutes as many characters as you want with an asterisk, and
only one character with a question mark.
All sorts of substitutions can be used simultaneously; the shell is very
logical about it. The Bash shell, for instance, has no problem with expressions
like ls dirname/*/*/*[2-3].
In other shells, the asterisk is commonly used to minimize the efforts of
typing: people would enter cd dir* instead of cd directory. In Bash however,
this is not necessary because the GNU shell has a feature called file name
completion. It means that you can type the first few characters of a command
(anywhere) or a file (in the current directory) and if no confusion is
possible, the shell will find out what you mean. For example in a directory
containing many files, you can check if there are any files beginning with the
letter A just by typing ls A and pressing the Tab key twice, rather than
pressing Enter. If there is only one file starting with “Aâ€, this file will be
shown as the argument to ls (or any shell command, for that matter)
immediately.
3.3.2. Which
A very simple way of looking up files is using the
which
command, to look in the directories listed in the user's search path for the
required file. Of course, since the search path contains only paths to
directories containing executable programs, which doesn't work for ordinary
files. The which command is useful when troubleshooting “Command not Foundâ€
problems. In the example below, user tina can't use the acroread program, while
her colleague has no troubles whatsoever on the same system. The problem is
similar to the PATH problem in the previous part: Tina's colleague tells her
that he can see the required program in /opt/acroread/bin, but this directory
is not in her path:
tina:~> which acroread
/usr/bin/which: no acroread in (/bin:/usr/bin:/usr/bin/X11)
The problem can be solved by giving the full path to the command to run, or by
re-exporting the content of the
PATH
variable:
tina:~> export PATH=$PATH:/opt/acroread/bin
tina:~> echo $PATH
/bin:/usr/bin:/usr/bin/X11:/opt/acroread/bin
Using the which command also checks to see if a command is an alias for another
command:
gerrit:~> which -a ls
ls is aliased to `ls -F --color=auto'
ls is /bin/ls
If this does not work on your system, use the alias command:
tille@www:~/mail$ alias ls
alias ls='ls --color'
3.3.3. Find and locate
These are the real tools, used when searching other paths beside those listed
in the search path. The
find
tool, known from UNIX, is very powerful, which may be the cause of a somewhat
more difficult syntax. GNU
find
, however, deals with the syntax problems. This command not only allows you to
search file names, it can also accept file size, date of last change and other
file properties as criteria for a search. The most common use is for finding
file names:
find <path> -name <searchstring>
This can be interpreted as “Look in all files and subdirectories contained in a
given path, and print the names of the files containing the search string in
their name†(not in their content).
Another application of find is for searching files of a certain size, as in the
example below, where user peter wants to find all files in the current
directory or one of its subdirectories, that are bigger than 5 MB:
peter:~> find . -size +5000k
psychotic_chaos.mp3
If you dig in the man pages, you will see that find can also perform operations
on the found files. A common example is removing files. It is best to first
test without the -exec option that the correct files are selected, after that
the command can be rerun to delete the selected files. Below, we search for
files ending in .tmp:
peter:~> find . -name "*.tmp" -exec rm {} \;
peter:~>
Optimize!
This command will call on rm as many times as a file answering the requirements
is found. In the worst case, this might be thousands or millions of times. This
is quite a load on your system.
A more realistic way of working would be the use of a pipe (|) and the
xargs
tool with rm as an argument. This way, the rm command is only called when the
command line is full, instead of for every file. See Chapter 5,_I/O_redirection
for more on using I/O redirection to ease everyday tasks.
Later on (in 1999 according to the man pages, after 20 years of find),
locate
was developed. This program is easier to use, but more restricted than find,
since its output is based on a file index database that is updated only once
every day. On the other hand, a search in the locate database uses less
resources than find and therefore shows the results nearly instantly.
Most Linux distributions use slocate these days, security enhanced locate, the
modern version of locate that prevents users from getting output they have no
right to read. The files in root's home directory are such an example, these
are not normally accessible to the public. A user who wants to find someone who
knows about the C shell may issue the command locate .cshrc, to display all
users who have a customized configuration file for the C shell. Supposing the
users root and jenny are running C shell, then only the file /home/jenny/.cshrc
will be displayed, and not the one in root's home directory. On most systems,
locate is a symbolic link to the slocate program:
billy:~> ls -l /usr/bin/locate
lrwxrwxrwx 1 root slocate 7 Oct 28 14:18 /usr/bin/locate -> slocate*
User tina could have used locate to find the application she wanted:
tina:~> locate acroread
/usr/share/icons/hicolor/16x16/apps/acroread.png
/usr/share/icons/hicolor/32x32/apps/acroread.png
/usr/share/icons/locolor/16x16/apps/acroread.png
/usr/share/icons/locolor/32x32/apps/acroread.png
/usr/local/bin/acroread
/usr/local/Acrobat4/Reader/intellinux/bin/acroread
/usr/local/Acrobat4/bin/acroread
Directories that don't contain the name bin can't contain the program - they
don't contain executable files. There are three possibilities left. The file in
/usr/local/bin is the one tina would have wanted: it is a link to the shell
script that starts the actual program:
tina:~> file /usr/local/bin/acroread
/usr/local/bin/acroread: symbolic link to ../Acrobat4/bin/acroread
tina:~> file /usr/local/Acrobat4/bin/acroread
/usr/local/Acrobat4/bin/acroread: Bourne shell script text executable
tina:~> file /usr/local/Acrobat4/Reader/intellinux/bin/acroread
/usr/local/Acrobat4/Reader/intellinux/bin/acroread: ELF 32-bit LSB
executable, Intel 80386, version 1, dynamically linked (uses
shared libs), not stripped
In order to keep the path as short as possible, so the system doesn't have to
search too long every time a user wants to execute a command, we add /usr/
local/bin to the path and not the other directories, which only contain the
binary files of one specific program, while /usr/local/bin contains other
useful programs as well.
Again, a description of the full features of find and locate can be found in
the Info pages.
3.3.4. The grep command
3.3.4.1. General line filtering
A simple but powerful program,
grep
is used for filtering input lines and returning certain patterns to the
output. There are literally thousands of applications for the grep program. In
the example below, jerry uses grep to see how he did the thing with find:
jerry:~> grep -a find .bash_history
find . -name userinfo
man find
find ../ -name common.cfg
Search history
Also useful in these cases is the search function in bash, activated by
pressing Ctrl+R at once, such as in the example where we want to check how we
did that last find again:
thomas ~> ^R
(reverse-i-search)`find': find `/home/thomas` -name *.xml
Type your search string at the search prompt. The more characters you type, the
more restricted the search gets. This reads the command history for this shell
session (which is written to .bash_history in your home directory when you quit
that session). The most recent occurrence of your search string is shown. If
you want to see previous commands containing the same string, type Ctrl+R
again.
See the Info pages on bash for more.
All UNIXes with just a little bit of decency have an online dictionary. So does
Linux. The dictionary is a list of known words in a file named words, located
in /usr/share/dict. To quickly check the correct spelling of a word, no
graphical application is needed:
william:~> grep pinguin /usr/share/dict/words
william:~> grep penguin /usr/share/dict/words
penguin
penguins
Dictionary vs. word list
Some distributions offer the
dict
command, which offers more features than simply searching words in a list.
Who is the owner of that home directory next to mine? Hey, there's his
telephone number!
lisa:~> grep gdbruyne /etc/passwd
gdbruyne:x:981:981:Guy Debruyne, tel 203234:/home/gdbruyne:/bin/bash
And what was the E-mail address of Arno again?
serge:~/mail> grep -i arno *
sent-mail: To: <Arno.Hintjens@celeb.com>
sent-mail: On Mon, 24 Dec 2001, Arno.Hintjens@celeb.com wrote:
find and locate are often used in combination with grep to define some serious
queries. For more information, see Chapter 5,_I/O_redirection on I/
O redirection.
3.3.4.2. Special characters
Characters that have a special meaning to the shell have to be
escaped
. The escape character in Bash is backslash, as in most shells; this takes away
the special meaning of the following character. The shell knows about quite
some special characters, among the most common /, ., ? and *. A full list can
be found in the Info pages and documentation for your shell.
For instance, say that you want to display the file “*†instead of all the
files in a directory, you would have to use
less \*
The same goes for filenames containing a space:
cat This\ File
3.4. More ways to view file content
3.4.1. General
Apart from cat, which really doesn't do much more than sending files to the
standard output, there are other tools to view file content.
The easiest way of course would be to use graphical tools instead of command
line tools. In the introduction we already saw a glimpse of an office
application, OpenOffice.org. Other examples are the GIMP (start up with
gimp
from the command line), the GNU Image Manipulation Program;
xpdf
to view Portable Document Format files (PDF); GhostView (
gv
) for viewing PostScript files; Mozilla/FireFox, links (a text mode browser),
Konqueror, Opera and many others for web content; XMMS, CDplay and others for
multimedia file content; AbiWord, Gnumeric, KOffice etc. for all kinds of
office applications and so on. There are thousands of Linux applications; to
list them all would take days.
Instead we keep concentrating on shell- or text-mode applications, which form
the basics for all other applications. These commands work best in a text
environment on files containing text. When in doubt, check first using the
file
command.
So let's see what text tools we have that are useful to look inside files.
Font problems
Plain text tools such as the ones we will now be discussing, often have
problems with “plain†text files because of the font encoding used in those
files. Special characters, such as accented alphabetical characters, Chinese
characters and other characters from languages using different character sets
than the default en_US encoding and so on, are then displayed the wrong way or
replaced by unreadable rubbish. These problems are discussed in Section 4,
“Region_specific_settingsâ€.
3.4.2. “less is moreâ€
Undoubtedly you will hear someone say this phrase sooner or later when working
in a UNIX environment. A little bit of UNIX history explains this:
* First there was
cat
. Output was streamed in an uncontrollable way.
* Then there was
pg
, which may still be found on older UNIXes. This command puts text to the
output one page at the time.
* The
more
program was a revised version of pg. This command is still available on
every Linux system.
* less
is the GNU version of more and has extra features allowing highlighting of
search strings, scrolling back etc. The syntax is very simple:
less name_of_file
More information is located in the Info pages.
You already know about pagers by now, because they are used for viewing the man
pages.
3.4.3. The head and tail commands
These two commands display the n first/last lines of a file respectively. To
see the last ten commands entered:
tony:~> tail -10 .bash_history
locate configure | grep bin
man bash
cd
xawtv &
grep usable /usr/share/dict/words
grep advisable /usr/share/dict/words
info quota
man quota
echo $PATH
frm
head
works similarly. The tail command has a handy feature to continuously show the
last n lines of a file that changes all the time. This -f option is often used
by system administrators to check on log files. More information is located in
the system documentation files.
3.5. Linking files
3.5.1. Link types
Since we know more about files and their representation in the file system,
understanding links (or shortcuts) is a piece of cake. A link is nothing more
than a way of matching two or more file names to the same set of file data.
There are two ways to achieve this:
* Hard link: Associate two or more file names with the same inode. Hard links
share the same data blocks on the hard disk, while they continue to behave as
independent files.
There is an immediate disadvantage: hard links can't span partitions, because
inode numbers are only unique within a given partition.
* Soft link or symbolic link (or for short: symlink): a small file that is a
pointer to another file. A symbolic link contains the path to the target file
instead of a physical location on the hard disk. Since inodes are not used in
this system, soft links can span across partitions.
The two link types behave similar, but are not the same, as illustrated in the
scheme below:
Figure 3.2. Hard and soft link mechanism
Hard links share the same inode, symbolic links have their own inode that
points to datablocks holding the filename of the original file that the link is
pointing to.
Note that removing the target file for a symbolic link makes the link useless.
Each regular file is in principle a hardlink. Hardlinks can not span across
partitions, since they refer to inodes, and inode numbers are only unique
within a given partition.
It may be argued that there is a third kind of link, the user-space link, which
is similar to a shortcut in MS Windows. These are files containing meta-data
which can only be interpreted by the graphical file manager. To the kernel and
the shell these are just normal files. They may end in a .desktop or .lnk
suffix; an example can be found in ~/.gnome-desktop:
[dupont@boulot .gnome-desktop]$ cat La\ Maison\ Dupont
[Desktop Entry]
Encoding=Legacy-Mixed
Name=La Maison Dupont
Type=X-nautilus-home
X-Nautilus-Icon=temp-home
URL=file:///home/dupont
This example is from a KDE desktop:
[lena@venus Desktop]$ cat camera
[Desktop Entry]
Dev=/dev/sda1
FSType=auto
Icon=memory
MountPoint=/mnt/camera
Type=FSDevice
X-KDE-Dynamic-Device=true
Creating this kind of link is easy enough using the features of your graphical
environment. Should you need help, your system documentation should be your
first resort.
In the next section, we will study the creation of UNIX-style symbolic links
using the command line.
3.5.2. Creating symbolic links
The symbolic link is particularly interesting for beginning users: they are
fairly obvious to see and you don't need to worry about partitions.
The command to make links is
ln
. In order to create symlinks, you need to use the -s option:
ln -s targetfile linkname
In the example below, user freddy creates a link in a subdirectory of his home
directory to a directory on another part of the system:
freddy:~/music> ln -s /opt/mp3/Queen/ Queen
freddy:~/music> ls -l
lrwxrwxrwx 1 freddy freddy 17 Jan 22 11:07 Queen -> /opt/mp3/Queen
Symbolic links are always very small files, while hard links have the same size
as the original file.
The application of symbolic links is widespread. They are often used to save
disk space, to make a copy of a file in order to satisfy installation
requirements of a new program that expects the file to be in another location,
they are used to fix scripts that suddenly have to run in a new environment and
can generally save a lot of work. A system admin may decide to move the home
directories of the users to a new location, disk2 for instance, but if he wants
everything to work like before, like the /etc/passwd file, with a minimum of
effort he will create a symlink from /home to the new location /disk2/home.
4. File security
4.1. Access rights: Linux's first line of defense
The Linux security model is based on the one used on UNIX systems, and is as
rigid as the UNIX security model (and sometimes even more), which is already
quite robust. On a Linux system, every file is owned by a user and a group
user. There is also a third category of users, those that are not the user
owner and don't belong to the group owning the file. For each category of
users, read, write and execute permissions can be granted or denied.
We already used the long option to list files using the ls -l command, though
for other reasons. This command also displays file permissions for these three
user categories; they are indicated by the nine characters that follow the
first character, which is the file type indicator at the beginning of the file
properties line. As seen in the examples below, the first three characters in
this series of nine display access rights for the actual user that owns the
file. The next three are for the group owner of the file, the last three for
other users. The permissions are always in the same order: read, write, execute
for the user, the group and the others. Some examples:
marise:~> ls -l To_Do
-rw-rw-r-- 1 marise users 5 Jan 15 12:39 To_Do
marise:~> ls -l /bin/ls
-rwxr-xr-x 1 root root 45948 Aug 9 15:01 /bin/ls*
The first file is a regular file (first dash). Users with user name marise or
users belonging to the group users can read and write (change/move/delete) the
file, but they can't execute it (second and third dash). All other users are
only allowed to read this file, but they can't write or execute it (fourth and
fifth dash).
The second example is an executable file, the difference: everybody can run
this program, but you need to be root to change it.
The Info pages explain how the ls command handles display of access rights in
detail, see the section What information is listed.
For easy use with commands, both access rights or modes and user groups have a
code. See the tables below.
Table 3.7. Access mode codes
_____________________________________________________________________________
|Code__|Meaning_______________________________________________________________|
|0_or_-|The_access_right_that_is_supposed_to_be_on_this_place_is_not_granted._|
|4_or_r|read_access_is_granted_to_the_user_category_defined_in_this_place_____|
|2_or_w|write_permission_is_granted_to_the_user_category_defined_in_this_place|
|1 or x|execute permission is granted to the user category defined in this |
|______|place_________________________________________________________________|
Table 3.8. User group codes
___________________________
|Code|Meaning_______________|
|u___|user_permissions______|
|g___|group_permissions_____|
|o___|permissions_for_others|
This straight forward scheme is applied very strictly, which allows a high
level of security even without network security. Among other functions, the
security scheme takes care of user access to programs, it can serve files on a
need-to-know basis and protect sensitive data such as home directories and
system configuration files.
You should know what your user name is. If you don't, it can be displayed using
the
id
command, which also displays the default group you belong to and eventually
other groups of which you are a member:
tilly:~> id
uid=504(tilly) gid=504(tilly) groups=504(tilly),100(users),2051(org)
Your user name is also stored in the environment variable USER:
tilly:~> echo $USER
tilly
4.2. The tools
4.2.1. The chmod command
A normal consequence of applying strict file permissions, and sometimes a
nuisance, is that access rights will need to be changed for all kinds of
reasons. We use the
chmod
command to do this, and eventually
to chmod
has become an almost acceptable English verb, meaning the changing of the
access mode of a file. The chmod command can be used with alphanumeric or
numeric options, whatever you like best.
The example below uses alphanumeric options in order to solve a problem that
commonly occurs with new users:
asim:~> ./hello
bash: ./hello: bad interpreter: Permission denied
asim:~> cat hello
#!/bin/bash
echo "Hello, World"
asim:~> ls -l hello
-rw-rw-r-- 1 asim asim 32 Jan 15 16:29 hello
asim:~> chmod u+x hello
asim:~> ./hello
Hello, World
asim:~> ls -l hello
-rwxrw-r-- 1 asim asim 32 Jan 15 16:29 hello*
The + and - operators are used to grant or deny a given right to a given group.
Combinations separated by commas are allowed. The Info and man pages contain
useful examples. Here's another one, which makes the file from the previous
example a private file to user asim:
asim:~> chmod u+rwx,go-rwx hello
asim:~> ls -l hello
-rwx------ 1 asim asim 32 Jan 15 16:29 hello*
The kind of problem resulting in an error message saying that permission is
denied somewhere is usually a problem with access rights in most cases. Also,
comments like, “It worked yesterday,†and “When I run this as root it works,â€
are most likely caused by the wrong file permissions.
When using chmod with numeric arguments, the values for each granted access
right have to be counted together per group. Thus we get a 3-digit number,
which is the symbolic value for the settings chmod has to make. The following
table lists the most common combinations:
Table 3.9. File protection with chmod
____________________________________________________________________________
|Command____________|Meaning_________________________________________________|
|chmod_400_file_____|To_protect_a_file_against_accidental_overwriting._______|
|chmod 500 directory|To protect yourself from accidentally removing, renaming|
|___________________|or_moving_files_from_this_directory.____________________|
|chmod 600 file |A private file only changeable by the user who entered |
|___________________|this_command.___________________________________________|
|chmod 644 file |A publicly readable file that can only be changed by the|
|___________________|issuing_user.___________________________________________|
|chmod 660 file |Users belonging to your group can change this file, |
|___________________|others_don't_have_any_access_to_it_at_all.______________|
|chmod 700 file |Protects a file against any access from other users, |
|___________________|while_the_issuing_user_still_has_full_access.___________|
|chmod 755 directory|For files that should be readable and executable by |
|___________________|others,_but_only_changeable_by_the_issuing_user.________|
|chmod_775_file_____|Standard_file_sharing_mode_for_a_group._________________|
|chmod_777_file_____|Everybody_can_do_everything_to_this_file._______________|
If you enter a number with less than three digits as an argument to chmod,
omitted characters are replaced with zeros starting from the left. There is
actually a fourth digit on Linux systems, that precedes the first three and
sets special access modes. Everything about these and many more are located in
the Info pages.
4.2.2. Logging on to another group
When you type
id
on the command line, you get a list of all the groups that you can possibly
belong to, preceded by your user name and ID and the group name and ID that you
are currently connected with. However, on many Linux systems you can only be
actively logged in to one group at the time. By default, this active or
primary group
is the one that you get assigned from the
/etc/passwd
file. The fourth field of this file holds users' primary group ID, which is
looked up in the
/etc/group
file. An example:
asim:~> id
uid=501(asim) gid=501(asim) groups=100(users),501(asim),3400(web)
asim:~> grep asim /etc/passwd
asim:x:501:501:Asim El Baraka:/home/asim:/bin/bash
asim:~> grep 501 /etc/group
asim:x:501:
The fourth field in the line from /etc/passwd contains the value “501â€, which
represents the group asim in the above example. From /etc/group we can get the
name matching this group ID. When initially connecting to the system, this is
the group that asim will belong to.
User private group scheme
In order to allow more flexibility, most Linux systems follow the so-called
user private group scheme, that assigns each user primarily to his or her own
group. This group is a group that only contains this particular user, hence the
name “private groupâ€. Usually this group has the same name as the user login
name, which can be a bit confusing.
Apart from his own private group, user asim can also be in the groups users and
web. Because these are secondary groups to this user, he will need to use the
newgrp
to log into any of these groups (use gpasswd for setting the group password
first). In the example, asim needs to create files that are owned by the group
web.
asim:/var/www/html> newgrp web
asim:/var/www/html> id
uid=501(asim) gid=3400(web) groups=100(users),501(asim),3400(web)
When asim creates new files now, they will be in group ownership of the group
web instead of being owned by the group asim:
asim:/var/www/html> touch test
asim:/var/www/html> ls -l test
-rw-rw-r-- 1 asim web 0 Jun 10 15:38 test
Logging in to a new group prevents you from having to use
chown
(see Section 4.2.4,_“Changing_user_and_group_ownershipâ€) or calling your
system administrator to change ownerships for you.
See the manpage for newgrp for more information.
4.2.3. The file mask
When a new file is saved somewhere, it is first subjected to the standard
security procedure. Files without permissions don't exist on Linux. The
standard file permission is determined by the
mask
for new file creation. The value of this mask can be displayed using the
umask
command:
bert:~> umask
0002
Instead of adding the symbolic values to each other, as with chmod, for
calculating the permission on a new file they need to be subtracted from the
total possible access rights. In the example above, however, we see 4 values
displayed, yet there are only 3 permission categories: user, group and other.
The first zero is part of the special file attributes settings, which we will
discuss in Section 4.2.4,_“Changing_user_and_group_ownership†and Section 1.6,
“SUID_and_SGIDâ€. It might just as well be that this first zero is not displayed
on your system when entering the umask command, and that you only see 3 numbers
representing the default file creation mask.
Each UNIX-like system has a system function for creating new files, which is
called each time a user uses a program that creates new files, for instance,
when downloading a file from the Internet, when saving a new text document and
so on. This function creates both new files and new directories. Full read,
write and execute permission is granted to everybody when creating a new
directory. When creating a new file, this function will grant read and write
permissions for everybody, but set execute permissions to none for all user
categories. This, before the mask is applied, a directory has permissions 777
or rwxrwxrwx, a plain file 666 or rw-rw-rw-.
The umask value is subtracted from these default permissions after the function
has created the new file or directory. Thus, a directory will have permissions
of 775 by default, a file 664, if the mask value is (0)002. This is
demonstrated in the example below:
bert:~> mkdir newdir
bert:~> ls -ld newdir
drwxrwxr-x 2 bert bert 4096 Feb 28 13:45 newdir/
bert:~> touch newfile
bert:~> ls -l newfile
-rw-rw-r-- 1 bert bert 0 Feb 28 13:52 newfile
Files versus directories
A directory gets more permissions by default: it always has the execute
permission. If it wouldn't have that, it would not be accessible. Try this out
by chmodding a directory 644!
If you log in to another group using the newgrp command, the mask remains
unchanged. Thus, if it is set to 002, files and directories that you create
while being in the new group will also be accessible to the other members of
that group; you don't have to use chmod.
The
root
user usually has stricter default file creation permissions:
[root@estoban root]# umask
022
These defaults are set system-wide in the shell resource configuration files,
for instance
/etc/bashrc
or
/etc/profile
. You can change them in your own shell configuration file, see Chapter 7,_Home
sweet_/home on customizing your shell environment.
4.2.4. Changing user and group ownership
When a file is owned by the wrong user or group, the error can be repaired with
the
chown
(change owner) and
chgrp
(change group) commands. Changing file ownership is a frequent system
administrative task in environments where files need to be shared in a group.
Both commands are very flexible, as you can find out by using the --help
option.
The chown command can be applied to change both user and group ownership of a
file, while chgrp only changes group ownership. Of course the system will check
if the user issuing one of these commands has sufficient permissions on the
file(s) she wants to change.
In order to only change the user ownership of a file, use this syntax:
chown newuser file
If you use a colon after the user name (see the Info pages), group ownership
will be changed as well, to the primary group of the user issuing the command.
On a Linux system, each user has his own group, so this form can be used to
make files private:
jacky:~> id
uid=1304(jacky) gid=(1304) groups=1304(jacky),2034(pproject)
jacky:~> ls -l my_report
-rw-rw-r-- 1 jacky project 29387 Jan 15 09:34 my_report
jacky:~> chown jacky: my_report
jacky:~> chmod o-r my_report
jacky:~> ls -l my_report
-rw-rw---- 1 jacky jacky 29387 Jan 15 09:34 my_report
If jacky would like to share this file, without having to give everybody
permission to write it, he can use the chgrp command:
jacky:~> ls -l report-20020115.xls
-rw-rw---- 1 jacky jacky 45635 Jan 15 09:35 report-20020115.xls
jacky:~> chgrp project report-20020115.xls
jacky:~> chmod o= report-20020115.xls
jacky:~> ls -l report-20020115.xls
-rw-rw---- 1 jacky project 45635 Jan 15 09:35 report-20020115.xls
This way, users in the group project will be able to work on this file. Users
not in this group have no business with it at all.
Both chown and chgrp can be used to change ownership recursively, using the -
R option. In that case, all underlying files and subdirectories of a given
directory will belong to the given user and/or group.
Restrictions
On most systems, the use of the chown and chgrp commands is restricted for non-
privileged users. If you are not the administrator of the system, you can not
change user nor group ownerships for security reasons. If the usage of these
commands would not be restricted, malicious users could assign ownership of
files to other users and/or groups and change behavior of those users'
environments and even cause damage to other users' files.
4.2.5. Special modes
For the system admin to not be bothered solving permission problems all the
time, special access rights can be given to entire directories, or to separate
programs. There are three special modes:
* Sticky bit mode: After execution of a job, the command is kept in the system
memory. Originally this was a feature used a lot to save memory: big jobs are
loaded into memory only once. But these days memory is inexpensive and there
are better techniques to manage it, so it is not used anymore for its
optimizing capabilities on single files. When applied to an entire directory,
however, the sticky bit has a different meaning. In that case, a user can
only change files in this directory when she is the user owner of the file or
when the file has appropriate permissions. This feature is used on
directories like /var/tmp, that have to be accessible for everyone, but where
it is not appropriate for users to change or delete each other's data. The
sticky bit is indicated by a t at the end of the file permission field:
mark:~> ls -ld /var/tmp
drwxrwxrwt 19 root root 8192 Jan 16 10:37 /var/tmp/
The sticky bit is set using the command chmod o+t directory. The historic
origin of the “t†is in UNIX' save Text access feature.
* SUID (set user ID) and SGID (set group ID): represented by the character s in
the user or group permission field. When this mode is set on an executable
file, it will run with the user and group permissions on the file instead of
with those of the user issuing the command, thus giving access to system
resources. We will discuss this further in Chapter 4,_Processes.
* SGID (set group ID) on a directory: in this special case every file created
in the directory will have the same group owner as the directory itself
(while normal behavior would be that new files are owned by the users who
create them). This way, users don't need to worry about file ownership when
sharing directories:
mimi:~> ls -ld /opt/docs
drwxrws--- 4 root users 4096 Jul 25 2001 docs/
mimi:~> ls -l /opt/docs
-rw-rw---- 1 mimi users 345672 Aug 30 2001-Council.doc
This is the standard way of sharing files in UNIX.
Existing files are left unchanged!
Files that are being moved to a SGID directory but were created elsewhere
keep their original user and group owner. This may be confusing.
5. Summary
On UNIX, as on Linux, all entities are in some way or another presented to the
system as files with the appropriate file properties. Use of (predefined) paths
allows the users and the system admin to find, read and manipulate files.
We've made our first steps toward becoming an expert: we discussed the real and
the fake structure of the file system, and we know about the Linux file
security model, as well as several other security precautions that are taken on
every system by default.
The shell is the most important tool for interaction with the system. We
learned several shell commands in this chapter, which are listed in the table
below.
Table 3.10. New commands in chapter 3: Files and the file system
_____________________________________________________________________________
|Command____________________________|Meaning__________________________________|
|bash_______________________________|GNU_shell_program._______________________|
|cat file(s) |Send content of file(s) to standard |
|___________________________________|output.__________________________________|
|cd directory |Enter directory. cd is a bash built-in |
|___________________________________|command._________________________________|
|chgrp newgroup file(s) |Change the group ownership of file(s) to |
|___________________________________|newgroup_________________________________|
|chmod_mode_file(s)_________________|Change_access_permissions_on_file(s)_____|
|chown_newowner[:[newgroup]]_file(s)|Change_file_owner_and_group_ownership.___|
|cp_sourcefile_targetfile___________|Copy_sourcefile_to_targetfile.___________|
|df file |Reports on used disk space on the |
|___________________________________|partition_containing_file._______________|
|echo_string________________________|Display_a_line_of_text___________________|
|export |Part of bash that announces variables and|
|___________________________________|their_values_to_the_system.______________|
|file_filename______________________|Determine_file_type_of_filename._________|
|find_path_expression_______________|Find_files_in_the_file_system_hierarchy__|
|grep PATTERN file |Print lines in file containing the search|
|___________________________________|pattern._________________________________|
|head file |Send the first part of file to standard |
|___________________________________|output___________________________________|
|id |Prints real and effective user name and |
|___________________________________|groups.__________________________________|
|info_command_______________________|Read_documentation_about_command.________|
|less_file__________________________|View_file_with_a_powerful_viewer.________|
|ln targetfile linkname |Make a link with name linkname to |
|___________________________________|targetfile.______________________________|
|locate searchstring |Print all accessible files matching the |
|___________________________________|search_pattern.__________________________|
|ls_file(s)_________________________|Prints_directory_content.________________|
|man command |Format and display online (system) manual|
|___________________________________|pages_for_command._______________________|
|mkdir_newdir_______________________|Make_a_new_empty_directory.______________|
|mv_oldfile_newfile_________________|Rename_or_move_oldfile.__________________|
|newgrp_groupname___________________|Log_in_to_a_new_group.___________________|
|pwd |Print the present or current working |
|___________________________________|directory._______________________________|
|quota______________________________|Show_disk_usage_and_limits.______________|
|rm_file____________________________|Removes_files_and_directories.___________|
|rmdir_file_________________________|Removes_directories._____________________|
|tail_file__________________________|Print_the_last_part_of_file._____________|
|umask_[value]______________________|Show_or_change_new_file_creation_mode.___|
|wc file |Counts lines, words and characters in |
|___________________________________|file.____________________________________|
|which_command______________________|Shows_the_full_path_to_command.__________|
We also stressed the fact that you should READ THE MAN PAGES. This
documentation is your first-aid kit and contains the answers to many questions.
The above list contains the basic commands that you will use on a daily basis,
but they can do much more than the tasks we've discussed here. Reading the
documentation will give you the control you need.
Last but not least, a handy overview of file permissions:
Table 3.11. File permissions
___________________________________
|Who\What|r(ead)|w(rite)|(e)x(ecute)|
|u(ser)__|4_____|2______|1__________|
|g(roup)_|4_____|2______|1__________|
|o(ther)_|4_____|2______|1__________|
6. Exercises
Just login with your common user ID.
6.1. Partitions
* On which partition is your home directory?
* How many partitions are on your system?
* What is the total size of your Linux installation?
6.2. Paths
* Display your search path.
* Export a senseless path by entering, for instance, export PATH=blah and try
listing directory content.
* What is the path to your home directory? How would another user reach your
home directory starting from his own home directory, using a relative path?
* Go to the tmp directory in /var.
* Now go to share in /usr using only one command. Change to doc. What is your
present working directory?
6.3. Tour of the system
* Change to the /proc directory.
* What CPU(s) is the system running on?
* How much RAM does it currently use?
* How much swap space do you have?
* What drivers are loaded?
* How many hours has the system been running?
* Which filesystems are known by your system?
* Change to /etc/rc.d | /etc/init.d | /etc/runlevels and choose the directory
appropriate for your run level.
* What services should be running in this level?
* Which services run in graphical mode that don't run in text mode?
* Change to /etc
* How long does the system keep the log file in which user logins are
monitored?
* Which release are you running?
* Are there any issues or messages of the day?
* How many users are defined on your system? Don't count them, let the computer
do it for you!
* How many groups?
* Where is the time zone information kept?
* Are the HOWTOs installed on your system?
* Change to /usr/share/doc.
* Name three programs that come with the GNU coreutils package.
* Which version of bash is installed on this system?
6.4. Manipulating files
* Create a new directory in your home directory.
* Can you move this directory to the same level as your home directory?
* Copy all XPM files from /usr/share/pixmaps to the new directory. What does
XPM mean?
* List the files in reverse alphabetical order.
* Change to your home directory. Create a new directory and copy all the files
of the /etc directory into it. Make sure that you also copy the files and
directories which are in the subdirectories of /etc! (recursive copy)
* Change into the new directory and make a directory for files starting with an
upper case character and one for files starting with a lower case character.
Move all the files to the appropriate directories. Use as few commands as
possible.
* Remove the remaining files.
* Delete the directory and its entire content using a single command.
* Use grep to find out which script starts the Font Server in the graphical run
level.
* Where is the sendmail server program?
* Make a symbolic link in your home directory to /var/tmp. Check that it really
works.
* Make another symbolic link in your home directory to this link. Check that it
works. Remove the first link and list directory content. What happened to the
second link?
6.5. File permissions
* Can you change file permissions on /home?
* What is your standard file creation mode?
* Change ownership of /etc to your own user and group.
* Change file permissions of ~/.bashrc so that only you and your primary group
can read it.
* Issue the command locate root. Do you notice anything special?
* Make a symbolic link to /root. Can it be used?
Chapter 4. Processes
Table of Contents
1._Processes_inside_out
1.1._Multi-user_and_multi-tasking
1.2._Process_types
1.3._Process_attributes
1.4._Displaying_process_information
1.5._Life_and_death_of_a_process
1.6._SUID_and_SGID
2._Boot_process,_Init_and_shutdown
2.1._Introduction
2.2._The_boot_process
2.3._GRUB_features
2.4._Init
2.5._Init_run_levels
2.6._Shutdown
3._Managing_processes
3.1._Work_for_the_system_admin
3.2._How_long_does_it_take?
3.3._Performance
3.4._Load
3.5._Can_I_do_anything_as_a_user?
4._Scheduling_processes
4.1._Use_that_idle_time!
4.2._The_sleep_command
4.3._The_at_command
4.4._Cron_and_crontab
5._Summary
6._Exercises
6.1._General
6.2._Booting,_init_etc.
6.3._Scheduling
Abstract
Next to files, processes are the most important things on a UNIX/Linux system.
In this chapter, we will take a closer look at those processes. We will learn
more about:
* Multi-user processing and multi-tasking
* Process types
* Controlling processes with different signals
* Process attributes
* The life cycle of a process
* System startup and shutdown
* SUID and SGID
* System speed and response
* Scheduling processes
* The Vixie cron system
* How to get the most out of your system
1. Processes inside out
1.1. Multi-user and multi-tasking
Now that we are more used to our environment and we are able to communicate a
little bit with our system, it is time to study the processes we can start in
more detail. Not every command starts a single process. Some commands initiate
a series of processes, such as mozilla; others, like ls, are executed as a
single command.
Furthermore, Linux is based on UNIX, where it has been common policy to have
multiple users running multiple commands, at the same time and on the same
system. It is obvious that measures have to be taken to have the CPU manage all
these processes, and that functionality has to be provided so users can switch
between processes. In some cases, processes will have to continue to run even
when the user who started them logs out. And users need a means to reactivate
interrupted processes.
We will explain the structure of Linux processes in the next sections.
1.2. Process types
1.2.1. Interactive processes
Interactive processes are initialized and controlled through a terminal
session. In other words, there has to be someone connected to the system to
start these processes; they are not started automatically as part of the system
functions. These processes can run in the foreground, occupying the terminal
that started the program, and you can't start other applications as long as
this process is running in the foreground. Alternatively, they can run in the
background, so that the terminal in which you started the program can accept
new commands while the program is running. Until now, we mainly focussed on
programs running in the foreground - the length of time taken to run them was
too short to notice - but viewing a file with the less command is a good
example of a command occupying the terminal session. In this case, the
activated program is waiting for you to do something. The program is still
connected to the terminal from where it was started, and the terminal is only
useful for entering commands this program can understand. Other commands will
just result in errors or unresponsiveness of the system.
While a process runs in the background, however, the user is not prevented from
doing other things in the terminal in which he started the program, while it is
running.
The shell offers a feature called
job control
which allows easy handling of multiple processes. This mechanism switches
processes between the foreground and the background. Using this system,
programs can also be started in the background immediately.
Running a process in the background is only useful for programs that don't need
user input (via the shell). Putting a job in the background is typically done
when execution of a job is expected to take a long time. In order to free the
issuing terminal after entering the command, a trailing ampersand is added. In
the example, using graphical mode, we open an extra terminal window from the
existing one:
billy:~> xterm &
[1] 26558
billy:~>
jobs
[1]+ Running xterm &
The full job control features are explained in detail in the bash Info pages,
so only the frequently used job control applications are listed here:
Table 4.1. Controlling processes
____________________________________________________________________________
|(part_of)_command|Meaning___________________________________________________|
|regular_command__|Runs_this_command_in_the_foreground.______________________|
|command_&____|Run_this_command_in_the_background_(release_the_terminal)_|
|jobs_____________|Show_commands_running_in_the_background.__________________|
|Ctrl+Z |Suspend (stop, but not quit) a process running in the |
|_________________|foreground_(suspend)._____________________________________|
|Ctrl+C |Interrupt (terminate and quit) a process running in the |
|_________________|foreground._______________________________________________|
| |Every process running in the background gets a number |
|%n |assigned to it. By using the % expression a job can be |
|_________________|referred_to_using_its_number,_for_instance_fg_%2._________|
|bg_______________|Reactivate_a_suspended_program_in_the_background._________|
|fg_______________|Puts_the_job_back_in_the_foreground.______________________|
|kill |End a process (also see Shell Builtin Commands in the Info|
|_________________|pages_of_bash)____________________________________________|
More practical examples can be found in the exercises.
Most UNIX systems are likely to be able to run
screen
, which is useful when you actually want another shell to execute commands.
Upon calling screen, a new session is created with an accompanying shell and/or
commands as specified, which you can then put out of the way. In this new
session you may do whatever it is you want to do. All programs and operations
will run independent of the issuing shell. You can then detach this session,
while the programs you started in it continue to run, even when you log out of
the originating shell, and pick your screen up again any time you like.
This program originates from a time when virtual consoles were not invented
yet, and everything needed to be done using one text terminal. To addicts, it
still has meaning in Linux, even though we've had virtual consoles for almost
ten years.
1.2.2. Automatic processes
Automatic or batch processes are not connected to a terminal. Rather, these are
tasks that can be queued into a spooler area, where they wait to be executed on
a FIFO (first-in, first-out) basis. Such tasks can be executed using one of two
criteria:
* At a certain date and time: done using the at command, which we will discuss
in the second part of this chapter.
* At times when the total system load is low enough to accept extra jobs: done
using the
batch
command. By default, tasks are put in a queue where they wait to be executed
until the system load is lower than 0.8. In large environments, the system
administrator may prefer batch processing when large amounts of data have to
be processed or when tasks demanding a lot of system resources have to be
executed on an already loaded system. Batch processing is also used for
optimizing system performance.
1.2.3. Daemons
Daemons are server processes that run continuously. Most of the time, they are
initialized at system startup and then wait in the background until their
service is required. A typical example is the networking daemon, xinetd, which
is started in almost every boot procedure. After the system is booted, the
network daemon just sits and waits until a client program, such as an FTP
client, needs to connect.
1.3. Process attributes
A process has a series of characteristics, which can be viewed with the
ps
command:
* The process ID or PID: a unique identification number used to refer to the
process.
* The parent process ID or PPID: the number of the process (PID) that started
this process.
* Nice number: the degree of friendliness of this process toward other
processes (not to be confused with process priority, which is calculated
based on this nice number and recent CPU usage of the process).
* Terminal or TTY: terminal to which the process is connected.
* User name of the real and effective user (RUID and EUID): the owner of the
process. The real owner is the user issuing the command, the effective user
is the one determining access to system resources. RUID and EUID are usually
the same, and the process has the same access rights the issuing user would
have. An example to clarify this: the browser mozilla in /usr/bin is owned by
user root:
theo:~> ls -l /usr/bin/mozilla
-rwxr-xr-x 1 root root 4996 Nov 20 18:28 /usr/bin/mozilla*
theo:~> mozilla &
[1] 26595
theo:~> ps -af
UID PID PPID C STIME TTY TIME CMD
theo 26601 26599 0 15:04 pts/5 00:00:00 /usr/lib/mozilla/mozilla-bin
theo 26613 26569 0 15:04 pts/5 00:00:00 ps -af
When user theo starts this program, the process itself and all processes
started by the initial process, will be owned by user theo and not by the
system administrator. When mozilla needs access to certain files, that access
will be determined by theo's permissions and not by root's.
* Real and effective group owner (RGID and EGID): The real group owner of a
process is the primary group of the user who started the process. The
effective group owner is usually the same, except when SGID access mode has
been applied to a file.
1.4. Displaying process information
The ps command is one of the tools for visualizing processes. This command has
several options which can be combined to display different process attributes.
With no options specified, ps only gives information about the current shell
and eventual processes:
theo:~> ps
PID TTY TIME CMD
4245 pts/7 00:00:00 bash
5314 pts/7 00:00:00 ps
Since this does not give enough information - generally, at least a hundred
processes are running on your system - we will usually select particular
processes out of the list of all processes, using the grep command in a pipe,
see Section 1.2.1,_“Output_redirection_with_>_and_|â€, as in this line, which
will select and display all processes owned by a particular user:
ps -ef | grep username
This example shows all processes with a process name of bash, the most common
login shell on Linux systems:
theo:> ps auxw | grep bash
brenda 31970 0.0 0.3 6080 1556 tty2 S Feb23 0:00 -bash
root 32043 0.0 0.3 6112 1600 tty4 S Feb23 0:00 -bash
theo 32581 0.0 0.3 6384 1864 pts/1 S Feb23 0:00 bash
theo 32616 0.0 0.3 6396 1896 pts/2 S Feb23 0:00 bash
theo 32629 0.0 0.3 6380 1856 pts/3 S Feb23 0:00 bash
theo 2214 0.0 0.3 6412 1944 pts/5 S 16:18 0:02 bash
theo 4245 0.0 0.3 6392 1888 pts/7 S 17:26 0:00 bash
theo 5427 0.0 0.1 3720 548 pts/7 S 19:22 0:00 grep bash
In these cases, the grep command finding lines containing the string bash is
often displayed as well on systems that have a lot of idletime. If you don't
want this to happen, use the
pgrep
command.
Bash shells are a special case: this process list also shows which ones are
login shells (where you have to give your username and password, such as when
you log in in textmode or do a remote login, as opposed to non-login shells,
started up for instance by clicking a terminal window icon). Such login shells
are preceded with a dash (-).
|?
We will explain about the | operator in the next chapter, see Chapter 5,_I/
O_redirection.
More info can be found the usual way: ps --help or man ps. GNU ps supports
different styles of option formats; the above examples don't contain errors.
Note that ps only gives a momentary state of the active processes, it is a one-
time recording. The
top
program displays a more precise view by updating the results given by ps (with
a bunch of options) once every five seconds, generating a new list of the
processes causing the heaviest load periodically, meanwhile integrating more
information about the swap space in use and the state of the CPU, from the proc
file system:
12:40pm up 9 days, 6:00, 4 users, load average: 0.21, 0.11, 0.03
89 processes: 86 sleeping, 3 running, 0 zombie, 0 stopped
CPU states: 2.5% user, 1.7% system, 0.0% nice, 95.6% idle
Mem: 255120K av, 239412K used, 15708K free, 756K shrd, 22620K buff
Swap: 1050176K av, 76428K used, 973748K free, 82756K cached
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
5005 root 14 0 91572 15M 11580 R 1.9 6.0 7:53 X
19599 jeff 14 0 1024 1024 796 R 1.1 0.4 0:01 top
19100 jeff 9 0 5288 4948 3888 R 0.5 1.9 0:24 gnome-terminal
19328 jeff 9 0 37884 36M 14724 S 0.5 14.8 1:30 mozilla-bin
1 root 8 0 516 472 464 S 0.0 0.1 0:06 init
2 root 9 0 0 0 0 SW 0.0 0.0 0:02 keventd
3 root 9 0 0 0 0 SW 0.0 0.0 0:00 kapm-idled
4 root 19 19 0 0 0 SWN 0.0 0.0 0:00 ksoftirqd_CPU0
5 root 9 0 0 0 0 SW 0.0 0.0 0:33 kswapd
6 root 9 0 0 0 0 SW 0.0 0.0 0:00 kreclaimd
7 root 9 0 0 0 0 SW 0.0 0.0 0:00 bdflush
8 root 9 0 0 0 0 SW 0.0 0.0 0:05 kupdated
9 root -1-20 0 0 0 SW< 0.0 0.0 0:00 mdrecoveryd
13 root 9 0 0 0 0 SW 0.0 0.0 0:01 kjournald
89 root 9 0 0 0 0 SW 0.0 0.0 0:00 khubd
219 root 9 0 0 0 0 SW 0.0 0.0 0:00 kjournald
220 root 9 0 0 0 0 SW 0.0 0.0 0:00 kjournald
The first line of top contains the same information displayed by the
uptime
command:
jeff:~> uptime
3:30pm, up 12 days, 23:29, 6 users, load average: 0.01, 0.02, 0.00
The data for these programs is stored among others in /var/run/utmp
(information about currently connected users) and in the virtual file system
/proc
, for example /proc/loadavg (average load information). There are all sorts of
graphical applications to view this data, such as the Gnome System Monitor and
lavaps. Over at FreshMeat and SourceForge you will find tens of applications
that centralize this information along with other server data and logs from
multiple servers on one (web) server, allowing monitoring of the entire IT
infrastructure from one workstation.
The relations between processes can be visualized using the
pstree
command:
sophie:~> pstree
init-+-amd
|-apmd
|-2*[artsd]
|-atd
|-crond
|-deskguide_apple
|-eth0
|-gdm---gdm-+-X
| `-gnome-session-+-Gnome
| |-ssh-agent
| `-true
|-geyes_applet
|-gkb_applet
|-gnome-name-serv
|-gnome-smproxy
|-gnome-terminal-+-bash---vim
| |-bash
| |-bash---pstree
| |-bash---ssh
| |-bash---mozilla-bin---mozilla-bin---3*[mozilla-bin]
| `-gnome-pty-helper
|-gpm
|-gweather
|-kapm-idled
|-3*[kdeinit]
|-keventd
|-khubd
|-5*[kjournald]
|-klogd
|-lockd---rpciod
|-lpd
|-mdrecoveryd
|-6*[mingetty]
|-8*[nfsd]
|-nscd---nscd---5*[nscd]
|-ntpd
|-3*[oafd]
|-panel
|-portmap
|-rhnsd
|-rpc.mountd
|-rpc.rquotad
|-rpc.statd
|-sawfish
|-screenshooter_a
|-sendmail
|-sshd---sshd---bash---su---bash
|-syslogd
|-tasklist_applet
|-vmnet-bridge
|-xfs
`-xinetd-ipv6
The -u and -a options give additional information. For more options and what
they do, refer to the Info pages.
In the next section, we will see how one process can create another.
1.5. Life and death of a process
1.5.1. Process creation
A new process is created because an existing process makes an exact copy of
itself. This child process has the same environment as its parent, only the
process ID number is different. This procedure is called
forking
.
After the forking process, the address space of the child process is
overwritten with the new process data. This is done through an
exec
call to the system.
The
fork-and-exec
mechanism thus switches an old command with a new, while the environment in
which the new program is executed remains the same, including configuration of
input and output devices, environment variables and priority. This mechanism is
used to create all UNIX processes, so it also applies to the Linux operating
system. Even the first process,
init
, with process ID 1, is forked during the boot procedure in the so-called
bootstrapping
procedure.
This scheme illustrates the fork-and-exec mechanism. The process ID changes
after the fork procedure:
Figure 4.1. Fork-and-exec mechanism
Fork creates a new process with the same content as the parent in memory but a
different PID, exec replaces the content with the actual data to be processed,
PID stays the same.
There are a couple of cases in which init becomes the parent of a process,
while the process was not started by init, as we already saw in the pstree
example. Many programs, for instance,
daemonize
their child processes, so they can keep on running when the parent stops or is
being stopped. A window manager is a typical example; it starts an xterm
process that generates a shell that accepts commands. The window manager then
denies any further responsibility and passes the child process to init. Using
this mechanism, it is possible to change window managers without interrupting
running applications.
Every now and then things go wrong, even in good families. In an exceptional
case, a process might finish while the parent does not wait for the completion
of this process. Such an unburied process is called a zombie process.
1.5.2. Ending processes
When a process ends normally (it is not killed or otherwise unexpectedly
interrupted), the program returns its
exit status
to the parent. This exit status is a number returned by the program providing
the results of the program's execution. The system of returning information
upon executing a job has its origin in the C programming language in which UNIX
has been written.
The return codes can then be interpreted by the parent, or in scripts. The
values of the return codes are program-specific. This information can usually
be found in the man pages of the specified program, for example the grep
command returns -1 if no matches are found, upon which a message on the lines
of “No files found†can be printed. Another example is the Bash builtin command
true, which does nothing except return an exit status of 0, meaning success.
1.5.3. Signals
Processes end because they receive a signal. There are multiple signals that
you can send to a process. Use the
kill
command to send a signal to a process. The command kill -l shows a list of
signals. Most signals are for internal use by the system, or for programmers
when they write code. As a user, you will need the following signals:
Table 4.2. Common signals
__________________________________________________________________________
|Signal_name|Signal_number|Meaning_________________________________________|
|SIGTERM____|15___________|Terminate_the_process_in_an_orderly_way.________|
|SIGINT |2 |Interrupt the process. A process can ignore this|
|___________|_____________|signal._________________________________________|
|SIGKILL |9 |Interrupt the process. A process can not ignore |
|___________|_____________|this_signal.____________________________________|
|SIGHUP_____|1____________|For_daemons:_reread_the_configuration_file._____|
You can read more about default actions that are taken when sending a signal to
a process in man 7 signal.
1.6. SUID and SGID
As promised in the previous chapter, we will now discuss the special modes SUID
and SGID in more detail. These modes exist to provide normal users the ability
to execute tasks they would normally not be able to do because of the tight
file permission scheme used on UNIX based systems. In the ideal situation
special modes are used as sparsely as possible, since they include security
risks. Linux developers have generally tried to avoid them as much as possible.
The Linux ps version, for example, uses the information stored in the /proc
file system, which is accessible to everyone, thus avoiding exposition of
sensitive system data and resources to the general public. Before that, and
still on older UNIX systems, the ps program needed access to files such as
/dev/mem
and
/dev/kmem
, which had disadvantages because of the permissions and ownerships on these
files:
rita:~> ls -l /dev/*mem
crw-r----- 1 root kmem 1, 2 Aug 30 22:30 /dev/kmem
crw-r----- 1 root kmem 1, 1 Aug 30 22:30 /dev/mem
With older versions of ps, it was not possible to start the program as a common
user, unless special modes were applied to it.
While we generally try to avoid applying any special modes, it is sometimes
necessary to use an SUID. An example is the mechanism for changing passwords.
Of course users will want to do this themselves instead of having their
password set by the system administrator. As we know, user names and passwords
are listed in the
/etc/passwd
file, which has these access permissions and owners:
bea:~> ls -l /etc/passwd
-rw-r--r-- 1 root root 1267 Jan 16 14:43 /etc/passwd
Still, users need to be able to change their own information in this file. This
is achieved by giving the passwd program special permissions:
mia:~> which passwd
passwd is /usr/bin/passwd
mia:~> ls -l /usr/bin/passwd
-r-s--x--x 1 root root 13476 Aug 7 06:03 /usr/bin/passwd*
When called, the passwd command will run using the access permissions of root,
thus enabling a common user to edit the password file which is owned by the
system admin.
SGID modes on a file don't occur nearly as frequently as SUID, because SGID
often involves the creation of extra groups. In some cases, however, we have to
go through this trouble in order to build an elegant solution (don't worry
about this too much - the necessary groups are usually created upon
installation). This is the case for the
write
and
wall
programs, which are used to send messages to other users' terminals (ttys).
The write command writes a message to a single user, while wall writes to all
connected users.
Sending text to another user's terminal or graphical display is normally not
allowed. In order to bypass this problem, a group has been created, which owns
all terminal devices. When the write and wall commands are granted SGID
permissions, the commands will run using the access rights as applicable to
this group, tty in the example. Since this group has write access to the
destination terminal, also a user having no permissions to use that terminal in
any way can send messages to it.
In the example below, user joe first finds out on which terminal his
correspondent is connected, using the who command. Then he sends her a message
using the write command. Also illustrated are the access rights on the write
program and on the terminals occupied by the receiving user: it is clear that
others than the user owner have no permissions on the device, except for the
group owner, which can write to it.
joe:~> which write
write is /usr/bin/write
joe:~> ls -l /usr/bin/write
-rwxr-sr-x 1 root tty 8744 Dec 5 00:55 /usr/bin/write*
joe:~>
who
jenny tty1 Jan 23 11:41
jenny pts/1 Jan 23 12:21 (:0)
jenny pts/2 Jan 23 12:22 (:0)
jenny pts/3 Jan 23 12:22 (:0)
joe pts/0 Jan 20 10:13 (lo.callhost.org)
joe:~> ls -l /dev/tty1
crw--w---- 1 jenny tty 4, 1 Jan 23 11:41 /dev/tty1
joe:~> write jenny tty1
hey Jenny, shall we have lunch together?
^C
User jenny gets this on her screen:
Message from joe@lo.callhost.org on ptys/1 at 12:36 ...
hey Jenny, shall we have lunch together?
EOF
After receiving a message, the terminal can be cleared using the Ctrl+L key
combination. In order to receive no messages at all (except from the system
administrator), use the mesg command. To see which connected users accept
messages from others use who -w. All features are fully explained in the Info
pages of each command.
Group names may vary
The group scheme is specific to the distribution. Other distributions may use
other names or other solutions.
2. Boot process, Init and shutdown
2.1. Introduction
One of the most powerful aspects of Linux concerns its open method of starting
and stopping the operating system, where it loads specified programs using
their particular configurations, permits you to change those configurations to
control the boot process, and shuts down in a graceful and organized way.
Beyond the question of controlling the boot or shutdown process, the open
nature of Linux makes it much easier to determine the exact source of most
problems associated with starting up or shutting down your system. A basic
understanding of this process is quite beneficial to everybody who uses a Linux
system.
A lot of Linux systems use
lilo
, the LInux LOader for booting operating systems. We will only discuss GRUB,
however, which is easier to use and more flexible. Should you need information
about lilo, refer to the man pages and HOWTOs. Both systems support dual boot
installations, we refer to the HOWTOs on this subject for practical examples
and background information.
2.2. The boot process
When an x86 computer is booted, the processor looks at the end of the system
memory for the BIOS (Basic Input/Output System) and runs it. The BIOS program
is written into permanent read-only memory and is always available for use. The
BIOS provides the lowest level interface to peripheral devices and controls the
first step of the boot process.
The BIOS tests the system, looks for and checks peripherals, and then looks for
a drive to use to boot the system. Usually it checks the floppy drive (or CD-
ROM drive on many newer systems) for bootable media, if present, and then it
looks to the hard drive. The order of the drives used for booting is usually
controlled by a particular BIOS setting on the system. Once Linux is installed
on the hard drive of a system, the BIOS looks for a Master Boot Record (MBR)
starting at the first sector on the first hard drive, loads its contents into
memory, then passes control to it.
This MBR contains instructions on how to load the GRUB (or LILO) boot-loader,
using a pre-selected operating system. The MBR then loads the boot-loader,
which takes over the process (if the boot-loader is installed in the MBR). In
the default Red Hat Linux configuration, GRUB uses the settings in the MBR to
display boot options in a menu. Once GRUB has received the correct instructions
for the operating system to start, either from its command line or
configuration file, it finds the necessary boot file and hands off control of
the machine to that operating system.
2.3. GRUB features
This boot method is called
direct loading
because instructions are used to directly load the operating system, with no
intermediary code between the boot-loaders and the operating system's main
files (such as the kernel). The boot process used by other operating systems
may differ slightly from the above, however. For example, Microsoft's DOS and
Windows operating systems completely overwrite anything on the MBR when they
are installed without incorporating any of the current MBR's configuration.
This destroys any other information stored in the MBR by other operating
systems, such as Linux. The Microsoft operating systems, as well as various
other proprietary operating systems, are loaded using a chain loading boot
method. With this method, the MBR points to the first sector of the partition
holding the operating system, where it finds the special files necessary to
actually boot that operating system.
GRUB supports both boot methods, allowing you to use it with almost any
operating system, most popular file systems, and almost any hard disk your BIOS
can recognize.
GRUB contains a number of other features; the most important include:
* GRUB provides a true command-based, pre-OS environment on x86 machines to
allow maximum flexibility in loading operating systems with certain options
or gathering information about the system.
* GRUB supports Logical Block Addressing (LBA) mode, needed to access many IDE
and all SCSI hard disks. Before LBA, hard drives could encounter a 1024-
cylinder limit, where the BIOS could not find a file after that point.
* GRUB's configuration file is read from the disk every time the system boots,
preventing you from having to write over the MBR every time you change the
boot options.
A full description of GRUB may be found by issuing the info grub command or at
the_GRUB_site. The Linux Documentation Project has a Multiboot_with_GRUB_Mini-
HOWTO.
2.4. Init
The kernel, once it is loaded, finds
init
in sbin and executes it.
When init starts, it becomes the parent or grandparent of all of the processes
that start up automatically on your Linux system. The first thing init does, is
reading its initialization file,
/etc/inittab
. This instructs init to read an initial configuration script for the
environment, which sets the path, starts swapping, checks the file systems, and
so on. Basically, this step takes care of everything that your system needs to
have done at system initialization: setting the clock, initializing serial
ports and so forth.
Then init continues to read the /etc/inittab file, which describes how the
system should be set up in each run level and sets the default
run level
. A run level is a configuration of processes. All UNIX-like systems can be run
in different process configurations, such as the single user mode, which is
referred to as run level 1 or run level S (or s). In this mode, only the system
administrator can connect to the system. It is used to perform maintenance
tasks without risks of damaging the system or user data. Naturally, in this
configuration we don't need to offer user services, so they will all be
disabled. Another run level is the reboot run level, or run level 6, which
shuts down all running services according to the appropriate procedures and
then restarts the system.
Use the
who
to check what your current run level is:
willy@ubuntu:~$ who -r
run-level 2 2006-10-17 23:22 last=S
More about run levels in the next section, see Section 2.5,_“Init_run_levelsâ€.
After having determined the default run level for your system, init starts all
of the background processes necessary for the system to run by looking in the
appropriate rc directory for that run level. init runs each of the kill scripts
(their file names start with a K) with a stop parameter. It then runs all of
the start scripts (their file names start with an S) in the appropriate run
level directory so that all services and applications are started correctly. In
fact, you can execute these same scripts manually after the system is finished
booting with a command like /etc/init.d/httpd stop or service httpd stop logged
in as root, in this case stopping the web server.
Special case
Note that on system startup, the scripts in rc2.d and rc3.d are usually
executed. In that case, no services are stopped (at least not permanently).
There are only services that are started.
None of the scripts that actually start and stop the services are located in /
etc/rc<x>.d. Rather, all of the files in /etc/rc<x>.d are symbolic links that
point to the actual scripts located in /etc/init.d. A symbolic link is nothing
more than a file that points to another file, and is used in this case because
it can be created and deleted without affecting the actual scripts that kill or
start the services. The symbolic links to the various scripts are numbered in a
particular order so that they start in that order. You can change the order in
which the services start up or are killed by changing the name of the symbolic
link that refers to the script that actually controls the service. You can use
the same number multiple times if you want a particular service started or
stopped right before or after another service, as in the example below, listing
the content of /etc/rc5.d, where crond and xfs are both started from a linkname
starting with “S90â€. In this case, the scripts are started in alphabetical
order.
[jean@blub /etc/rc5.d] ls
K15httpd@ K45named@ S08ipchains@ S25netfs@ S85gpm@
K16rarpd@ K46radvd@ S08iptables@ S26apmd@ S90crond@
K20nfs@ K61ldap@ S09isdn@ S28autofs@ S90xfs@
K20rstatd@ K65identd@ S10network@ S30nscd@ S95anacron@
K20rusersd@ K74ntpd@ S12syslog@ S55sshd@ S95atd@
K20rwalld@ K74ypserv@ S13portmap@ S56rawdevices@ S97rhnsd@
K20rwhod@ K74ypxfrd@ S14nfslock@ S56xinetd@ S99local@
K25squid@ K89bcm5820@ S17keytable@ S60lpd@
K34yppasswdd@ S05kudzu@ S20random@ S80sendmail@
After init has progressed through the run levels to get to the default run
level, the /etc/inittab script forks a
getty
process for each virtual console (login prompt in text mode). getty opens tty
lines, sets their modes, prints the login prompt, gets the user's name, and
then initiates a login process for that user. This allows users to authenticate
themselves to the system and use it. By default, most systems offer 6 virtual
consoles, but as you can see from the inittab file, this is configurable.
/etc/inittab can also tell init how it should handle a user pressing
Ctrl+Alt+Delete at the console. As the system should be properly shut down and
restarted rather than immediately power-cycled, init is told to execute the
command /sbin/shutdown -t3 -r now, for instance, when a user hits those keys.
In addition, /etc/inittab states what init should do in case of power failures,
if your system has a UPS unit attached to it.
On most RPM-based systems the graphical login screen is started in run level 5,
where /etc/inittab runs a script called
/etc/X11/prefdm
. The prefdm script runs the preferred X display manager, based on the contents
of the /etc/sysconfig/desktop directory. This is typically
gdm
if you run GNOME or
kdm
if you run KDE, but they can be mixed, and there's also the xdm that comes
with a standard X installation.
But there are other possibilities as well. On Debian, for instance, there is an
initscript for each of the display managers, and the content of the /etc/X11/
default-display-manager is used to determine which one to start. More about the
graphical interface can be read in Section 3,_“The_graphical_environmentâ€.
Ultimately, your system documentation will explain the details about the higher
level aspects of init.
The
/etc/default
and/or
/etc/sysconfig
directories contain entries for a range of functions and services, these are
all read at boot time. The location of the directory containing system defaults
might be somewhat different depending on your Linux distribution.
Besides the graphical user environment, a lot of other services may be started
as well. But if all goes well, you should be looking at a login prompt or login
screen when the boot process has finished.
Other procedures
We explained how SysV init works on x86 based machines. Startup procedures may
vary on other architectures and distributions. Other systems may use the BSD-
style init, where startup files are not split up into multiple /etc/rc<LEVEL>.d
directories. It might also be possible that your system uses /etc/rc.d/init.d
instead of /etc/init.d.
2.5. Init run levels
The idea behind operating different services at different run levels
essentially revolves around the fact that different systems can be used in
different ways. Some services cannot be used until the system is in a
particular state, or mode, such as being ready for more than one user or having
networking available.
There are times in which you may want to operate the system in a lower mode.
Examples are fixing disk corruption problems in run level 1 so no other users
can possibly be on the system, or leaving a server in run level 3 without an X
session running. In these cases, running services that depend upon a higher
system mode to function does not make sense because they will not work
correctly anyway. By already having each service assigned to start when its
particular run level is reached, you ensure an orderly start up process, and
you can quickly change the mode of the machine without worrying about which
services to manually start or stop.
Available run levels are generally described in /etc/inittab, which is
partially shown below:
#
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
# Default run level. The run levels are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS
# (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
id:5:initdefault:
<--cut-->
Feel free to configure unused run levels (commonly run level 4) as you see fit.
Many users configure those run levels in a way that makes the most sense for
them while leaving the standard run levels as they are by default. This allows
them to quickly move in and out of their custom configuration without
disturbing the normal set of features at the standard run levels.
If your machine gets into a state where it will not boot due to a bad /etc/
inittab or will not let you log in because you have a corrupted /etc/passwd
file (or if you have simply forgotten your password), boot into single-user
mode.
No graphics?
When you are working in text mode because you didn't get presented a graphical
login screen on the console of your machine, you can normally switch to console
7 or up to have a graphical login. If this is not the case, check the current
run level using the command who -r. If it is set to something else than the
original default from /etc/inittab, chances are that the system does not start
up in graphical mode by default. Contact your system administrator or read man
init in that case. Note that switching run levels is done preferably using the
telinit
command; switching from a text to a graphical console or vice versa does not
involve a run level switch.
The discussion of run levels, scripts and configurations in this guide tries to
be as general as possible. Lots of variations exist. For instance, Gentoo Linux
stores scripts in /etc/run levels. Other systems might first run through (a)
lower run level(s) and execute all the scripts in there before arriving at the
final run level and executing those scripts. Refer to your system documentation
for more information. You might also read through the scripts that are refered
to in /etc/inittab to get a better comprehension of what happens on your
system.
2.5.1. Tools
The
chkconfig
or
update-rc.d
utilities, when installed on your system, provide a simple command-line tool
for maintaining the
/etc/init.d
directory hierarchy. These relieve system administrators from having to
directly manipulate the numerous symbolic links in the directories under /etc/
rc[x].d.
In addition, some systems offer the
ntsysv
tool, which provides a text-based interface; you may find this easier to use
than chkconfig's command-line interface. On SuSE Linux, you will find the
yast
and
insserv
tools. For Mandrake easy configuration, you may want to try DrakConf, which
allows among other features switching between run levels 3 and 5. In Mandriva
this became the Mandriva Linux Control Center.
Most distributions provide a graphical user interface for configuring
processes, check with your system documentation.
All of these utilities must be run as root. The system administrator may also
manually create the appropriate links in each run level directory in order to
start or stop a service in a certain run level.
2.6. Shutdown
UNIX was not made to be shut down, but if you really must, use the
shutdown
command. After completing the shutdown procedure, the -h option will halt the
system, while -r will reboot it.
The reboot and
halt
commands are now able to invoke shutdown if run when the system is in run
levels 1-5, and thus ensure proper shutdown of the system,but it is a bad habit
to get into, as not all UNIX/Linux versions have this feature.
If your computer does not power itself down, you should not turn off the
computer until you see a message indicating that the system is halted or
finished shutting down, in order to give the system the time to unmount all
partitions. Being impatient may cause data loss.
3. Managing processes
3.1. Work for the system admin
While managing system resources, including processes, is a task for the local
system administrator, it doesn't hurt a common user to know something about it,
especially where his or her own processes and their optimal execution are
concerned.
We will explain a little bit on a theoretical level about system performance,
though not as far as hardware optimization and other advanced procedures.
Instead, we will study the daily problems a common user is confronted with, and
actions such a user can take to optimally use the resources available. As we
learn in the next section, this is mainly a matter of thinking before acting.
Figure 4.2. Can't you go faster?
Man-powered computer: one person works the pedals, one person works with the
comp.
3.2. How long does it take?
Bash offers a built-in
time
command that displays how long a command takes to execute. The timing is
highly accurate and can be used on any command. In the example below, it takes
about a minute and a half to make this book:
tilly:~/xml/src> time make
Output written on abook.pdf (222 pages, 1619861 bytes).
Transcript written on abook.log.
real 1m41.056s
user 1m31.190s
sys 0m1.880s
The GNU time command in /usr/bin (as opposed to the shell built-in version)
displays more information that can be formatted in different ways. It also
shows the exit status of the command, and the total elapsed time. The same
command as the above using the independent time gives this output:
tilly:~/xml/src> /usr/bin/time make
Output written on abook.pdf (222 pages, 1595027 bytes).
Transcript written on abook.log.
Command exited with non-zero status 2
88.87user 1.74system 1:36.21elapsed 94%CPU
(0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (2192major+30002minor)pagefaults 0swaps
Refer again to the Info pages for all the information.
3.3. Performance
To a user, performance means quick execution of commands. To a system manager,
on the other hand, it means much more: the system admin has to optimize system
performance for the whole system, including users, all programs and daemons.
System performance can depend on a thousand tiny things which are not accounted
for with the time command:
* the program executing is badly written or doesn't use the computer
appropriately
* access to disks, controllers, display, all kinds of interfaces, etc.
* reachability of remote systems (network performance)
* amount of users on the system, amount of users actually working
simultaneously
* time of day
* ...
3.4. Load
In short: the load depends on what is normal for your system. My old P133
running a firewall, SSH server, file server, a route daemon, a sendmail server,
a proxy server and some other services doesn't complain with 7 users connected;
the load is still 0 on average. Some (multi-CPU) systems I've seen were quite
happy with a load of 67. There is only one way to find out - check the load
regularly if you want to know what's normal. If you don't, you will only be
able to measure system load from the response time of the command line, which
is a very rough measurement since this speed is influenced by a hundred other
factors.
Keep in mind that different systems will behave different with the same load
average. For example, a system with a graphics card supporting hardware
acceleration will have no problem rendering 3D images, while the same system
with a cheap VGA card will slow down tremendously while rendering. My old P133
will become quite uncomfortable when I start the X server, but on a modern
system you hardly notice the difference in the system load.
3.5. Can I do anything as a user?
A big environment can slow you down. If you have lots of environment variables
set (instead of shell variables), long search paths that are not optimized
(errors in setting the path environment variable) and more of those settings
that are usually made “on the flyâ€, the system will need more time to search
and read data.
In X, window managers and desktop environments can be real CPU-eaters. A really
fancy desktop comes with a price, even when you can download it for free, since
most desktops provide add-ons ad infinitum. Modesty is a virtue if you don't
buy a new computer every year.
3.5.1. Priority
The priority or importance of a job is defined by it's nice number. A program
with a high nice number is friendly to other programs, other users and the
system; it is not an important job. The lower the nice number, the more
important a job is and the more resources it will take without sharing them.
Making a job nicer by increasing its nice number is only useful for processes
that use a lot of CPU time (compilers, math applications and the like).
Processes that always use a lot of I/O time are automatically rewarded by the
system and given a higher priority (a lower nice number), for example keyboard
input always gets highest priority on a system.
Defining the priority of a program is done with the
nice
command.
Most systems also provide the BSD
renice
command, which allows you to change the niceness of a running command. Again,
read the man page for your system-specific information.
Interactive programs
It is NOT a good idea to nice or renice an interactive program or a job running
in the foreground.
Use of these commands is usually a task for the system administrator. Read the
man page for more info on extra functionality available to the system
administrator.
3.5.2. CPU resources
On every Linux system, many programs want to use the CPU(s) at the same time,
even if you are the only user on the system. Every program needs a certain
amount of cycles on the CPU to run. There may be times when there are not
enough cycles because the CPU is too busy. The
uptime
command is wildly inaccurate (it only displays averages, you have to know what
is normal), but far from being useless. There are some actions you can
undertake if you think your CPU is to blame for the unresponsiveness of your
system:
* Run heavy programs when the load is low. This may be the case on your system
during the night. See next section for scheduling.
* Prevent the system from doing unnecessary work: stop daemons and programs
that you don't use, use locate instead of a heavy find, ...
* Run big jobs with a low priority
If none of these solutions are an option in your particular situation, you may
want to upgrade your CPU. On a UNIX machine this is a job for the system admin.
3.5.3. Memory resources
When the currently running processes expect more memory than the system has
physically available, a Linux system will not crash; it will start paging, or
swapping
, meaning the process uses the memory on disk or in swap space, moving contents
of the physical memory (pieces of running programs or entire programs in the
case of swapping) to disk, thus reclaiming the physical memory to handle more
processes. This slows the system down enormously since access to disk is much
slower than access to memory. The
top
command can be used to display memory and swap use. Systems using glibc offer
the memusage and memusagestat commands to visualize memory usage.
If you find that a lot of memory and swap space are being used, you can try:
* Killing, stopping or renicing those programs that use a big chunk of memory
* Adding more memory (and in some cases more swap space) to the system.
* Tuning system performance, which is beyond the scope of this document. See
the reading list in Appendix A,_Where_to_go_from_here? for more.
3.5.4. I/O resources
While I/O limitations are a major cause of stress for system admins, the Linux
system offers rather poor utilities to measure I/O performance. The
ps
,
vmstat
and
top
tools give some indication about how many programs are waiting for I/O;
netstat
displays network interface statistics, but there are virtually no tools
available to measure the I/O response to system load, and the
iostat
command gives a brief overview of general I/O usage. Various graphical front-
ends exist to put the output of these commands in a humanly understandable
form.
Each device has its own problems, but the bandwidth available to network
interfaces and the bandwidth available to disks are the two primary causes of
bottlenecks in I/O performance.
Network I/O problems:
* Network overload:
The amount of data transported over the network is larger than the network's
capacity, resulting in slow execution of every network related task for all
users. They can be solved by cleaning up the network (which mainly involves
disabling protocols and services that you don't need) or by reconfiguring the
network (for example use of subnets, replacing hubs with switches, upgrading
interfaces and equipment).
* Network integrity problems:
Occurs when data is transferred incorrectly. Solving this kind of problem can
only be done by isolating the faulty element and replacing it.
Disk I/O problems:
* per-process transfer rate too low:
Read or write speed for a single process is not sufficient.
* aggregate transfer rate too low:
The maximum total bandwidth that the system can provide to all programs that
run is not enough.
This kind of problem is more difficult to detect, and usually takes extra
hardware in order to re-divide data streams over buses, controllers and disks,
if overloaded hardware is cause of the problem. One solution to solve this is a
RAID array configuration optimized for input and output actions. This way, you
get to keep the same hardware. An upgrade to faster buses, controlers and disks
is usually the other option.
If overload is not the cause, maybe your hardware is gradually failing, or not
well connected to the system. Check contacts, connectors and plugs to start
with.
3.5.5. Users
Users can be divided in several classes, depending on their behavior with
resource usage:
* Users who run a (large) number of small jobs: you, the beginning Linux user,
for instance.
* Users who run relatively few but large jobs: users running simulations,
calculations, emulators or other programs that eat a lot of memory, and
usually these users have accompanying large data files.
* Users who run few jobs but use a lot of CPU time (developers and the like).
You can see that system requirements may vary for each class of users, and that
it can be hard to satisfy everyone. If you are on a multi-user system, it is
useful (and fun) to find out habits of other users and the system, in order to
get the most out of it for your specific purposes.
3.5.6. Graphical tools
For the graphical environment, there are a whole bunch of monitoring tools
available. Below is a screen shot of the Gnome System Monitor, which has
features for displaying and searching process information, and monitoring
system resources:
Figure 4.3. Gnome System Monitor
This GUI shows a graphic of how load and memory usage vary with time.
There are also a couple of handy icons you can install in the task bar, such as
a disk, memory and load monitor.
xload
is another small X application for monitoring system load. Find your favorite!
3.5.7. Interrupting your processes
As a non-privileged user, you can only influence your own processes. We already
saw how you can display processes and filter out processes that belong to a
particular user, and what possible restrictions can occur. When you see that
one of your processes is eating too much of the system's resources, there are
two things that you can do:
1. Make the process use less resources without interrupting it;
2. Stop the process altogether.
In the case that you want the process to continue to run, but you also want to
give the other processes on the system a chance, you can renice the process.
Appart from using the nice or renice commands,
top
is an easy way of spotting the troublesome process(es) and reducing priority.
Identify the process in the “NI†column, it will most likely have a negative
priority. Type r and enter the process ID of the process that you want to
renice. Then enter the nice value, for instance “20â€. That means that from now
on, this process will take 1/5 of the CPU cycles at the most.
Examples of processes that you want to keep on running are emulators, virtual
machines, compilers and so on.
If you want to stop a process because it hangs or is going totally berserk in
the way of I/O consumption, file creation or use of other system resources, use
the kill command. If you have the opportunity, first try to kill the process
softly, sending it the
SIGTERM
signal. This is an instruction to terminate whatever it is doing, according to
procedures as described in the code of the program:
joe:~> ps -ef | grep mozilla
joe 25822 1 0 Mar11 ? 00:34:04 /usr/lib/mozilla-1.4.1/mozilla-
joe:~> kill -15 25822
In the example above, user joe stopped his Mozilla browser because it hung.
Some processes are a little bit harder to get rid of. If you have the time, you
might want to send them the SIGINT signal to interrupt them. If that does not
do the trick either, use the strongest signal, SIGKILL. In the example below,
joe stops a Mozilla that is frozen:
joe:~> ps -ef | grep mozilla
joe 25915 1 0 Mar11 ? 00:15:06 /usr/lib/mozilla-1.4.1/mozilla-
joe:~> kill -9 25915
joe:~> ps -ef | grep 25915
joe 2634 32273 0 18:09 pts/4 00:00:00 grep 25915
In such cases, you might want to check that the process is really dead, using
the grep filter again on the PID. If this only returns the grep process, you
can be sure that you succeeded in stopping the process.
Among processes that are hard to kill is your shell. And that is a good thing:
if they would be easy to kill, you woud loose your shell every time you type
Ctrl-C on the command line accidentally, since this is equivalent to sending a
SIGINT.
UNIX without pipes is almost unthinkable
The usage of pipes (|) for using output of one command as input of another is
explained in the next chapter, Chapter 5,_I/O_redirection.
In a graphical environment, the
xkill
program is very easy to use. Just type the name of the command, followed by an
Enter and select the window of the application that you want to stop. It is
rather dangerous because it sends a SIGKILL by default, so only use it when an
application hangs.
4. Scheduling processes
4.1. Use that idle time!
A Linux system can have a lot to suffer from, but it usually suffers only
during office hours. Whether in an office environment, a server room or at
home, most Linux systems are just idling away during the morning, the evening,
the nights and weekends. Using this idle time can be a lot cheaper than buying
those machines you'd absolutely need if you want everything done at the same
time.
There are three types of delayed execution:
* Waiting a little while and then resuming job execution, using the
sleep
command. Execution time depends on the system time at the moment of
submission.
* Running a command at a specified time, using the at command. Execution of the
job(s) depends on system time, not the time of submission.
* Regularly running a command on a monthly, weekly, daily or hourly basis,
using the
cron
facilities.
The following sections discuss each possibility.
4.2. The sleep command
The Info page on sleep is probably one of the shortest there is. All
sleep
does is wait. By default the time to wait is expressed in seconds.
So why does it exist? Some practical examples:
Somebody calls you on the phone, you say "Yes I'll be with you in half an hour"
but you're about drowned in work as it is and bound to forget your lunch:
(sleep 1800; echo "Lunch time..") &
When you can't use the at command for some reason, it's five o'clock, you want
to go home but there's still work to do and right now somebody is eating system
resources:
(sleep 10000; myprogram) &
Make sure there's an auto-logout on your system, and that you log out or lock
your desktop/office when submitting this kind of job, or run it in a screen
session.
When you run a series of printouts of large files, but you want other users to
be able to print in between:
lp lotoftext; sleep 900; lp hugefile; sleep 900; lp anotherlargefile
Printing files is discussed in Chapter 8,_Printers_and_printing.
Programmers often use the sleep command to halt script or program execution for
a certain time.
4.3. The at command
The at command executes commands at a given time, using your default shell
unless you tell the command otherwise (see the man page).
The options to at are rather user-friendly, which is demonstrated in the
examples below:
steven@home:~> at tomorrow + 2 days
warning: commands will be executed using (in order) a) $SHELL
b) login shell c) /bin/sh
at> cat reports | mail myboss@mycompany
at> <EOT>
job 1 at 2001-06-16 12:36
Typing Ctrl+D quits the at utility and generates the “EOT†message.
User steven does a strange thing here combining two commands; we will study
this sort of practice in Chapter 5,_I/O_redirection, Redirecting Input and
Output.
steven@home:~> at 0237
warning: commands will be executed using (in order) a) $SHELL
b) login shell c) /bin/sh
at> cd new-programs
at> ./configure; make
at> <EOT>
job 2 at 2001-06-14 02:00
The -m option sends mail to the user when the job is done, or explains when a
job can't be done. The command atq lists jobs; perform this command before
submitting jobs in order prevent them from starting at the same time as others.
With the atrm command you can remove scheduled jobs if you change your mind.
It is a good idea to pick strange execution times, because system jobs are
often run at “round†hours, as you can see in Section 4.4,_“Cron_and_crontabâ€
the next section. For example, jobs are often run at exactly 1 o'clock in the
morning (e.g. system indexing to update a standard locate database), so
entering a time of 0100 may easily slow your system down rather than fire it
up. To prevent jobs from running all at the same time, you may also use the
batch
command, which queues processes and feeds the work in the queue to the system
in an evenly balanced way, preventing excessive bursts of system resource
usage. See the Info pages for more information.
4.4. Cron and crontab
The cron system is managed by the cron daemon. It gets information about which
programs and when they should run from the system's and users' crontab entries.
Only the root user has access to the system crontabs, while each user should
only have access to his own crontabs. On some systems (some) users may not have
access to the cron facility.
At system startup the cron daemon searches /var/spool/cron/ for crontab entries
which are named after accounts in /etc/passwd, it searches
/etc/cron.d/
and it searches
/etc/crontab
, then uses this information every minute to check if there is something to be
done. It executes commands as the user who owns the crontab file and mails any
output of commands to the owner.
On systems using Vixie cron, jobs that occur hourly, daily, weekly and monthly
are kept in separate directories in /etc to keep an overview, as opposed to the
standard UNIX cron function, where all tasks are entered into one big file.
Example of a Vixie crontab file:
[root@blob /etc]# more crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# run-parts
# commands to execute every hour
01 * * * * root run-parts /etc/cron.hourly
# commands to execute every day
02 4 * * * root run-parts /etc/cron.daily
# commands to execute every week
22 4 * * 0 root run-parts /etc/cron.weekly
commands to execute every month
42 4 1 * * root run-parts /etc/cron.monthly
Alternative
You could also use the crontab -l command to display crontabs.
Some variables are set, and after that there's the actual scheduling, one line
per job, starting with 5 time and date fields. The first field contains the
minutes (from 0 to 59), the second defines the hour of execution (0-23), the
third is day of the month (1-31), then the number of the month (1-12), the last
is day of the week (0-7, both 0 and 7 are Sunday). An asterisk in these fields
represents the total acceptable range for the field. Lists are allowed; to
execute a job from Monday to Friday enter 1-5 in the last field, to execute a
job on Monday, Wednesday and Friday enter 1,3,5.
Then comes the user who should run the processes which are listed in the last
column. The example above is from a Vixie cron configuration where root runs
the program
run-parts
on regular intervals, with the appropriate directories as options. In these
directories, the actual jobs to be executed at the scheduled time are stored as
shell scripts, like this little script that is run daily to update the database
used by the locate command:
billy@ahost cron.daily]$ cat slocate.cron
#!/bin/sh
renice +19 -p $$ >/dev/null 2>&1
/usr/bin/updatedb -f "nfs,smbfs,ncpfs,proc,devpts" -e \
"/tmp,/var/tmp, /usr/tmp,/afs,/net"
Users are supposed to edit their crontabs in a safe way using the crontab -
e command. This will prevent a user from accidentally opening more than one
copy of his/her crontab file. The default editor is vi (see Chapter 6,_Text
editors, but you can use any text editor, such as gvim or gedit if you feel
more comfortable with a GUI editor.
When you quit, the system will tell you that a new crontab is installed.
This crontab entry reminds billy to go to his sports club every Thursday night:
billy:~> crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.20264 installed on Sun Jul 20 22:35:14 2003)
# (Cron version -- $Id$)
38 16 * * 3 mail -s "sports evening" billy
After adding a new scheduled task, the system will tell you that a new crontab
is installed. You do not need to restart the cron daemon for the changes to
take effect. In the example, billy added a new line pointing to a backup
script:
billy:~> crontab -e
45 15 * * 3 mail -s "sports evening" billy
4 4 * * 4,7 /home/billy/bin/backup.sh
<--write and quit-->
crontab: installing new crontab
billy:~>
The backup.sh script is executed every Thursday and Sunday. See Section 2.5,
“Shell_scripts†for an introduction to shell scripting. Keep in mind that
output of commands, if any, is mailed to the owner of the crontab file. If no
mail service is configured, you might find the output of your commands in your
local mailbox, /var/spool/mail/<your_username>, a plain text file.
Who runs my commands?
You don't have to specify the user who should run the commands. They are
executed with the user's own permissions by default.
5. Summary
Linux is a multi-user, multi-tasking operating system that has a UNIX-like way
of handling processes. Execution speed of commands can depend on a thousand
tiny things. Among others, we learned a lot of new commands to visualize and
handle processes. Here's a list:
Table 4.3. New commands in chapter 4: Processes
____________________________________________________________________________
|Command_______|Meaning______________________________________________________|
|at____________|Queue_jobs_for_later_execution.______________________________|
|atq___________|Lists_the_user's_pending_jobs._______________________________|
|atrm__________|Deletes_jobs,_determined_by_their_job_number.________________|
|batch_________|Executes_commands_when_system_load_level_permits.____________|
|crontab_______|Maintain_crontab_files_for_individual_users._________________|
|halt__________|Stop_the_system._____________________________________________|
|init_run_level|Process_control_initialization.______________________________|
|jobs__________|Lists_currently_executing_jobs.______________________________|
|kill__________|Terminate_a_process._________________________________________|
|mesg__________|Control_write_access_to_your_terminal._______________________|
|netstat |Display network connections, routing tables, interface |
|______________|statistics,_masquerade_connections_and_multicast_memberships.|
|nice__________|Run_a_program_with_modified_scheduling_priority._____________|
|pgrep_________|Display_processes.___________________________________________|
|ps____________|Report_process_status._______________________________________|
|pstree________|Display_a_tree_of_processes._________________________________|
|reboot________|Stop_the_system._____________________________________________|
|renice________|Alter_priority_of_running_processes._________________________|
|shutdown______|Bring_the_system_down._______________________________________|
|sleep_________|Delay_for_a_specified_time.__________________________________|
|time__________|Time_a_command_or_report_resource_usage._____________________|
|top___________|Display_top_CPU_processes.___________________________________|
|uptime________|Show_how_long_the_system_has_been_running.___________________|
|vmstat________|Report_virtual_memory_statistics.____________________________|
|w_____________|Show_who_is_logged_on_and_what_they_are_doing._______________|
|wall__________|Send_a_message_to_everybody's_terminals._____________________|
|who___________|Show_who_is_logged_on._______________________________________|
|write_________|Send_a_message_to_another_user.______________________________|
6. Exercises
These are some exercises that will help you get the feel for processes running
on your system.
6.1. General
* Run top in one terminal while you do the exercises in another.
* Run the ps command.
* Read the man pages to find out how to display all your processes.
* Run the command find /. What effect does it have on system load? Stop this
command.
* In graphical mode, start the xclock program in the foreground. Then let it
run in the background. Stop the program using the kill command.
* Run the xcalc directly in the background, so that the prompt of the issuing
terminal is released.
* What does kill -9 -1 do?
* Open two terminals or terminal windows again and use write to send a message
from one to the other.
* Issue the dmesg command. What does it tell?
* How long does it take to execute ls in the current directory?
* Based on process entries in /proc, owned by your UID, how would you work to
find out which processes these actually represent?
* How long has your system been running?
* Which is your current TTY?
* Name 3 processes that couldn't have had init as an initial parent.
* Name 3 commands which use SUID mode. Explain why this is so.
* Name the commands that are generally causing the highest load on your system.
6.2. Booting, init etc.
* Can you reboot the system as a normal user? Why is that?
* According to your current run level, name the steps that are taken during
shutdown.
* How do you change the system run level? Switch from your default run level to
run level 1 and vice versa.
* Make a list of all the services and daemons that are started up when your
system has booted.
* Which kernel is currently load at startup?
* Suppose you have to start some exotic server at boot time. Up until now, you
logged in after booting the system and started this server manually using a
script named deliver_pizza in your home directory. What do you have to do in
order to have the service start up automatically in run level 4, which you
defined for this purpose only?
6.3. Scheduling
* Use sleep to create a reminder that your pasta is ready in ten minutes.
* Create an at job that copies all files in your home directory to /var/tmp
within half an hour. You may want to create a sub-directory in /var/tmp.
* Make a cronjob that does this task every Monday to Friday during lunch.
* Check that it works.
* Make a mistake in the crontab entry, like issuing the nonexistent command
coppy instead of cp. What happens upon execution of the task?
Chapter 5. I/O redirection
Table of Contents
1._Simple_redirections
1.1._What_are_standard_input_and_standard_output?
1.2._The_redirection_operators
2._Advanced_redirection_features
2.1._Use_of_file_descriptors
2.2._Examples
3._Filters
3.1._More_about_grep
3.2._Filtering_output
4._Summary
5._Exercises
Abstract
This chapter describes more about the powerful UNIX mechanism of redirecting
input, output and errors. Topics include:
* Standard input, output and errors
* Redirection operators
* How to use output of one command as input for another
* How to put output of a command in a file for later referrence
* How to append output of multiple commands to a file
* Input redirection
* Handling standard error messages
* Combining redirection of input, output and error streams
* Output filters
1. Simple redirections
1.1. What are standard input and standard output?
Most Linux commands read input, such as a file or another attribute for the
command, and write output. By default, input is being given with the keyboard,
and output is displayed on your screen. Your keyboard is your standard input
(stdin) device, and the screen or a particular terminal window is the
standard output
(stdout) device.
However, since Linux is a flexible system, these default settings don't
necessarily have to be applied. The standard output, for example, on a heavily
monitored server in a large environment may be a printer.
1.2. The redirection operators
1.2.1. Output redirection with > and |
Sometimes you will want to put output of a command in a file, or you may want
to issue another command on the output of one command. This is known as
redirecting output. Redirection is done using either the “>†(greater-than
symbol), or using the “|†(pipe) operator which sends the standard output of
one command to another command as standard input.
As we saw before, the cat command concatenates files and puts them all together
to the standard output. By redirecting this output to a file, this file name
will be created - or overwritten if it already exists, so take care.
nancy:~> cat test1
some words
nancy:~> cat test2
some other words
nancy:~> cat test1 test2 > test3
nancy:~> cat test3
some words
some other words
Don't overwrite!
Be careful not to overwrite existing (important) files when redirecting output.
Many shells, including Bash, have a built-in feature to protect you from that
risk:
noclobber
. See the Info pages for more information. In Bash, you would want to add the
set -o
noclobber
command to your
.bashrc
configuration file in order to prevent accidental overwriting of files.
Redirecting “nothing†to an existing file is equal to emptying the file:
nancy:~> ls -l list
-rw-rw-r-- 1 nancy nancy 117 Apr 2 18:09 list
nancy:~> > list
nancy:~> ls -l list
-rw-rw-r-- 1 nancy nancy 0 Apr 4 12:01 list
This process is called
truncating
.
The same redirection to an nonexistent file will create a new empty file with
the given name:
nancy:~> ls -l newlist
ls: newlist: No such file or directory
nancy:~> > newlist
nancy:~> ls -l newlist
-rw-rw-r-- 1 nancy nancy 0 Apr 4 12:05 newlist
Chapter 7,_Home_sweet_/home gives some more examples on the use of this sort of
redirection.
Some examples using piping of commands:
To find a word within some text, display all lines matching “pattern1â€, and
exclude lines also matching “pattern2†from being displayed:
grep pattern1 file | grep -v pattern2
To display output of a directory listing one page at a time:
ls -la | less
To find a file in a directory:
ls -l | grep part_of_file_name
1.2.2. Input redirection
In another case, you may want a file to be the input for a command that
normally wouldn't accept a file as an option. This redirecting of input is done
using the “<†(less-than symbol) operator.
Below is an example of sending a file to somebody, using input redirection.
andy:~> mail mike@somewhere.org < to_do
If the user mike exists on the system, you don't need to type the full address.
If you want to reach somebody on the Internet, enter the fully qualified
address as an argument to mail.
This reads a bit more difficult than the beginner's cat file | mail someone,
but it is of course a much more elegant way of using the available tools.
1.2.3. Combining redirections
The following example combines input and output redirection. The file text.txt
is first checked for spelling mistakes, and the output is redirected to an
error log file:
spell < text.txt > error.log
The following command lists all commands that you can issue to examine another
file when using less:
mike:~> less --help | grep -i examine
:e [file] Examine a new file.
:n * Examine the (N-th) next file from the command line.
:p * Examine the (N-th) previous file from the command line.
:x * Examine the first (or N-th) file from the command line.
The -i option is used for case-insensitive searches - remember that UNIX
systems are very case-sensitive.
If you want to save output of this command for future reference, redirect the
output to a file:
mike:~> less --help | grep -i examine > examine-files-in-less
mike:~> cat examine-files-in-less
:e [file] Examine a new file.
:n * Examine the (N-th) next file from the command line.
:p * Examine the (N-th) previous file from the command line.
:x * Examine the first (or N-th) file from the command line.
Output of one command can be piped into another command virtually as many times
as you want, just as long as these commands would normally read input from
standard input and write output to the standard output. Sometimes they don't,
but then there may be special options that instruct these commands to behave
according to the standard definitions; so read the documentation (man and Info
pages) of the commands you use if you should encounter errors.
Again, make sure you don't use names of existing files that you still need.
Redirecting output to existing files will replace the content of those files.
1.2.4. The >> operator
Instead of overwriting file data, you can also append text to an existing file
using two subsequent greater-than signs:
Example:
mike:~> cat wishlist
more money
less work
mike:~> date >> wishlist
mike:~> cat wishlist
more money
less work
Thu Feb 28 20:23:07 CET 2002
The date command would normally put the last line on the screen; now it is
appended to the file wishlist.
2. Advanced redirection features
2.1. Use of file descriptors
There are three types of I/O, which each have their own identifier, called a
file descriptor:
* standard input: 0
* standard output: 1
* standard error: 2
In the following descriptions, if the file descriptor number is omitted, and
the first character of the redirection operator is <, the redirection refers to
the standard input (file descriptor 0). If the first character of the
redirection operator is >, the redirection refers to the standard output (file
descriptor 1).
Some practical examples will make this more clear:
ls > dirlist 2>&1
will direct both standard output and standard error to the file dirlist, while
the command
ls 2>&1 > dirlist
will only direct standard output to dirlist. This can be a useful option for
programmers.
Things are getting quite complicated here, don't confuse the use of the
ampersand here with the use of it in Section 1.2.1,_“Interactive_processesâ€,
where the ampersand is used to run a process in the background. Here, it merely
serves as an indication that the number that follows is not a file name, but
rather a location that the data stream is pointed to. Also note that the
bigger-than sign should not be separated by spaces from the number of the file
descriptor. If it would be separated, we would be pointing the output to a file
again. The example below demonstrates this:
[nancy@asus /var/tmp]$ ls 2> tmp
[nancy@asus /var/tmp]$ ls -l tmp
-rw-rw-r-- 1 nancy nancy 0 Sept 7 12:58 tmp
[nancy@asus /var/tmp]$ ls 2 > tmp
ls: 2: No such file or directory
The first command that nancy executes is correct (eventhough no errors are
generated and thus the file to which standard error is redirected is empty).
The second command expects that 2 is a file name, which does not exist in this
case, so an error is displayed.
All these features are explained in detail in the Bash Info pages.
2.2. Examples
2.2.1. Analyzing errors
If your process generates a lot of errors, this is a way to thoroughly examine
them:
command 2>&1 | less
This is often used when creating new software using the make command, such as
in:
andy:~/newsoft> make all 2>&1 | less
--output ommitted--
2.2.2. Separating standard output from standard error
Constructs like these are often used by programmers, so that output is
displayed in one terminal window, and errors in another. Find out which pseudo
terminal you are using issuing the tty command first:
andy:~/newsoft> make all 2> /dev/pts/7
2.2.3. Writing to output and files simultaneously
You can use the
tee
command to copy input to standard output and one or more output files in one
move. Using the -a option to tee results in appending input to the file(s).
This command is useful if you want to both see and save output. The > and >>
operators do not allow to perform both actions simultaneously.
This tool is usually called on through a pipe (|), as demonstrated in the
example below:
mireille ~/test> date | tee file1 file2
Thu Jun 10 11:10:34 CEST 2004
mireille ~/test> cat file1
Thu Jun 10 11:10:34 CEST 2004
mireille ~/test> cat file2
Thu Jun 10 11:10:34 CEST 2004
mireille ~/test> uptime | tee -a file2
11:10:51 up 21 days, 21:21, 57 users, load average: 0.04, 0.16, 0.26
mireille ~/test> cat file2
Thu Jun 10 11:10:34 CEST 2004
11:10:51 up 21 days, 21:21, 57 users, load average: 0.04, 0.16, 0.26
3. Filters
When a program performs operations on input and writes the result to the
standard output, it is called a filter. One of the most common uses of filters
is to restructure output. We'll discuss a couple of the most important filters
below.
3.1. More about grep
As we saw in Section 3.3.4,_“The_grep_commandâ€,
grep
scans the output line per line, searching for matching patterns. All lines
containing the pattern will be printed to standard output. This behavior can be
reversed using the -v option.
Some examples: suppose we want to know which files in a certain directory have
been modified in February:
jenny:~> ls -la | grep Feb
The grep command, like most commands, is case sensitive. Use the -i option to
make no difference between upper and lower case. A lot of GNU extensions are
available as well, such as --colour, which is helpful to highlight searchterms
in long lines, and --after-context, which prints the number of lines after the
last matching line. You can issue a recursive grep that searches all
subdirectories of encountered directories using the -r option. As usual,
options can be combined.
Regular expressions can be used to further detail the exact character matches
you want to select out of all the input lines. The best way to start with
regular expressions is indeed to read the grep documentation. An excellent
chapter is included in the grep Info page. Since it would lead us too far
discussing the ins and outs of regular expressions, it is strongly advised to
start here if you want to know more about them.
Play around a bit with grep, it will be worth the trouble putting some time in
this most basic but very powerful filtering command. The exercises at the end
of this chapter will help you to get started, see Section 5,_“Exercisesâ€.
3.2. Filtering output
The command
sort
arranges lines in alphabetical order by default:
thomas:~> cat people-I-like | sort
Auntie Emmy
Boyfriend
Dad
Grandma
Mum
My boss
But there are many more things sort can do. Looking at the file size, for
instance. With this command, directory content is sorted smallest files first,
biggest files last:
ls -la | sort -nk 5
Old sort syntax
You might obtain the same result with ls -la | sort +4n, but this is an old
form which does not comply with the current standards.
The sort command is also used in combination with the uniq program (or sort -u)
to sort output and filter out double entries:
thomas:~> cat itemlist
1
4
2
5
34
567
432
567
34
555
thomas:~> sort itemlist | uniq
1
2
34
4
432
5
555
567
4. Summary
In this chapter we learned how commands can be linked to each other, and how
input from one command can be used as output for another command.
Input/output redirection is a common task on UNIX and Linux machines. This
powerful mechanism allows flexible use of the building blocks UNIX is made of.
The most commonly used redirections are > and |. Refer to Appendix C,_Shell
Features for an overview of redirection commands and other shell constructs.
Table 5.1. New commands in chapter 5: I/O redirection
__________________________________________________
|Command|Meaning___________________________________|
|date___|Display_time_and_date_information.________|
|set____|Configure_shell_options.__________________|
|sort___|Sort_lines_of_text._______________________|
|uniq___|Remove_duplicate_lines_from_a_sorted_file.|
5. Exercises
These exercises give more examples on how to combine commands. The main goal is
to try and use the Enter key as little as possible.
All exercises are done using a normal user ID, so as to generate some errors.
While you're at it, don't forget to read those man pages!
* Use the cut command on the output of a long directory listing in order to
display only the file permissions. Then pipe this output to sort and uniq to
filter out any double lines. Then use the wc to count the different
permission types in this directory.
* Put the output of date in a file. Append the output of ls to this file. Send
this file to your local mailbox (don't specify anything <@domain>, just the
user name will do). When using Bash, you will see a new mail notice upon
success.
* List the devices in /dev which are currently used by your UID. Pipe through
less to view them properly.
* Issue the following commands as a non-privileged user. Determine standard
input, output and error for each command.
o cat nonexistentfile
o file /sbin/ifconfig
o grep root /etc/passwd /etc/nofiles > grepresults
o /etc/init.d/sshd start > /var/tmp/output
o /etc/init.d/crond start > /var/tmp/output 2>&1
Now check your results by issuing the commands again, now redirecting
standardoutput to the file /var/tmp/output and standard error to the file /
var/tmp/error.
* How many processes are you currently running?
* How many invisible files are in your home directory?
* Use locate to find documentation about the kernel.
* Find out which file contains the following entry:
root:x:0:0:root:/root:/bin/bash
And this one:
system: root
* See what happens upon issuing this command:
> time; date >> time; cat < time
* What command would you use to check which script in /etc/init.d starts a
given process?
Chapter 6. Text editors
Table of Contents
1._Text_editors
1.1._Why_should_I_use_an_editor?
1.2._Which_editor_should_I_use?
2._Using_the_Vim_editor
2.1._Two_modes
2.2._Basic_commands
2.3._The_easy_way
3._Linux_in_the_office
3.1._History
3.2._Suites_and_programs
3.3._Remarks
4._Summary
5._Exercises
Abstract
In this chapter, we will discuss the importance of mastering an editor. We will
focus mainly on the Improved vi editor.
After finishing this chapter, you will be able to:
* Open and close files in text mode
* Edit files
* Search text
* Undo errors
* Merge files
* Recover lost files
* Find a program or suite for office use
1. Text editors
1.1. Why should I use an editor?
It is very important to be able to use at least one text mode editor. Knowing
how to use an editor on your system is the first step to independence.
We will need to master an editor by the next chapter as we need it to edit
files that influence our environment. As an advanced user, you may want to
start writing scripts, or books, develop websites or new programs. Mastering an
editor will immensely improve your productivity as well as your capabilities.
1.2. Which editor should I use?
Our focus is on text editors, which can also be used on systems without a
graphical environment and in terminal windows. The additional advantage of
mastering a text editor is in using it on remote machines. Since you don't need
to transfer the entire graphical environment over the network, working with
text editors tremendously improves network speed.
There are, as usual, multiple ways to handle the problem. Let's see what
editors are commonly available:
1.2.1. GNU Emacs
Emacs is the extensible, customizable, self-documenting, real-time display
editor, known on many UNIX and other systems. The text being edited is visible
on the screen and is updated automatically as you type your commands. It is a
real-time editor because the display is updated very frequently, usually after
each character or pair of characters you type. This minimizes the amount of
information you must keep in your head as you edit. Emacs is called advanced
because it provides facilities that go beyond simple insertion and deletion:
controlling subprocesses; automatic indentation of programs; viewing two or
more files at once; editing formatted text; and dealing in terms of characters,
words, lines, sentences, paragraphs, and pages, as well as expressions and
comments in several different programming languages.
Self-documenting means that at any time you can type a special character,
Ctrl+H, to find out what your options are. You can also use it to find out what
any command does, or to find all the commands that pertain to a topic.
Customizable means that you can change the definitions of Emacs commands in
little ways. For example, if you use a programming language in which comments
start with “<**†and end with “**>â€, you can tell the Emacs comment
manipulation commands to use those strings. Another sort of customization is
rearrangement of the command set. For example, if you prefer the four basic
cursor motion commands (up, down, left and right) on keys in a diamond pattern
on the keyboard, you can rebind the keys that way.
Extensible means that you can go beyond simple customization and write entirely
new commands, programs in the Lisp language that are run by Emacs's own Lisp
interpreter. Emacs is an online extensible system, which means that it is
divided into many functions that call each other, any of which can be redefined
in the middle of an editing session. Almost any part of Emacs can be replaced
without making a separate copy of all of Emacs. Most of the editing commands of
Emacs are written in Lisp already; the few exceptions could have been written
in Lisp but are written in C for efficiency. Although only a programmer can
write an extension, anybody can use it afterward.
When run under the X Window System (started as xemacs) Emacs provides its own
menus and convenient bindings to mouse buttons. But Emacs can provide many of
the benefits of a window system on a text-only terminal. For instance, you can
look at or edit several files at once, move text between files, and edit files
while running shell commands.
1.2.2. Vi(m)
Vim stands for “Vi IMprovedâ€. It used to be “Vi IMitationâ€, but there are so
many improvements that a name change was appropriate. Vim is a text editor
which includes almost all the commands from the UNIX program vi and a lot of
new ones.
Commands in the
vi
editor are entered using only the keyboard, which has the advantage that you
can keep your fingers on the keyboard and your eyes on the screen, rather than
moving your arm repeatedly to the mouse. For those who want it, mouse support
and a GUI version with scrollbars and menus can be activated.
We will refer to vi or vim throughout this book for editing files, while you
are of course free to use the editor of your choice. However, we recommend to
at least get the vi basics in the fingers, because it is the standard text
editor on almost all UNIX systems, while emacs can be an optional package.
There may be small differences between different computers and terminals, but
the main point is that if you can work with vi, you can survive on any UNIX
system.
Apart from the vim command, the vIm packages may also provide
gvim
, the Gnome version of vim. Beginning users might find this easier to use,
because the menus offer help when you forgot or don't know how to perform a
particular editing task using the standard vim commands.
2. Using the Vim editor
2.1. Two modes
The vi editor is a very powerful tool and has a very extensive built-in manual,
which you can activate using the :help command when the program is started
(instead of using man or info, which don't contain nearly as much information).
We will only discuss the very basics here to get you started.
What makes vi confusing to the beginner is that it can operate in two modes:
command mode and insert mode. The editor always starts in command mode.
Commands move you through the text, search, replace, mark blocks and perform
other editing tasks, and some of them switch the editor to insert mode.
This means that each key has not one, but likely two meanings: it can either
represent a command for the editor when in command mode, or a character that
you want in a text when in insert mode.
Pronunciation
It's pronounced “vee-eyeâ€.
2.2. Basic commands
2.2.1. Moving through the text
Moving through the text is usually possible with the arrow keys. If not, try:
* h to move the cursor to the left
* l to move it to the right
* k to move up
* j to move down
SHIFT-G will put the prompt at the end of the document.
2.2.2. Basic operations
These are some popular vi commands:
* n dd will delete n lines starting from the current cursor position.
* n dw will delete n words at the right side of the cursor.
* x will delete the character on which the cursor is positioned
* :n moves to line n of the file.
* :w will save (write) the file
* :q will exit the editor.
* :q! forces the exit when you want to quit a file containing unsaved changes.
* :wq will save and exit
* :w newfile will save the text to newfile.
* :wq! overrides read-only permission (if you have the permission to override
permissions, for instance when you are using the root account.
* /astring will search the string in the file and position the cursor on the
first match below its position.
* / will perform the same search again, moving the cursor to the next match.
* :1, $s/word/anotherword/g will replace word with anotherword throughout the
file.
* yy will copy a block of text.
* n p will paste it n times.
* :recover will recover a file after an unexpected interruption.
2.2.3. Commands that switch the editor to insert mode
* a will append: it moves the cursor one position to the right before switching
to insert mode
* i will insert
* o will insert a blank line under the current cursor position and move the
cursor to that line.
Pressing the Esc key switches back to command mode. If you're not sure what
mode you're in because you use a really old version of vi that doesn't display
an “INSERT†message, type Esc and you'll be sure to return to command mode. It
is possible that the system gives a little alert when you are already in
command mode when hitting Esc, by beeping or giving a visual bell (a flash on
the screen). This is normal behavior.
2.3. The easy way
Instead of reading the text, which is quite boring, you can use the vimtutor to
learn you first Vim commands. This is a thirty minute tutorial that teaches the
most basic Vim functionality in eight easy exercises. While you can't learn
everything about vim in just half an hour, the tutor is designed to describe
enough of the commands that you will be able to easily use Vim as an all-
purpose editor.
In UNIX and MS Windows, if Vim has been properly installed, you can start this
program from the shell or command line, entering the
vimtutor
command. This will make a copy of the tutor file, so that you can edit it
without the risk of damaging the original. There are a few translated versions
of the tutor. To find out if yours is available, use the two-letter language
code. For French this would be vimtutor fr (if installed on the system).
3. Linux in the office
3.1. History
Throughout the last decade the office domain has typically been dominated by MS
Office, and, let's face it: the Microsoft Word, Excel and PowerPoint formats
are industry standards that you will have to deal with sooner or later.
This monopoly situation of Microsoft proved to be a big disadvantage for
getting new users to Linux, so a group of German developers started the
StarOffice project, that was, and is still, aimed at making an MS Office clone.
Their company, StarDivision, was acquired by Sun Microsystems by the end of the
1990s, just before the 5.2 release. Sun continues development but restricted
access to the sources. Nevertheless, development on the original set of sources
continues in the Open Source community, which had to rename the project to
OpenOffice. OpenOffice is now available for a variety of platforms, including
MS Windows, Linux, MacOS and Solaris. There is a screenshot in Section 3.2,
“Ten_years_of_experience_at_your_serviceâ€.
Almost simultaneously, a couple of other quite famous projects took off. Also a
very common alternative to using MS Office is KOffice, the office suite that
used to be popular among SuSE users. Like the original, this clone incorporates
an MS Word and Excel compatible program, and much more.
Smaller projects deal with particular programs of the MS example suite, such as
Abiword and MS Wordview for compatibility with MS Word documents, and Gnumeric
for viewing and creating Excel compatible spreadsheets.
3.2. Suites and programs
Current distributions usually come with all the necessary tools. Since these
provide excellent guidelines and searchable indexes in the Help menus, we won't
discuss them in detail. For references, see you system documentation or the web
sites of the projects, such as
* http://www.openoffice.org
* http://www.koffice.org
* Freshmeat and SourceForge for various other projects.
3.3. Remarks
3.3.1. General use of office documents
Try to limit the use of office documents for the purposes they were meant for:
the office.
An example: it drives most Linux users crazy if you send them a mail that says
in the body something like: “Hello, I want to tell you something, see attachâ€,
and then the attachement proves to be an MS Word compatible document like:
“Hello my friend, how is your new job going and will you have time to have
lunch with me tomorrow?†Also a bad idea is the attachment of your signature in
such a file, for instance. If you want to sign messages or files, use GPG, the
PGP-compatible GNU Privacy Guard or SSL (Secure Socket Layer) certificates.
These users are not annoyed because they are unable to read these documents, or
because they are worried that these formats typically generate much larger
files, but rather because of the implication that they are using MS Windows,
and possibly because of the extra work of starting some additional programs.
3.3.2. System and user configuration files
In the next chapter, we start configuring our environment, and this might
include editing all kinds of files that determine how a program behave.
Don't edit these files with any office component!
The default file format specification would make the program add several lines
of code, defining the format of the file and the fonts used. These lines won't
be interpreted in the correct way by the programs depending on them, resulting
in errors or a crash of the program reading the file. In some cases, you can
save the file as plain text, but you'll run into trouble when making this a
habit.
3.3.3. But I want a graphical text editor!
If you really insist, try gedit, kedit, kwrite or xedit; these programs only do
text files, which is what we will be needing. If you plan on doing anything
serious, though, stick to a real text mode editor such as vim or emacs.
An acceptable alternative is gvim, the Gnome version of vim. You still need to
use vi commands, but if you are stuck, you can look them up in the menus.
4. Summary
In this chapter we learned to use an editor. While it depends on your own
individual preference which one you use, it is necessary to at least know how
to use one editor.
The vi editor is available on every UNIX system.
Most Linux distributions include an office suite and a graphical text editor.
5. Exercises
This chapter has only one exercise: start the Vim tutor by entering vimtutor in
a terminal session, and get started.
You may alternatively start emacs and type Ctrl+H and then T to invoke the
self-paced Emacs tutorial.
Practice is the only way!
Chapter 7. Home sweet /home
Table of Contents
1._General_good_housekeeping
1.1._Introduction
1.2._Make_space
2._Your_text_environment
2.1._Environment_variables
2.2._Shell_setup_files
2.3._A_typical_set_of_setup_files
2.4._The_Bash_prompt
2.5._Shell_scripts
3._The_graphical_environment
3.1._Introduction
3.2._The_X_Window_System
3.3._X_server_configuration
4._Region_specific_settings
4.1._Keyboard_setup
4.2._Fonts
4.3._Date_and_time_zone
4.4._Language
4.5._Country-specific_Information
5._Installing_new_software
5.1._General
5.2._Package_formats
5.3._Automating_package_management_and_updates
5.4._Upgrading_your_kernel
5.5._Installing_extra_packages_from_the_installation_CDs
6._Summary
7._Exercises
7.1._Shell_environment
7.2._Graphical_environment
Abstract
This chapter is about configuring your environment. Now that we know how to use
an editor, we can change all kinds of files to make ourselves feel better at
home. After completing this chapter, you will know more about:
* Organizing your environment
* Common shell setup files
* Shell configuration
* Configuring the prompt
* Configuring the graphical environment
* Sound and video applications
* Display and window managers
* How the X client-server system works
* Language and font settings
* Installing new software
* Updating existing packages
1. General good housekeeping
1.1. Introduction
As we mentioned before, it is easy enough to make a mess of the system. We
can't put enough stress on the importance of keeping the place tidy. When you
learn this from the start, it will become a good habit that will save you time
when programming on a Linux or UNIX system or when confronted with system
management tasks. Here are some ways of making life easier on yourself:
* Make a bin directory for your program files and scripts.
* Organize non-executable files in appropriate directories, and make as many
directories as you like. Examples include separate directories for images,
documents, projects, downloaded files, spreadsheets, personal files, and so
on.
* Make directories private with the chmod 700 dirname command.
* Give your files sensible names, such as Complaint to the prime minister
050302 rather than letter1.
1.2. Make space
On some systems, the quota system may force you to clean up from time to time,
or the physical limits of your hard disk may force you to make more space
without running any monitoring programs. This section discusses a number of
ways, besides using the rm command, to reclaim disk space.
Run the quota -v command to see how much space is left.
1.2.1. Emptying files
Sometimes the content of a file doesn't interest you, but you need the file
name as a marker (for instance, you just need the timestamp of a file, a
reminder that the file was there or should be there some time in the future).
Redirecting the output of a null command is how this is done in the Bourne and
Bash shells:
andy:~> cat wishlist > placeholder
andy:~> ls -la placeholder
-rw-rw-r-- 1 andy andy 200 Jun 12 13:34 placeholder
andy:~> > placeholder
andy:~> ls -la placeholder
-rw-rw-r-- 1 andy andy 0 Jun 12 13:35 placeholder
The process of reducing an existing file to a file with the same name that is 0
bytes large is called
truncating
.
For creating a new empty file, the same effect is obtained with the
touch
command. On an existing file, touch will only update the timestamp. See the
Info pages on touch for more details.
To “almost†empty a file, use the
tail
command. Suppose user andy's wishlist becomes rather long because he always
adds stuff at the end but never deletes the things he actually gets. Now he
only wants to keep the last five items:
andy:~> tail -5 wishlist > newlist
andy:~> cat newlist > wishlist
andy:~> rm newlist
1.2.2. More about log files
Some Linux programs insist on writing all sorts of output in a log file.
Usually there are options to only log errors, or to log a minimal amount of
information, for example setting the debugging level of the program. But even
then, you might not care about the log file. Here are some ways to get rid of
them or at least set some limits to their size:
* Try removing the log file when the program is not running, if you are sure
that you won't need it again. Some programs may even see, when restarted,
that there is no log file and will therefore not log.
* If you remove the log file and the program recreates it, read the
documentation for this particular program in search for command options that
avoid making log files.
* Try making smaller log files by logging only the information that is relevant
to you, or by logging only significant information.
* Try replacing the log file with a symbolic link to /dev/null; if you're lucky
the program won't complain. Don't do this with the log files of programs that
run at system boot or programs that run from cron (see Chapter 4,_Processes).
These programs might replace the symbolic link with a small file that starts
growing again.
1.2.3. Mail
Regularly clean out your mailbox, make sub-folders and automatic redirects
using procmail (see the Info pages) or the filters of your favorite mail
reading application. If you have a trash folder, clean it out on a regular
basis.
To redirect mail, use the
.forward
file in your home directory. The Linux mail service looks for this file
whenever it has to deliver local mail. The content of the file defines what the
mail system should do with your mail. It can contain a single line holding a
fully qualified E-mail address. In that case the system will send all your mail
to this address. For instance, when renting space for a website, you might want
to forward the mail destined for the webmaster to your own account in order not
to waste disk space. The webmaster's .forward may look like this:
webmaster@www ~/> cat .forward
mike@pandora.be
Using mail forwarding is also useful to prevent yourself from having to check
several different mailboxes. You can make every address point to a central and
easily accessible account.
You can ask your system administrator to define a forward for you in the local
mail aliases file, like when an account is being closed but E-mail remains
active for a while.
1.2.4. Save space with a link
When several users need access to the same file or program, when the original
file name is too long or too difficult to remember, use a symbolic link instead
of a separate copy for each user or purpose.
Multiple symbolic links may have different names, e.g. a link may be called
monfichier in one user's directory, and mylink in another's. Multiple links
(different names) to the same file may also occur in the same directory. This
is often done in the /lib directory: when issuing the command
ls -l /lib
you will see that this directory is plenty of links pointing to the same files.
These are created so that programs searching for one name would not get stuck,
so they are pointed to the correct/current name of the libraries they need.
1.2.5. Limit file sizes
The shell contains a built-in command to limit file sizes,
ulimit
, which can also be used to display limitations on system resources:
cindy:~> ulimit -a
core file size (blocks) 0
data seg size (kbytes) unlimited
file size (blocks) unlimited
max locked memory (kbytes) unlimited
max memory size (kbytes) unlimited
open files 1024
pipe size (512 bytes) 8
stack size (kbytes) 8192
cpu time (seconds) unlimited
max user processes 512
virtual memory (kbytes) unlimited
Cindy is not a developer and doesn't care about core dumps, which contain
debugging information on a program. If you do want core dumps, you can set
their size using the ulimit command. Read the Info pages on bash for a detailed
explanation.
Core file?
A core file or core dump is sometimes generated when things go wrong with a
program during its execution. The core file contains a copy of the system's
memory, as it was at the time that the error occured.
1.2.6. Compressed files
Compressed files are useful because they take less space on your hard disk.
Another advantage is that it takes less bandwidth to send a compressed file
over your network. A lot of files, such as the man pages, are stored in a
compressed format on your system. Yet unpacking these to get a little bit of
information and then having to compress them again is rather time-consuming.
You don't want to unpack a man page, for instance, read about an option to a
command and then compress the man page again. Most people will probably forget
to clean up after they found the information they needed.
So we have tools that work on compressed files, by uncompressing them only in
memory. The actual compressed file stays on your disk as it is. Most systems
support zgrep, zcat, bzless and other members of the z-family to prevent
unnecessary decompressing/compressing actions. See your system's binary
directory and the Info pages.
See Chapter 9,_Fundamental_Backup_Techniques for more on the actual compressing
of files and examples on making archives.
2. Your text environment
2.1. Environment variables
2.1.1. General
We already mentioned a couple of environment variables, such as PATH and HOME.
Until now, we only saw examples in which they serve a certain purpose to the
shell. But there are many other Linux utilities that need information about you
in order to do a good job.
What other information do programs need apart from paths and home directories?
A lot of programs want to know about the kind of terminal you are using; this
information is stored in the
TERM
variable. In text mode, this will be the linux terminal emulation, in
graphical mode you are likely to use xterm. Lots of programs want to know what
your favorite editor is, in case they have to start an editor in a subprocess.
The shell you are using is stored in the
SHELL
variable, the operating system type in
OS
and so on. A list of all variables currently defined for your session can be
viewed entering the
printenv
command.
The environment variables are managed by the shell. As opposed to regular shell
variables, environment variables are inherited by any program you start,
including another shell. New processes are assigned a copy of these variables,
which they can read, modify and pass on in turn to their own child processes.
There is nothing special about variable names, except that the common ones are
in upper case characters by convention. You may come up with any name you want,
although there are standard variables that are important enough to be the same
on every Linux system, such as PATH and HOME.
2.1.2. Exporting variables
An individual variable's content is usually displayed using the
echo
command, as in these examples:
debby:~> echo $PATH
/usr/bin:/usr/sbin:/bin:/sbin:/usr/X11R6/bin:/usr/local/bin
debby:~> echo $MANPATH
/usr/man:/usr/share/man/:/usr/local/man:/usr/X11R6/man
If you want to change the content of a variable in a way that is useful to
other programs, you have to export the new value from your environment into the
environment that runs these programs. A common example is exporting the PATH
variable. You may declare it as follows, in order to be able to play with the
flight simulator software that is in /opt/FlightGear/bin:
debby:~> PATH=$PATH:/opt/FlightGear/bin
This instructs the shell to not only search programs in the current path,
$PATH, but also in the additional directory /opt/FlightGear/bin.
However, as long as the new value of the PATH variable is not known to the
environment, things will still not work:
debby:~> runfgfs
bash: runfgfs: command not found
Exporting variables is done using the shell built-in command
export
:
debby:~> export PATH
debby:~> runfgfs
--flight simulator starts--
In Bash, we normally do this in one elegant step:
export VARIABLE=value
The same technique is used for the
MANPATH
variable, that tells the man command where to look for compressed man pages.
If new software is added to the system in new or unusual directories, the
documentation for it will probably also be in an unusual directory. If you want
to read the man pages for the new software, extend the MANPATH variable:
debby:~> export MANPATH=$MANPATH:/opt/FlightGear/man
debby:~> echo $MANPATH
/usr/man:/usr/share/man:/usr/local/man:/usr/X11R6/man:/opt/FlightGear/man
You can avoid retyping this command in every window you open by adding it to
one of your shell setup files, see Section 2.2,_“Shell_setup_filesâ€.
2.1.3. Reserved variables
The following table gives an overview of the most common predefined variables:
Table 7.1. Common environment variables
__________________________________________________________________________
|Variable_name__|Stored_information________________________________________|
|DISPLAY________|used_by_the_X_Window_system_to_identify_the_display_server|
|DOMAIN_________|domain_name_______________________________________________|
|EDITOR_________|stores_your_favorite_line_editor__________________________|
|HISTSIZE_______|size_of_the_shell_history_file_in_number_of_lines_________|
|HOME___________|path_to_your_home_directory_______________________________|
|HOSTNAME_______|local_host_name___________________________________________|
|INPUTRC |location of definition file for input devices such as |
|_______________|keyboard__________________________________________________|
|LANG___________|preferred_language________________________________________|
|LD_LIBRARY_PATH|paths_to_search_for_libraries_____________________________|
|LOGNAME________|login_name________________________________________________|
|MAIL___________|location_of_your_incoming_mail_folder_____________________|
|MANPATH________|paths_to_search_for_man_pages_____________________________|
|OS_____________|string_describing_the_operating_system____________________|
|OSTYPE_________|more_information_about_version_etc._______________________|
|PAGER |used by programs like man which need to know what to do in|
|_______________|case_output_is_more_than_one_terminal_window._____________|
|PATH___________|search_paths_for_commands_________________________________|
|PS1____________|primary_prompt____________________________________________|
|PS2____________|secondary_prompt__________________________________________|
|PWD____________|present_working_directory_________________________________|
|SHELL__________|current_shell_____________________________________________|
|TERM___________|terminal_type_____________________________________________|
|UID____________|user_ID___________________________________________________|
|USER(NAME)_____|user_name_________________________________________________|
|VISUAL_________|your_favorite_full-screen_editor__________________________|
|XENVIRONMENT___|location_of_your_personal_settings_for_X_behavior_________|
|XFILESEARCHPATH|paths_to_search_for_graphical_libraries___________________|
A lot of variables are not only predefined but also preset, using configuration
files. We discuss these in the next section.
2.2. Shell setup files
When entering the ls -al command to get a long listing of all files, including
the ones starting with a dot, in your home directory, you will see one or more
files starting with a . and ending in rc. For the case of bash, this is
.bashrc
. This is the counterpart of the system-wide configuration file
/etc/bashrc
.
When logging into an interactive login shell, login will do the authentication,
set the environment and start your shell. In the case of bash, the next step is
reading the general
profile
from /etc, if that file exists. bash then looks for
~/.bash_profile
,
~/.bash_login
and
~/.profile
, in that order, and reads and executes commands from the first one that exists
and is readable. If none exists, /etc/bashrc is applied.
When a login shell exits, bash reads and executes commands from the file
~/.bash_logout
, if it exists.
This procedure is explained in detail in the
login
and bash man pages.
2.3. A typical set of setup files
2.3.1. /etc/profile example
Let's look at some of these config files. First /etc/profile is read, in which
important variables such as PATH, USER and HOSTNAME are set:
debby:~> cat /etc/profile
# /etc/profile
# System wide environment and startup programs, for login setup
# Functions and aliases go in /etc/bashrc
# Path manipulation
if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/sbin" ; then
PATH=/sbin:$PATH
fi
if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/usr/sbin" ; then
PATH=/usr/sbin:$PATH
fi
if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/usr/local/sbin"
then
PATH=/usr/local/sbin:$PATH
fi
if ! echo $PATH | /bin/grep -q "/usr/X11R6/bin" ; then
PATH="$PATH:/usr/X11R6/bin"
fi
These lines check the path to set: if root opens a shell (user ID 0), it is
checked that /sbin, /usr/sbin and /usr/local/sbin are in the path. If not, they
are added. It is checked for everyone that /usr/X11R6/bin is in the path.
# No core files by default
ulimit -S -c 0 > /dev/null 2>&1
All trash goes to /dev/null if the user doesn't change this setting.
USER=`id -un`
LOGNAME=$USER
MAIL="/var/spool/mail/$USER"
HOSTNAME=`/bin/hostname`
HISTSIZE=1000
Here general variables are assigned their proper values.
if [ -z "$INPUTRC" -a ! -f "$HOME/.inputrc" ]; then
INPUTRC=/etc/inputrc
fi
If the variable INPUTRC is not set, and there is no .inputrc in the user's home
directory, then the default input control file is loaded.
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC
All variables are exported, so that they are available to other programs
requesting information about your environment.
2.3.2. The profile.d directory
for i in /etc/profile.d/*.sh ; do
if [ -r $i ]; then
. $i
fi
done
unset i
All readable shell scripts from the
/etc/profile.d
directory are read and executed. These do things like enabling color-ls,
aliasing vi to vim, setting locales etc. The temporary variable i is unset to
prevent it from disturbing shell behavior later on.
2.3.3. .bash_profile example
Then bash looks for a .bash_profile in the user's home directory:
debby:~> cat .bash_profile
#################################################################
# #
# .bash_profile file #
# #
# Executed from the bash shell when you log in. #
# #
#################################################################
source ~/.bashrc
source ~/.bash_login
This very straight forward file instructs your shell to first read ~/.bashrc
and then ~/.bash_login. You will encounter the
source
built-in shell command regularly when working in a shell environment: it is
used to apply configuration changes to the current environment.
2.3.4. .bash_login example
The ~/.bash_login file defines default file protection by setting the umask
value, see Section 4.2.2,_“Logging_on_to_another_groupâ€. The ~/.bashrc file is
used to define a bunch of user-specific aliases and functions and personal
environment variables. It first reads /etc/bashrc, which describes the default
prompt (PS1) and the default umask value. After that, you can add your own
settings. If no ~/.bashrc exists, /etc/bashrc is read by default.
2.3.5. /etc/bashrc example
Your /etc/bashrc file might look like this:
debby:~> cat /etc/bashrc
# /etc/bashrc
# System wide functions and aliases
# Environment stuff goes in /etc/profile
# by default, we want this to get set.
# Even for non-interactive, non-login shells.
if [ `id -gn` = `id -un` -a `id -u` -gt 99 ]; then
umask 002
else
umask 022
fi
These lines set the umask value. Then, depending on the type of shell, the
prompt is set:
# are we an interactive shell?
if [ "$PS1" ]; then
if [ -x /usr/bin/tput ]; then
if [ "x`tput kbs`" != "x" ]; then
# We can't do this with "dumb" terminal
stty erase `tput kbs`
elif [ -x /usr/bin/wc ]; then
if [ "`tput kbs|wc -c `" -gt 0 ]; then
# We can't do this with "dumb" terminal
stty erase `tput kbs`
fi
fi
fi
case $TERM in
xterm*)
if [ -e /etc/sysconfig/bash-prompt-xterm ]; then
PROMPT_COMMAND=/etc/sysconfig/bash-prompt-xterm
else
PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:\
${PWD/$HOME/~}\007"'
fi
;;
*)
[ -e /etc/sysconfig/bash-prompt-default ] && PROMPT_COMMAND=\
/etc/sysconfig/bash-prompt-default
;;
esac
[ "$PS1" = "\\s-\\v\\\$ " ] && PS1="[\u@\h \W]\\$ "
if [ "x$SHLVL" != "x1" ]; then # We're not a login shell
for i in /etc/profile.d/*.sh; do
if [ -x $i ]; then
. $i
fi
done
fi
fi
2.3.6. .bash_logout example
Upon logout, the commands in ~/.bash_logout are executed, which can for
instance clear the terminal, so that you have a clean window upon logging out
of a remote session, or upon leaving the system console:
debby:~> cat .bash_logout
# ~/.bash_logout
clear
Let's take a closer look at how these scripts work in the next section. Keep
info bash close at hand.
2.4. The Bash prompt
2.4.1. Introduction
The Bash prompt can do much more than displaying such simple information as
your user name, the name of your machine and some indication about the present
working directory. We can add other information such as the current date and
time, number of connected users etc.
Before we begin, however, we will save our current prompt in another
environment variable:
[jerry@nowhere jerry]$ MYPROMPT=$PS1
[jerry@nowhere jerry]$ echo $MYPROMPT
[\u@\h \W]\$
[jerry@nowhere jerry]$
When we change the prompt now, for example by issuing the command PS1="->", we
can always get our original prompt back with the command PS1=$MYPROMPT. You
will, of course, also get it back when you reconnect, as long as you just
fiddle with the prompt on the command line and avoid putting it in a shell
configuration file.
2.4.2. Some examples
In order to understand these prompts and the escape sequences used, we refer to
the Bash Info or man pages.
* export PS1="[\t \j] "
Displays time of day and number of running jobs
* export PS1="[\d][\u@\h \w] : "
Displays date, user name, host name and current working directory. Note that
\W displays only base names of the present working directory.
* export PS1="{\!} "
Displays history number for each command.
* export PS1="\[\033[1;35m\]\u@\h\[\033[0m\] "
Displays user@host in pink.
* export PS1="\[\033[1;35m\]\u\[\033[0m\] \[\033[1;34m\]\w\[\033[0m\] "
Sets the user name in pink and the present working directory in blue.
* export PS1="\[\033[1;44m\]$USER is in \w\[\033[0m\] "
Prompt for people who have difficulties seeing the difference between the
prompt and what they type.
* export PS1="\[\033[4;34m\]\u@\h \w \[\033[0m\]"
Underlined prompt.
* export PS1="\[\033[7;34m\]\u@\h \w \[\033[0m\] "
White characters on a blue background.
* export PS1="\[\033[3;35m\]\u@\h \w \[\033[0m\]\a"
Pink prompt in a lighter font that alerts you when your commands have
finished.
* export PS1=...
Variables are exported so the subsequently executed commands will also know
about the environment. The prompt configuration line that you want is best put
in your shell configuration file,
~/.bashrc
.
If you want, prompts can execute shell scripts and behave different under
different conditions. You can even have the prompt play a tune every time you
issue a command, although this gets boring pretty soon. More information can be
found in the Bash-Prompt_HOWTO.
2.5. Shell scripts
2.5.1. What are scripts?
A shell script is, as we saw in the shell configuration examples, a text file
containing shell commands. When such a file is used as the first non-option
argument when invoking Bash, and neither the -c nor -s option is supplied, Bash
reads and executes commands from the file, then exits. This mode of operation
creates a non-interactive shell. When Bash runs a shell script, it sets the
special parameter 0 to the name of the file, rather than the name of the shell,
and the positional parameters (everything following the name of the script) are
set to the remaining arguments, if any are given. If no additional arguments
are supplied, the positional parameters are unset.
A shell script may be made executable by using the
chmod
command to turn on the execute bit. When Bash finds such a file while
searching the PATH for a command, it spawns a sub-shell to execute it. In other
words, executing
filename ARGUMENTS
is equivalent to executing
bash filename ARGUMENTS
if “filename†is an executable shell script. This sub-shell reinitializes
itself, so that the effect is as if a new shell had been invoked to interpret
the script, with the exception that the locations of commands remembered by the
parent (see hash in the Info pages) are retained by the child.
Most versions of UNIX make this a part of the operating system's command
execution mechanism. If the first line of a script begins with the two
characters “#!â€, the remainder of the line specifies an interpreter for the
program. Thus, you can specify bash, awk, perl or some other interpreter or
shell and write the rest of the script file in that language.
The arguments to the interpreter consist of a single optional argument
following the interpreter name on the first line of the script file, followed
by the name of the script file, followed by the rest of the arguments. Bash
will perform this action on operating systems that do not handle it themselves.
Bash scripts often begin with
#! /bin/bash
(assuming that Bash has been installed in /bin), since this ensures that Bash
will be used to interpret the script, even if it is executed under another
shell.
2.5.2. Some simple examples
A very simple script consisting of only one command, that says hello to the
user executing it:
[jerry@nowhere ~] cat hello.sh
#!/bin/bash
echo "Hello $USER"
The script actually consists of only one command, echo, which uses the value of
($) the USER environment variable to print a string customized to the user
issuing the command.
Another one-liner, used for displaying connected users:
#!/bin/bash
who | cut -d " " -f 1 | sort -u
Here is a script consisting of some more lines, that I use to make backup
copies of all files in a directory. The script first makes a list of all the
files in the current directory and puts it in the variable LIST. Then it sets
the name of the copy for each file, and then it copies the file. For each file,
a message is printed:
tille:~> cat bin/makebackupfiles.sh
#!/bin/bash
# make copies of all files in a directory
LIST=`ls`
for i in $LIST; do
ORIG=$i
DEST=$i.old
cp $ORIG $DEST
echo "copied $i"
done
Just entering a line like mv * *.old won't work, as you will notice when trying
this on a set of test files. An echo command was added in order to display some
activity. echo's are generally useful when a script won't work: insert one
after each doubted step and you will find the error in no time.
The /etc/rc.d/init.d directory contains loads of examples. Let's look at this
script that controls the fictive ICanSeeYou server:
#!/bin/sh
# description: ICanSeeYou allows you to see networked people
# process name: ICanSeeYou
# pidfile: /var/run/ICanSeeYou/ICanSeeYou.pid
# config: /etc/ICanSeeYou.cfg
# Source function library.
. /etc/rc.d/init.d/functions
# See how (with which arguments) we were called.
case "$1" in
start)
echo -n "Starting ICanSeeYou: "
daemon ICanSeeYou
echo
touch /var/lock/subsys/ICanSeeYou
;;
stop)
echo -n "Shutting down ICanSeeYou: "
killproc ICanSeeYou
echo
rm -f /var/lock/subsys/ICanSeeYou
rm -f /var/run/ICanSeeYou/ICanSeeYou.pid
;;
status)
status ICanSeeYou
;;
restart)
$0 stop
$0 start
;;
*)
echo "Usage: $0 {start|stop|restart|status}"
exit 1
esac
exit 0
First, with the . command (dot) a set of shell functions, used by almost all
shell scripts in /etc/rc.d/init.d, is loaded. Then a case command is issued,
which defines 4 different ways the script can execute. An example might be
ICanSeeYou start. The decision of which case to apply is made by reading the
(first) argument to the script, with the expression $1.
When no compliant input is given, the default case, marked with an asterisk, is
applied, upon which the script gives an error message. The case list is ended
with the esac statement. In the start case the server program is started as a
daemon, and a process ID and lock are assigned. In the stop case, the server
process is traced down and stopped, and the lock and the PID are removed.
Options, such as the daemon option, and functions like killproc, are defined in
the /etc/rc.d/init.d/functions file. This setup is specific to the distribution
used in this example. The initscripts on your system might use other functions,
defined in other files, or none at all.
Upon success, the script returns an exit code of zero to its parent.
This script is a fine example of using functions, which make the script easier
to read and the work done faster. Note that they use sh instead of bash, to
make them useful on a wider range of systems. On a Linux system, calling bash
as sh results in the shell running in POSIX-compliant mode.
The bash man pages contain more information about combining commands, for- and
while-loops and regular expressions, as well as examples. A comprehensible Bash
course for system administrators and power users, with exercises, from the same
author as this Introduction to Linux guide, is at http://tille.garrels.be/
training/bash/. Detailed description of Bash features and applications is in
the reference guide Advanced_Bash_Scripting.
3. The graphical environment
3.1. Introduction
The average user may not care too much about his login settings, but Linux
offers a wide variety of flashy window and desktop managers for use under X,
the graphical environment. The use and configuration of window managers and
desktops is straightforward and may even resemble the standard MS Windows,
Apple or UNIX CDE environment, although many Linux users prefer flashier
desktops and fancier window managers. We won't discuss the user specific
configuration here. Just experiment and read the documentation using the built-
in Help functions these managers provide and you will get along fine.
We will, however, take a closer look at the underlying system.
3.2. The X Window System
The X Window System is a network-transparent window system which runs on a wide
range of computing and graphics machines. X Window System servers run on
computers with bitmap displays. The X server distributes user input to and
accepts output requests from several client programs through a variety of
different interprocess communication channels. Although the most common case is
for the client programs to be running on the same machine as the server,
clients can be run transparently from other machines (including machines with
different architectures and operating systems) as well. We will learn how to do
this in Chapter 10,_Networking on networking and remote applications.
X supports overlapping hierarchical sub-windows and text and graphics
operations, on both monochrome and color displays. The number of X client
programs that use the X server is quite large. Some of the programs provided in
the core X Consortium distribution include:
* xterm: a terminal emulator
* twm: a minimalistic window manager
* xdm: a display manager
* xconsole: a console redirect program
* bitmap: a bitmap editor
* xauth, xhost and iceauth: access control programs
* xset, xmodmap and many others: user preference setting programs
* xclock: a clock
* xlsfonts and others: a font displayer, utilities for listing information
about fonts, windows and displays
* xfs: a font server
* ...
We refer again to the man pages of these commands for detailed information.
More explanations on available functions can be found in the Xlib - C language
X Interface manual that comes with your X distribution, the X Window System
Protocol specification, and the various manuals and documentation of X
toolkits. The /usr/share/doc directory contains references to these documents
and many others.
Many other utilities, window managers, games, toolkits and gadgets are included
as user-contributed software in the X Consortium distribution, or are available
using anonymous FTP on the Internet. Good places to start are http://www.x.org
and http://www.xfree.org.
Furthermore, all your graphical applications, such as your browser, your E-mail
program, your image viewing programs, sound playing tools and so on, are all
clients to your X server. Note that in normal operation, that is in graphical
mode, X clients and the X server on Linux run on the same machine.
3.2.1. Display names
From the user's perspective, every X server has a display name in the form of:
hostname:displaynumber.screennumber
This information is used by the application to determine how it should connect
to the X server and which screen it should use by default (on displays with
multiple monitors):
* hostname
: The host name specifies the name of the client machine to which the display
is physically connected. If the host name is not given, the most efficient
way of communicating to a server on the same machine will be used.
* displaynumber
: The phrase “display†is usually used to refer to a collection of monitors
that share a common key board and pointer (mouse, tablet, etc.). Most
workstations tend to only have one keyboard, and therefore, only one display.
Larger, multi-user systems, however, frequently have several displays so that
more than one person can be doing graphics work at once. To avoid confusion,
each display on a machine is assigned a display number (beginning at 0) when
the X server for that display is started. The display number must always be
given in a display name.
* screen number
: Some displays share a single keyboard and pointer among two or more
monitors. Since each monitor has its own set of windows, each screen is
assigned a screen number (beginning at 0) when the X server for that display
is started. If the screen number is not given, screen 0 will be used.
On POSIX systems, the default display name is stored in your
DISPLAY
environment variable. This variable is set automatically by the xterm terminal
emulator. However, when you log into another machine on a network, you might
need to set DISPLAY by hand to point to your display, see Section 4.3.2,
“Telnet_and_Xâ€.
More information can be found in the X man pages.
3.2.2. Window and desktop managers
The layout of windows on the screen is controlled by special programs called
window managers
. Although many window managers will honor geometry specifications as given,
others may choose to ignore them (requiring the user to explicitly draw the
window's region on the screen with the pointer, for example).
Since window managers are regular (albeit complex) client programs, a variety
of different user interfaces can be built. The X Consortium distribution comes
with a window manager named twm, but most users prefer something more fancy
when system resources permit. Sawfish and Enlightenment are popular examples
which allow each user to have a desktop according to mood and style.
A desktop manager makes use of one window manager or another for arranging your
graphical desktop in a convenient way, with menubars, drop-down menus,
informative messages, a clock, a program manager, a file manager and so on.
Among the most popular desktop managers are Gnome and KDE, which both run on
almost any Linux distribution and many other UNIX systems.
KDE applications in Gnome/Gnome applications in KDE
You don't need to start your desktop in KDE in order to be able to run KDE
applications. If you have the KDE libraries installed (the kdelibs package),
you can run these applications from the Gnome menus or start them from a Gnome
terminal.
Running Gnome applications in a KDE environment is a bit more tricky, because
there is no single set of base-libraries in Gnome. However, the dependencies
and thus extra packages you might have to install will become clear when
running or installing such an application.
3.3. X server configuration
The X distribution that used to come with Linux, XFree86, uses the
configuration file XF86Config for its initial setup. This file configures your
video card and is searched for in a number of locations, although it is usually
in /etc/X11.
If you see that the file
/etc/X11/XF86Config
is present on your system, a full description can be found in the Info or man
pages about XF86Config.
Because of licensing issues with XFree86, newer systems usually come with the
X.Org
distribution of the X server and tools. The main configuration file here is
xorg.conf
, usually also in /etc/X11. The file consists of a number of sections that may
occur in any order. The sections contain information about your monitor, your
video adaptor, the screen configuration, your keyboard etcetera. As a user, you
needn't worry too much about what is in this file, since everything is normally
determined at the time the system is installed.
Should you need to change graphical server settings, however, you can run the
configuration tools or edit the configuration files that set up the
infrastructure to use the XFree86 server. See the man pages for more
information; your distribution might have its own tools. Since misconfiguration
may result in unreadable garbage in graphical mode, you may want to make a
backup copy of the configuration file before attempting to change it, just to
be on the safe side.
4. Region specific settings
4.1. Keyboard setup
Setting the keyboard layout is done using the
loadkeys
command for text consoles. Use your local X configuration tool or edit the
Keyboard section in
XF86Config
manually to configure the layout for graphical mode. The
XkbdLayout
is the one you want to set:
XkbLayout "us"
This is the default. Change it to your local settings by replacing the quoted
value with any of the names listed in the subdirectories of your keymaps
directory. If you can't find the keymaps, try displaying their location on your
system issuing the command
locate keymaps
It is possible to combine layout settings, like in this example:
Xkblayout "us,ru"
Make a backup of the /etc/X11/XF86Config file before editing it! You will need
to use the root account to do this.
Log out and reconnect in order to reload X settings.
The Gnome Keyboard Applet enables real-time switching between layouts; no
special pemissions are needed for using this program. KDE has a similar tool
for switching between keyboard layouts.
4.2. Fonts
Use the
setfont
tool to load fonts in text mode. Most systems come with a standard
inputrc
file which enables combining of characters, such as the French “é†(meta
characters). The system admin should then add the line
export INPUTRC="/etc/inputrc"
to the /etc/bashrc file.
4.3. Date and time zone
Setting time information is usually done at installation time. After that, it
can be kept up to date using an
NTP
(Network Time Protocol) client. Most Linux systems run ntpd by default:
debby:~> ps -ef | grep ntpd
ntp 24678 1 0 2002 ? 00:00:33 ntpd -U ntp
You can run
ntpdate
manually to set the time, on condition that you can reach a time server. The
ntpd daemon should not be running when you adjust the time using ntpdate. Use a
time server as argument to the command:
root@box:~# ntpdate 10.2.5.200
26 Oct 14:35:42 ntpdate[20364]: adjust time server 10.2.5.200 offset
-0.008049 sec
See your system manual and the documentation that comes with the NTP package.
Most desktop managers include tools to set the system time, providing that you
have access to the system administrator's account.
For setting the time zone correct, you can use
tzconfig
or
timezone
commands. Timezone information is usually set during the installation of your
machine. Many systems have distribution-specific tools to configure it, see
your system documentation.
4.4. Language
If you'd rather get your messages from the system in Dutch or French, you may
want to set the
LANG
and
LANGUAGE
environment variables, thus enabling locale support for the desired language
and eventually the fonts related to character conventions in that language.
With most graphical login systems, such as gdm or kdm, you have the possibility
to configure these language settings before logging in.
Note that on most systems, the default tends to be
en_US.UTF-8
these days. This is not a problem, because systems where this is the default,
will also come with all the programs supporting this encoding. Thus, vi can
edit all the files on your system, cat won't behave strange and so on.
Trouble starts when you connect to an older system not supporting this font
encoding, or when you open a
UTF-8
encoded file on a system supporting only 1-byte character fonts. The
recode
utility might come in handy to convert files from one character set to
another. Read the man pages for an overview of features and usage. Another
solution might be to temporarily work with another encoding definition, by
setting the LANG environment variable:
debby:~> acroread /var/tmp/51434s.pdf
Warning: charset "UTF-8" not supported, using "ISO8859-1".
Aborted
debby:~> set | grep UTF
LANG=en_US.UTF-8
debby:~> export LANG=en_US
debby:~> acroread /var/tmp/51434s.pdf
<--new window opens-->
Refer to the Mozilla web site for guidance on how to get Firefox in your
language. The OpenOffice.org web site has information on localization of your
OpenOffice.org suite.
4.5. Country-specific Information
The list_of_HOWTOs contains references to Bangla, Belarusian, Chinese,
Esperanto, Finnish, Francophone, Hebrew, Hellenic, Latvian, Polish, Portugese,
Serbian, Slovak, Slovenian, Spanish, Thai and Turkish localization
instructions.
5. Installing new software
5.1. General
Most people are surprised to see that they have a running, usable computer
after installing Linux; most distributions contain ample support for video and
network cards, monitors and other external devices, so there is usually no need
to install extra drivers. Also common tools such as office suites, web
browsers, E-mail and other network client programs are included in the main
distributions. Even so, an initial installation might not meet your
requirements.
If you just can't find what you need, maybe it is not installed on your system.
It may also be that you have the required software, but it does not do what it
is supposed to do. Remember that Linux moves fast, and software improves on a
daily basis. Don't waste your time troubleshooting problems that might already
be resolved.
You can update your system or add packages to it at any time you want. Most
software comes in packages. Extra software may be found on your installation
CDs or on the Internet. The website of your Linux distribution is a good place
to start looking for additional software and contains instructions about how to
install it on your type of Linux, see Appendix A,_Where_to_go_from_here?.
Always read the documentation that comes with new software, and any
installation guidelines the package might contain. All software comes with a
README
file, which you are very strongly advised to read.
5.2. Package formats
5.2.1. RPM packages
5.2.1.1. What is RPM?
RPM, the RedHat Package Manager, is a powerful package manager that you can use
to install, update and remove packages. It allows you to search for packages
and keeps track of the files that come with each package. A system is built-in
so that you can verify the authenticity of packages downloaded from the
Internet. Advanced users can build their own packages with RPM.
An RPM package consists of an archive of files and meta-data used to install
and erase the archive files. The meta-data includes helper scripts, file
attributes, and descriptive information about the package. Packages come in two
varieties: binary packages, used to encapsulate software to be installed, and
source packages, containing the source code and recipe necessary to produce
binary packages.
Many other distributions support RPM packages, among the popular ones RedHat
Enterprise Linux, Mandriva (former Mandrake), Fedora Core and SuSE Linux. Apart
from the advice for your distribution, you will want to read man rpm.
5.2.1.2. RPM examples
Most packages are simply installed with the upgrade option, -U, whether the
package is already installed or not. The RPM package contains a complete
version of the program, which overwrites existing versions or installs as a new
package. The typical usage is as follows:
rpm -Uvh /path/to/rpm-package(s)
The -v option generates more verbose output, and -h makes rpm print a progress
bar:
[root@jupiter tmp]# rpm -Uvh totem-0.99.5-1.fr.i386.rpm
Preparing... ###########################################
[100%]
1:totem ###########################################
[100%]
[root@jupiter tmp]#
New kernel packages, however, are installed with the install option -i, which
does not overwrite existing version(s) of the package. That way, you will still
be able to boot your system with the old kernel if the new one does not work.
You can also use
rpm
to check whether a package is installed on your system:
[david@jupiter ~] rpm -qa | grep vim
vim-minimal-6.1-29
vim-X11-6.1-29
vim-enhanced-6.1-29
vim-common-6.1-29
Or you can find out which package contains a certain file or executable:
[david@jupiter ~] rpm -qf /etc/profile
setup-2.5.25-1
[david@jupiter ~] which cat
cat is /bin/cat
[david@jupiter ~] rpm -qf /bin/cat
coreutils-4.5.3-19
Note that you need not have access to administrative privileges in order to use
rpm to query the RPM database. You only need to be root when adding, modifying
or deleting packages.
Below is one last example, demonstrating how to uninstall a package using rpm:
[root@jupiter root]# rpm -e totem
[root@jupiter root]#
Note that uninstalling is not that verbose by default, it is normal that you
don't see much happening. When in doubt, use rpm -qa again to verify that the
package has been removed.
RPM can do much more than the couple of basic functions we discussed in this
introduction; the RPM_HOWTO contains further references.
5.2.2. DEB (.deb) packages
5.2.2.1. What are Debian packages?
This package format is the default on Debian GNU/Linux, where
dselect
, and, nowadays more common,
aptitude
, is the standard tool for managing the packages. It is used to select packages
that you want to install or upgrade, but it will also run during the
installation of a Debian system and help you to define the access method to
use, to list available packages and to configure packages.
The Debian_web_site contains all information you need, including a “dselect
Documentation for Beginnersâ€.
According to the latest news, the Debian package format is becoming more and
more popular. At the time of this writing, 5 of the top-10 distributions use
it. Also
apt-get
(see Section 5.3.2,_“APT†is becoming extremely popular, also on non-DEB
systems.
5.2.2.2. Examples with DEB tools
Checking whether a package is installed is done using the dpkg command. For
instance, if you want to know which version of the Gallery software is
installed on your machine:
nghtwsh@gorefest:~$ dpkg -l *gallery*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err:
uppercase=bad)
||/ Name Version Description
+++-==============-==============-
============================================
ii gallery 1.5-1sarge2 a web-based photo album written in php
The “ii†prefix means the package is installed. Should you see “un†as a
prefix, that means that the package is known in the list that your computer
keeps, but that it is not installed.
Searching which package a file belongs to is done using the -S to dpkg:
nghtwsh@gorefest:~$ dpkg -S /bin/cat
coreutils: /bin/cat
More information can be found in the Info pages for dpkg.
5.2.3. Source packages
The largest part of Linux programs is Free/Open Source, so source packages are
available for these programs. Source files are needed for compiling your own
program version. Sources for a program can be downloaded from its web site,
often as a compressed tarball (program-version.tar.gz or similar). For RPM-
based distributions, the source is often provided in the program-
version.src.rpm. Debian, and most distributions based on it, provide themselves
the adapted source which can be obtained using apt-get source.
Specific requirements, dependencies and installation instructions are provided
in the README file. You will probably need a C compiler, gcc. This GNU C
compiler is included in most Linux systems and is ported to many other
platforms.
5.3. Automating package management and updates
5.3.1. General remarks
The first thing you do after installing a new system is applying updates; this
applies to all operating systems and Linux is not different.
The updates for most Linux systems can usually be found on a nearby site
mirroring your distribution. Lists of sites offering this service can be found
at your distribution's web site, see Appendix A,_Where_to_go_from_here?.
Updates should be applied regularly, daily if possible - but every couple of
weeks would be a reasonable start. You really should try to have the most
recent version of your distribution, since Linux changes constantly. As we said
before, new features, improvements and bug fixes are supplied at a steady
rhythm, and sometimes important security problems are addressed.
The good news is that most Linux distributions provide tools so that you don't
have to upgrade tens of packages daily by hand. The following sections give an
overview of package manager managers. There is much more to this subject, even
regular updates of source packages is manageable automatically; we only list
the most commonly known systems. Always refer to the documentation for your
specific distribution for advised procedures.
5.3.2. APT
The Advanced Package Tool is a management system for software packages. The
command line tool for handling packages is
apt-get
, which comes with an excellent man page describing how to install and update
packages and how to upgrade singular packages or your entire distribution. APT
has its roots in the Debian GNU/Linux distribution, where it is the default
manager for the Debian packages. APT has been ported to work with RPM packages
as well. The main advantage of APT is that it is free and flexible to use. It
will allow you to set up systems similar to the distribution specific (and in
some cases commercial) ones listed in the next sections.
Generally, when first using apt-get, you will need to get an index of the
available packages. This is done using the command
apt-get update
After that, you can use apt-get to upgrade your system:
apt-get upgrade
Do this often, it's an easy way to keep your system up-to-date and thus safe.
Apart from this general usage, apt-get is also very fast for installing
individual packages. This is how it works:
[david@jupiter ~] su - -c "apt-get install xsnow"
Password:
Reading Package Lists... Done
Building Dependency Tree... Done
The following NEW packages will be installed:
xsnow
0 packages upgraded, 1 newly installed, 0 removed and 3 not upgraded.
Need to get 33.6kB of archives.
After unpacking 104kB of additional disk space will be used.
Get:1 http://ayo.freshrpms.net redhat/9/i386/os xsnow 1.42-10 [33.6kB]
Fetched 33.6kB in 0s (106kB/s)
Executing RPM (-Uvh)...
Preparing... ###########################################
[100%]
1:xsnow ###########################################
[100%]
Note the -c option to the su command, which indicates to the root shell to only
execute this command, and then return to the user's environment. This way, you
cannot forget to quit the root account.
If there are any dependencies on other packages, apt-get will download and
install these supporting packages.
More information can be found in the APT_HOWTO.
5.3.3. Systems using RPM packages
Update Agent, which originally only supported RedHat RPM packages, is now
ported to a wider set of software, including non-RedHat repositories. This tool
provides a complete system for updating the RPM packages on a RedHat or Fedora
Core system. On the command line, type
up2date
to update your system. On the desktop, by default a small icon is activated,
telleng you whether or not there are updates available for your system.
Yellowdog's Updater Modified (
yum
) is another tool that recently became more popular. It is an interactive but
automated update program for installing, updating or removing RPM packages on a
system. It is the tool of choice on Fedora systems.
On SuSE Linux, everything is done with YaST, Yet another Setup Tool, which
supports a wide variety of system administration tasks, among which updating
RPM packages. Starting from SuSE Linux 7.1 you can also upgrade using a web
interface and YOU, Yast Online Update.
Mandrake Linux and Mandriva provide so-called URPMI tools, a set of wrapper
programs that make installing new software easier for the user. These tools
combine with RPMDrake and MandrakeUpdate to provide everything needed for
smooth install and uninstall of software packages. MandrakeOnline offers an
extended range of services and can automatically notify administrators when
updates are available for your particular Mandrake system. See man urpmi, among
others, for more info.
Also the KDE and Gnome desktop suites have their own (graphical) versions of
package managers.
5.4. Upgrading your kernel
Most Linux installations are fine if you periodically upgrade your
distribution. The upgrade procedure will install a new kernel when needed and
make all necessary changes to your system. You should only compile or install a
new kernel manually if you need kernel features that are not supported by the
default kernel included in your Linux distribution.
Whether compiling your own optimized kernel or using a pre-compiled kernel
package, install it in co-existence with the old kernel until you are sure that
everything works according to plan.
Then create a dual boot system that will allow you to choose which kernel to
boot by updating your boot loader configuration file
grub.conf
. This is a simple example:
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making config changes.
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, e.g.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/hde8
# initrd /initrd-version.img
#boot=/dev/hde
default=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Linux new (2.4.9-31)
root (hd0,0)
kernel /vmlinuz-2.4.9-31 ro root=/dev/hde8
initrd /initrd-2.4.9-31.img
title old-kernel
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hde8
initrd /initrd-2.4.9-21.img
After the new kernel has proven to work, you may remove the lines for the old
one from the GRUB config file, although it is best to wait a couple of days
just to be sure.
5.5. Installing extra packages from the installation CDs
5.5.1. Mounting a CD
This is basically done in the same way as installing packages manually, except
that you have to append the file system of the CD to your machine's file system
to make it accessible. On most systems, this will be done automatically upon
insertion of a CD in the drive because the
automount
daemon is started up at boot time. If your CD is not made available
automatically, issue the
mount
command in a terminal window. Depending on your actual system configuration, a
line similar to this one will usually do the trick:
mount /dev/cdrom /mnt/cdrom
On some systems, only root can mount removable media; this depends on the
configuration.
For automation purposes, the CD drive usually has an entry in
/etc/fstab
, which lists the file systems and their mount points, that make up your file
system tree. This is such a line:
[david@jupiter ~] grep cdrom /etc/fstab
/dev/cdrom /mnt/cdrom iso9660 noauto,owner,ro 0 0
This indicates that the system will understand the command mount /mnt/cdrom.
The noauto option means that on this system, CDs are not mounted at boot time.
You may even try to right click on the CD icon on your desktop to mount the CD
if your file manager doesn't do it for you. You can check whether it worked
issuing the
mount
command with no arguments:
[david@jupiter ~] mount | grep cdrom
/dev/cdrom on /mnt/cdrom type iso9660 (ro,nosuid,nodev)
5.5.2. Using the CD
After mounting the CD, you can change directories, usually to the mount point /
mnt/cdrom, where you can access the content of the CD-ROM. Use the same
commands for dealing with files and directories as you would use for files on
the hard disk.
5.5.3. Ejecting the CD
In order to get the CD out of the drive after you've finished using it, the
file system on the CD should be unused. Even being in one of the subdirectories
of the mount point, /mnt/cdrom in our example, will be considered as “using the
file systemâ€, so you should get out of there. Do this for instance by typing cd
with no arguments, which will put you back in your home directory. After that,
you can either use the command
umount /mnt/cdrom
or
eject cdrom
Blocked drives
NEVER force the drive. The trick with the paperclip is a bad idea, because this
will eventually expunge the CD, but your system will think the CD is still
there because normal procedures were not followed. Chances are likely that you
will have to reboot to get the system back in a consistent state.
If you keep getting “device busy†messages, check first that all shell sessions
have left the CD file system and that no graphical applications are using it
anymore. When in doubt, use the lsof tool to trace down the process(es) still
using the CD resource.
6. Summary
When everything has its place, that means already half the work is done.
While keeping order is important, it is equally important to feel at home in
your environment, whether text or graphical. The text environment is controlled
through the shell setup files. The graphical environment is primarily dependent
on the X server configuration, on which a number of other applications are
built, such as window and desktop managers and graphical applications, each
with their own config files. You should read the system and program specific
documentation to find out about how to configure them.
Regional settings such as keyboard setup, installing appropriate fonts and
language support are best done at installation time.
Software is managed either automatically or manually using a package system.
The following commands were introduced in this chapter:
Table 7.2. New commands in chapter 7: Making yourself at home
_______________________________________________________________________
|Command__|Meaning______________________________________________________|
|aptitude_|Manage_packages_Debian-style.________________________________|
|automount|automatically_include_newly_inserted_file_systems.___________|
|dpkg_____|Debian_package_manager.______________________________________|
|dselect__|Manage_packages_Debian-style.________________________________|
|loadkeys_|Load_keyboard_configuration._________________________________|
|lsof_____|Identify_processes.__________________________________________|
|mount____|Include_a_new_file_system_into_the_existing_file_system_tree.|
|ntpdate__|Set_the_system_time_and_date_using_a_time_server.____________|
|quota____|Display_information_about_allowed_disk_space_usage.__________|
|recode___|Convert_files_to_another_character_set.______________________|
|rpm______|Manage_RPM_packages._________________________________________|
|setfont__|Choose_a_font._______________________________________________|
|timezone_|Set_the_timezone.____________________________________________|
|tzconfig_|Set_the_timezone.____________________________________________|
|ulimit___|Set_or_display_resource_limits.______________________________|
|up2date__|Manage_RPM_packages._________________________________________|
|urpmi____|Manage_RPM_packages._________________________________________|
|yum______|Manage_RPM_packages._________________________________________|
7. Exercises
7.1. Shell environment
* Print out your environment settings. Which variable may be used to store the
CPU type of your machine?
* Make a script that can say something on the lines of “hello, world.†Give it
appropriate permissions so it can be run. Test your script.
* Create a directory in your home directory and move the script to the new
directory. Permanently add this new directory to your search path. Test that
the script can be executed without giving a path to its actual location.
* Create subdirectories in your home directory to store various files, for
instance a directory music to keep audio files, a directory documents for
your notes, and so on. And use them!
* Create a personalized prompt.
* Display limits on resource usage. Can you change them?
* Try to read compressed man pages without decompressing them first.
* Make an alias lll which actually executes ls -la.
* Why does the command tail testfile > testfile not work?
* Mount a data CD, such as your Linux installation CD, and have a look around.
Don't forget to unmount when you don't need it anymore.
* The script from Section 2.5.2,_“Some_simple_examples†is not perfect. It
generates errors for files that are directories. Adapt the script so that it
only selects plain files for copying. Use find to make the selection. Do not
forget to make the script executable before you try to run it.
7.2. Graphical environment
* Try all the mouse buttons in different regions (terminal, background, task
bar).
* Explore the menus.
* Customize your terminal window.
* Use the mouse buttons to copy and paste text from one terminal to another.
* Find out how to configure your window manager; try different workspaces
(virtual screens).
* Add an applet, such as a load monitor, to the task bar.
* Apply a different theme.
* Enable the so-called sloppy focus - this is when a window is activated by
just moving the mouse over it, so that you do not need to click the window in
order to be able to use it.
* Switch to a different window manager.
* Log out and select a different session type, like KDE if you were using Gnome
before. Repeat the previous steps.
Chapter 8. Printers and printing
Table of Contents
1._Printing_files
1.1._Command_line_printing
1.2._Formatting
2._The_server_side
2.1._General
2.2._Graphical_printer_configuration
2.3._Buying_a_printer_for_Linux
3._Print_problems
3.1._Wrong_file
3.2._My_print_hasn't_come_out
4._Summary
5._Exercises
Abstract
In this chapter we will learn more about printers and printing files. After
reading this part, you will be able to:
* Format documents
* Preview documents before sending them to the printer
* Choose a good printer that works with your Linux system
* Print files and check on printer status
* Troubleshoot printing problems
* Find necessary documentation to install a printer
1. Printing files
1.1. Command line printing
1.1.1. Getting the file to the printer
Printing from within an application is very easy, selecting the Print option
from the menu.
From the command line, use the
lp
or
lpr
command.
lp file(s)
lpr file(s)
These commands can read from a pipe, so you can print the output of commands
using
command | lp
There are many options available to tune the page layout, the number of copies,
the printer that you want to print to if you have more than one available,
paper size, one-side or double-sided printing if your printer supports this
feature, margins and so on. Read the man pages for a complete overview.
1.1.2. Status of your print jobs
Once the file is accepted in the print queue, an identification number for the
print job is assigned:
davy:~> lp /etc/profile
request id is blob-253 (1 file(s))
To view (query) the print queue, use the
lpq
or
lpstat
command. When entered without arguments, it displays the contents of the
default print queue.
davy:~> lpq
blob is ready and printing
Rank Owner Job File(s) Total Size
active davy 253 profile 1024 bytes
davy:~> lpstat
blob-253 davy 1024 Tue 25 Jul 2006 10:20_01 AM CEST
1.1.3. Status of your printer
Which is the default printer on a system that has access to multiple printers?
lpstat -d
davy:~> lpstat -d
system default destination: blob
What is the status of my printer(s)?
lpstat -p
davy:~> lpstat -p
printer blob now printing blob-253. enabled since Jan 01 18:01
1.1.4. Removing jobs from the print queue
If you don't like what you see from the status commands, use
lprm
or
cancel
to delete jobs.
davy:~> lprm 253
In the graphical environment, you may see a popup window telling you that the
job has been canceled.
In larger environments, lpc may be used to control multiple printers. See the
Info or man pages on each command.
There are many GUI print tools used as a front-end to lp, and most graphical
applications have a print function that uses lp. See the built-in Help
functions and program specific documentation for more.
Why are there two commands for every task related to printing?
Printing on UNIX and alikes has a long history. There used to be two rather
different approaches: the BSD-style printing and the SystemV-style printing.
For compatibility, Linux with CUPS supports the commands from both styles. Also
note that lp does not behave exactly like lpr, lpq has somewhat different
options than lpstat and lprm is almost, but not quite, like cancel. Which one
you use is not important, just pick the commands that you are comfortable with,
or that you may know from previous experiences with UNIX-like systems.
1.2. Formatting
1.2.1. Tools and languages
If we want to get something sensible out of the printer, files should be
formatted first. Apart from an abundance of formatting software, Linux comes
with the basic UNIX formatting tools and languages.
Modern Linux systems support direct printing, without any formatting by the
user, of a range of file types: text, PDF, PostScript and several image formats
like PNG, JPEG, BMP and GIF.
For those file formats that do need formatting, Linux comes with a lot of
formatting tools, such as the pdf2ps, fax2ps and a2ps commands, that convert
other formats to PostScript. These commands can create files that can then be
used on other systems that don't have all the conversion tools installed.
Apart from these command line tools there are a lot of graphical word
processing programs. Several complete office suites are available, many are
free. These do the formatting automatically upon submission of a print job.
Just to name a few: OpenOffice.org, KOffice, AbiWord, WordPerfect, etc.
The following are common languages in a printing context:
* groff: GNU version of the UNIX roff command. It is a front-end to the groff
document formatting system. Normally it runs the troff command and a post-
processor appropriate for the selected device. It allows generation of
PostScript files.
* TeX and the macro package LaTeX: one of the most widely used markup languages
on UNIX systems. Usually invoked as tex, it formats files and outputs a
corresponding device-independent representation of the typeset document.
Technical works are still frequently written in LaTeX because of its support
for mathematic formulas, although efforts are being made at W3C (the World
Wide Web Consortium) to include this feature in other applications.
* SGML and XML: Free parsers are available for UNIX and Linux. XML is the next
generation SGML, it forms the basis for DocBook XML, a document system (this
book is written in XML, for instance).
Printing documentation
The man pages contain pre-formatted
troff
data which has to be formatted before it can roll out of your printer.
Printing is done using the -t option to the man command:
man -t command > man-command.ps
Then print the PostScript file. If a default print destination is configured
for your system/account, you can just issue the command man -t command to send
the formatted page to the printer directly.
1.2.2. Previewing formatted files
Anything that you can send to the printer can normally be sent to the screen as
well. Depending on the file format, you can use one of these commands:
* PostScript files: with the
gv
(GhostView) command.
* TeX dvi files: with
xdvi
, or with KDE's
kdvi
.
* PDF files:
xpdf
,
kpdf
,
gpdf
or Adobe's viewer,
acroread
, which is also available for free but is not free software. Adobe's reader
supports PDF 1.6, the others only support PDF versions up to 1.5. The version
of a PDF file can be determined using the file command.
* From within applications, such as Firefox or OpenOffice, you can usually
select Print Preview from one of the menus.
2. The server side
2.1. General
Until a couple of years ago, the choice for Linux users was simple: everyone
ran the same old LPD from BSD's Net-2 code. Then LPRng became more popular, but
nowadays most modern Linux distributions use CUPS, the Common UNIX Printing
System. CUPS is an implementation of the Internet Printing Protocol (IPP), an
HTTP-like RFC standard replacement protocol for the venerable (and clunky) LPD
protocol. CUPS is distributed under the GNU Public License. CUPS is also the
default print system on MacOS X.
2.2. Graphical printer configuration
Most distributions come with a GUI for configuring networked and local
(parallel port or USB) printers. They let you choose the printer type from a
list and allow easy testing. You don't have to bother about syntax and location
of configuration files. Check your system documentation before you attempt
installing your printer.
CUPS can also be configured using a web interface that runs on port 631 on your
computer. To check if this feature is enabled, try browsing to localhost:631/
help or localhost:631/.
2.3. Buying a printer for Linux
As more and more printer vendors make drivers for CUPS available, CUPS will
allow easy connection with almost any printer that you can plug into a serial,
parallel, or USB port, plus any printer on the network. CUPS will ensure a
uniform presentation to you and your applications of all different types of
printers.
Printers that only come with a Win9x driver could be problematic if they have
no other support. Check with http://linuxprinting.org/ when in doubt.
In the past, your best choice would have been a printer with native PostScript
support in the firmware, since nearly all UNIX or Linux software producing
printable output, produces it in PostScript, the publishing industry's printer
control language of choice. PostScript printers are usually a bit more
expensive, but it is a device-independent, open programming language and you're
always 100% sure that they will work. These days, however, the importance of
this rule of thumb is dwindling.
3. Print problems
In this section, we will discuss what you can do as a user when something goes
wrong. We won't discuss any problems that have to do with the daemon-part of
the printing service, as that is a task for system administrators.
3.1. Wrong file
If you print the wrong file, the job may be canceled using the command lprm
jobID, where jobID is in the form printername-printjobnumber (get it from
information displayed by lpq or lpstat). This will work when other jobs are
waiting to be printed in this printer's queue. However, you have to be really
quick if you are the only one using this printer, since jobs are usually
spooled and send to the printer in only seconds. Once they arrive on the
printer, it is too late to remove jobs using Linux tools.
What you can try in those cases, or in cases where the wrong print driver is
configured and only rubbish comes out of the printer, is power off the printer.
However, that might not be the best course of action, as you might cause paper
jams and other irregularities.
3.2. My print hasn't come out
Use the lpq command and see if you can spot your job:
elly:~> lpq
Printer: lp@blob
Queue: 2 printable jobs
Server: pid 29998 active
Unspooler: pid 29999 active
Status: waiting for subserver to exit at 09:43:20.699
Rank Owner/ID Class Job Files Size Time
1 elly@blob+997 A 997 (STDIN) 129 09:42:54
2 elly@blob+22 A 22 /etc/profile 917 09:43:20
Lots of printers have web interfaces these days, which can display status
information by typing the printer's IP address in your web browser:
Figure 8.1. Printer Status through web interface
This GUI shows printer status using your browser. There is a trafic light
indicating the status, and choices for managing the printer: view config,
control panel, printer settings, online support and admin settings.
CUPS web interface versus printer web interface
Note that this is not the CUPS web interface and only works for printers
supporting this feature. Check the documentation of your printer.
If your job ID is not there and not on the printer, contact your system
administrator. If your job ID is listed in the output, check that the printer
is currently printing. If so, just wait, your job will get done in due time.
If the printer is not printing, check that it has paper, check the physical
connections to both electricity and data network. If that's okay, the printer
may need restarting. Ask your system admin for advice.
In the case of a network printer, try printing from another host. If the
printer is reachable from your own host (see Chapter 10,_Networking for the
ping utility), you may try to put the formatted file on it, like file.ps in
case of a PostScript printer, using an FTP client. If that works, your print
system is misconfigured. If it doesn't work, maybe the printer doesn't
understand the format you are feeding it.
The GNU/Linux_Printing_site contains more tips and tricks.
4. Summary
The Linux print service comes with a set of printing tools based on the
standard UNIX LPD tools, whether it be the SystemV or BSD implementation. Below
is a list of print-related commands.
Table 8.1. New commands in chapter 8: Printing
_________________________________________
|Command_______|Meaning___________________|
|lpr_or_lp_____|Print_file________________|
|lpq_or_lpstat_|Query_print_queue_________|
|lprm_or_cancel|Remove_print_job__________|
|acroread______|PDF_viewer________________|
|groff_________|Formatting_tool___________|
|gv____________|PostScript_viewer_________|
|printconf_____|Configure_printers________|
|xdvi__________|DVI_viewer________________|
|xpdf__________|PDF_viewer________________|
|*2ps__________|Convert_file_to_PostScript|
5. Exercises
Configuring and testing printers involves being in the possession of one, and
having access to the root account. If so, you may try:
* Installing the printer using the GUI on your system.
* Printing a test page using the GUI.
* Printing a test page using the lp command.
* Print from within an application, for example Mozilla or OpenOffice, by
choosing File → Print from the menu.
* Disconnect the printer from the network or the local machine/print-server.
What happens when you try to print something?
The following exercises can be done without printer or root access.
* Try to make PostScript files from different source files, (e.g. HTML, PDF,
man pages). Test the results with the gv viewer.
* Check that the print daemon is running.
* Print the files anyway. What happens?
* Make a PostScript file using Mozilla. Test it with gv.
* Convert it to PDF format. Test with xpdf.
* How would you go about printing a GIF file from the command line?
* Use a2ps to print the /etc/profile file to an output file. Test again with
gv. What happens if you don't specify an output file?
Chapter 9. Fundamental Backup Techniques
Table of Contents
1._Introduction
1.1._Preparing_your_data
2._Moving_your_data_to_a_backup_device
2.1._Making_a_copy_on_a_floppy_disk
2.2._Making_a_copy_with_a_CD-writer
2.3._Backups_on/from_jazz_drives,_USB_devices_and_other_removables
2.4._Backing_up_data_using_a_tape_device
2.5._Tools_from_your_distribution
3._Using_rsync
3.1._Introduction
3.2._An_example:_rsync_to_a_USB_storage_device
4._Encryption
4.1._General_remarks
4.2._Generate_a_key
4.3._About_your_key
4.4._Encrypt_data
4.5._Decrypting_files
5._Summary
6._Exercises
Abstract
Accidents will happen sooner or later. In this chapter, we'll discuss how to
get data to a safe place using other hosts, floppy disks, CD-ROMs and tapes. We
will also discuss the most popular compressing and archiving commands.
Upon completion of this chapter, you will know how to:
* Make, query and unpack file archives
* Handle floppy disks and make a boot disk for your system
* Write CD-ROMs
* Make incremental backups
* Create Java archives
* Find documentation to use other backup devices and programs
* Encrypt your data
1. Introduction
Although Linux is one of the safest operating systems in existence, and even if
it is designed to keep on going, data can get lost. Data loss is most often the
consequence of user errors, but occasionally a system fault, such as a power or
disk failure, is the cause, so it's always a good idea to keep an extra copy of
sensitive and/or important data.
1.1. Preparing your data
1.1.1. Archiving with tar
In most cases, we will first collect all the data to back up in a single
archive file, which we will compress later on. The process of archiving
involves concatenating all listed files and taking out unnecessary blanks. In
Linux, this is commonly done with the
tar
command. tar was originally designed to archive data on tapes, but it can also
make archives, known as
tarballs
.
tar has many options, the most important ones are cited below:
* -v: verbose
* -t: test, shows content of a tarball
* -x: extract archive
* -c: create archive
* -f archivedevice: use archivedevice as source/destination for the tarball,
the device defaults to the first tape device (usually /dev/st0 or something
similar)
* -j: filter through bzip2, see Section 1.1.2,_“Incremental_backups_with_tarâ€
It is common to leave out the dash-prefix with tar options, as you can see from
the examples below.
Use GNU tar for compatibility
The archives made with a proprietary tar version on one system, may be
incompatible with tar on another proprietary system. This may cause much
headaches, such as if the archive needs to be recovered on a system that
doesn't exist anymore. Use the GNU tar version on all systems to prevent your
system admin from bursting into tears. Linux always uses GNU tar. When working
on other UNIX machines, enter tar --help to find out which version you are
using. Contact your system admin if you don't see the word GNU somewhere.
In the example below, an archive is created and unpacked.
gaby:~> ls images/
me+tux.jpg nimf.jpg
gaby:~> tar cvf images-in-a-dir.tar images/
images/
images/nimf.jpg
images/me+tux.jpg
gaby:~> cd images
gaby:~/images> tar cvf images-without-a-dir.tar *.jpg
me+tux.jpg
nimf.jpg
gaby:~/images> cd
gaby:~> ls */*.tar
images/images-without-a-dir.tar
gaby:~> ls *.tar
images-in-a-dir.tar
gaby:~> tar xvf images-in-a-dir.tar
images/
images/nimf.jpg
images/me+tux.jpg
gaby:~> tar tvf images/images-without-dir.tar
-rw-r--r-- gaby/gaby 42888 1999-06-30 20:52:25 me+tux.jpg
-rw-r--r-- gaby/gaby 7578 2000-01-26 12:58:46 nimf.jpg
gaby:~> tar xvf images/images-without-a-dir.tar
me+tux.jpg
nimf.jpg
gaby:~> ls *.jpg
me+tux.jpg nimf.jpg
This example also illustrates the difference between a tarred directory and a
bunch of tarred files. It is advisable to only compress directories, so files
don't get spread all over when unpacking the tarball (which may be on another
system, where you may not know which files were already there and which are the
ones from the archive).
When a tape drive is connected to your machine and configured by your system
administrator, the file names ending in .tar are replaced with the tape device
name, for example:
tar cvf /dev/tape mail/
The directory mail and all the files it contains are compressed into a file
that is written on the tape immediately. A content listing is displayed because
we used the verbose option.
1.1.2. Incremental backups with tar
The tar tool supports the creation of incremental backups, using the -N option.
With this option, you can specify a date, and tar will check modification time
of all specified files against this date. If files are changed more recent than
date, they will be included in the backup. The example below uses the timestamp
on a previous archive as the date value. First, the initial archive is created
and the timestamp on the initial backup file is shown. Then a new file is
created, upon which we take a new backup, containing only this new file:
jimmy:~> tar cvpf /var/tmp/javaproggies.tar java/*.java
java/btw.java
java/error.java
java/hello.java
java/income2.java
java/income.java
java/inputdevice.java
java/input.java
java/master.java
java/method1.java
java/mood.java
java/moodywaitress.java
java/test3.java
java/TestOne.java
java/TestTwo.java
java/Vehicle.java
jimmy:~> ls -l /var/tmp/javaproggies.tar
-rw-rw-r-- 1 jimmy jimmy 10240 Jan 21 11:58 /var/tmp/javaproggies.tar
jimmy:~> touch java/newprog.java
jimmy:~> tar -N /var/tmp/javaproggies.tar \
-cvp /var/tmp/incremental1-javaproggies.tar java/*.java 2> /dev/null
java/newprog.java
jimmy:~> cd /var/tmp/
jimmy:~> tar xvf incremental1-javaproggies.tar
java/newprog.java
Standard errors are redirected to /dev/null. If you don't do this, tar will
print a message for each unchanged file, telling you it won't be dumped.
This way of working has the disadvantage that it looks at timestamps on files.
Say that you download an archive into the directory containing your backups,
and the archive contains files that have been created two years ago. When
checking the timestamps of those files against the timestamp on the initial
archive, the new files will actually seem old to tar, and will not be included
in an incremental backup made using the -N option.
A better choice would be the -g option, which will create a list of files to
backup. When making incremental backups, files are checked against this list.
This is how it works:
jimmy:~> tar cvpf work-20030121.tar -g snapshot-20030121 work/
work/
work/file1
work/file2
work/file3
jimmy:~> file snapshot-20030121
snapshot-20030121: ASCII text
The next day, user jimmy works on file3 a bit more, and creates file4. At the
end of the day, he makes a new backup:
jimmy:~> tar cvpf work-20030122.tar -g snapshot-20030121 work/
work/
work/file3
work/file4
These are some very simple examples, but you could also use this kind of
command in a cronjob (see Section 4.4,_“Cron_and_crontabâ€), which specifies for
instance a snapshot file for the weekly backup and one for the daily backup.
Snapshot files should be replaced when taking full backups, in that case.
More information can be found in the tar documentation.
The real stuff
As you could probably notice, tar is OK when we are talking about a simple
directory, a set of files that belongs together. There are tools that are
easier to manage, however, when you want to archive entire partitions or disks
or larger projects. We just explain about tar here because it is a very popular
tool for distributing archives. It will happen quite often that you need to
install a software that comes in a so-called “compressed tarballâ€. See
Section 3,_“Using_rsync†for an easier way to perform regular backups.
1.1.3. Compressing and unpacking with gzip or bzip2
Data, including tarballs, can be compressed using zip tools. The
gzip
command will add the suffix .gz to the file name and remove the original file.
jimmy:~> ls -la | grep tar
-rw-rw-r-- 1 jimmy jimmy 61440 Jun 6 14:08 images-without-dir.tar
jimmy:~> gzip images-without-dir.tar
jimmy:~> ls -la images-without-dir.tar.gz
-rw-rw-r-- 1 jimmy jimmy 50562 Jun 6 14:08 images-without-dir.tar.gz
Uncompress gzipped files with the -d option.
bzip2
works in a similar way, but uses an improved compression algorithm, thus
creating smaller files. See the bzip2 info pages for more.
Linux software packages are often distributed in a gzipped tarball. The
sensible thing to do after unpacking that kind of archives is find the
README
and read it. It will generally contain guidelines to installing the package.
The GNU tar command is aware of gzipped files. Use the command
tar zxvf file.tar.gz
for unzipping and untarring .tar.gz or .tgz files. Use
tar jxvf file.tar.bz2
for unpacking tar archives that were compressed with bzip2.
1.1.4. Java archives
The GNU project provides us with the
jar
tool for creating Java archives. It is a Java application that combines
multiple files into a single JAR archive file. While also being a general
purpose archiving and compression tool, based on ZIP and the ZLIB compression
format, jar was mainly designed to facilitate the packing of Java code, applets
and/or applications in a single file. When combined in a single archive, the
components of a Java application, can be downloaded much faster.
Unlike tar, jar compresses by default, independent from other tools - because
it is basically the Java version of zip. In addition, it allows individual
entries in an archive to be signed by the author, so that origins can be
authenticated.
The syntax is almost identical as for the tar command, we refer to info jar for
specific differences.
tar, jar and symbolic links
One noteworthy feature not really mentioned in the standard documentation is
that jar will follow symbolic links. Data to which these links are pointing
will be included in the archive. The default in tar is to only backup the
symbolic link, but this behavior can be changed using the -h to tar.
1.1.5. Transporting your data
Saving copies of your data on another host is a simple but accurate way of
making backups. See Chapter 10,_Networking for more information on scp, ftp and
more.
In the next section we'll discuss local backup devices.
2. Moving your data to a backup device
2.1. Making a copy on a floppy disk
2.1.1. Formatting the floppy
On most Linux systems, users have access to the floppy disk device. The name of
the device may vary depending on the size and number of floppy drives, contact
your system admin if you are unsure. On some systems, there will likely be a
link /dev/floppy pointing to the right device, probably /dev/fd0 (the auto-
detecting floppy device) or /dev/fd0H1440 (set for 1,44MB floppies).
fdformat
is the low-level floppy disk formatting tool. It has the device name of the
floppy disk as an option. fdformat will display an error when the floppy is
write-protected.
emma:~> fdformat /dev/fd0H1440
Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB.
Formatting ... done
Verifying ... done
emma:~>
The
mformat
command (from the mtools package) is used to create DOS-compatible floppies
which can then be accessed using the mcopy, mdir and other m-commands.
Graphical tools are also available.
Figure 9.1. Floppy formatter
This GUI allows for chosing the FS type and density.
After the floppy is formatted, it can be mounted into the file system and
accessed as a normal, be it small, directory, usually via the /mnt/floppy
entry.
Should you need it, install the
mkbootdisk
utility, which makes a floppy from which the current system can boot.
2.1.2. Using the dd command to dump data
The
dd
command can be used to put data on a disk, or get it off again, depending on
the given input and output devices. An example:
gaby:~> dd if=images-without-dir.tar.gz of=/dev/fd0H1440
98+1 records in
98+1 records out
gaby~> dd if=/dev/fd0H1440 of=/var/tmp/images.tar.gz
2880+0 records in
2880+0 records out
gaby:~> ls /var/tmp/images*
/var/tmp/images.tar.gz
Note that the dumping is done on an unmounted device. Floppies created using
this method will not be mountable in the file system, but it is of course the
way to go for creating boot or rescue disks. For more information on the
possibilities of dd, read the man pages.
This tool is part of the GNU coreutils package.
Dumping disks
The dd command can also be used to make a raw dump of an entire hard disk.
2.2. Making a copy with a CD-writer
On some systems users are allowed to use the CD-writer device. Your data will
need to be formatted first. Use the
mkisofs
command to do this in the directory containing the files you want to backup.
Check with
df
that enough disk space is available, because a new file about the same size as
the entire current directory will be created:
[rose@blob recordables] df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/hde5 19G 15G 3.2G 82% /home
[rose@blob recordables] du -h -s .
325M .
[rose@blob recordables] mkisofs -J -r -o cd.iso .
<--snap-->
making a lot of conversions
<--/snap-->
98.95% done, estimate finish Fri Apr 5 13:54:25 2002
Total translation table size: 0
Total rockridge attributes bytes: 35971
Total directory bytes: 94208
Path table size(bytes): 452
Max brk space used 37e84
166768 extents written (325 Mb)
The -J and -r options are used to make the CD-ROM mountable on different
systems, see the man pages for more. After that, the CD can be created using
the
cdrecord
tool with appropriate options:
[rose@blob recordables] cdrecord -dev 0,0,0 -speed=8 cd.iso
Cdrecord 1.10 (i686-pc-linux-gnu) (C) 1995-2001 Joerg Schilling
scsidev: '0,0,0'
scsibus: 0 target: 0 lun: 0
Linux sg driver version: 3.1.20
Using libscg version 'schily-0.5'
Device type : Removable CD-ROM
Version : 0
Response Format: 1
Vendor_info : 'HP '
Identification : 'CD-Writer+ 8100 '
Revision : '1.0g'
Device seems to be: Generic mmc CD-RW.
Using generic SCSI-3/mmc CD-R driver (mmc_cdr).
Driver flags : SWABAUDIO
Starting to write CD/DVD at speed 4 in write mode for single session.
Last chance to quit, starting real write in 0 seconds.
Operation starts.
Depending on your CD-writer, you now have the time to smoke^H^H^H^H^H eat a
healthy piece of fruit and/or get a cup of coffee. Upon finishing the job, you
will get a confirmation message:
Track 01: Total bytes read/written: 341540864/341540864
(166768 sectors).
There are some graphical tools available to make it easier on you. One of the
popular ones is
xcdroast
, which is freely available from the_X-CD-Roast_web_site and is included on
most systems and in the GNU directory. Both the KDE and Gnome desktop managers
have facilities to make your own CDs.
2.3. Backups on/from jazz drives, USB devices and other removables
These devices are usually mounted into the file system. After the mount
procedure, they are accessed as normal directories, so you can use the standard
commands for manipulating files.
In the example below, images are copied from a USB camera to the hard disk:
robin:~> mount /mnt/camera
robin:~> mount | grep camera
/dev/sda1 on /mnt/camera type vfat (rw,nosuid,nodev)
If the camera is the only USB storage device that you ever connect to your
system, this is safe. But keep in mind that USB devices are assigned entries in
/dev as they are connected to the system. Thus, if you first connect a USB
stick to your system, it will be on the /dev/sda entry, and if you connect your
camera after that, it will be assigned to /dev/sdb - provided that you do not
have any SCSI disks, which are also on /dev/sd*. On newer systems, since kernel
2.6, a hotplug system called HAL (Hardware Abstraction Layer) ensures that
users don't have to deal with this burden. If you want to check where your
device is, type
dmesg
after inserting it.
You can now copy the files:
robin:~> cp -R /mnt/camera/* images/
robin:~> umount /mnt/camera
Likewise, a jazz drive may be mounted on /mnt/jazz.
Appropriate lines should be added in
/etc/modules.conf
and
/etc/fstab
to make this work. Refer to specific hardware HOWTOs for more information. On
systems with a 2.6.x kernel or higher, you may also want to check the man pages
for modprobe and modprobe.conf.
2.4. Backing up data using a tape device
This is done using tar (see above). The
mt
tool is used for controlling the magnetic tape device, like /dev/st0. Entire
books have been written about tape backup, therefore, refer to our reading-list
in Appendix B,_DOS_versus_Linux_commands for more information. Keep in mind
that databases might need other backup procedures because of their
architecture.
The appropriate backup commands are usually put in one of the cron directories
in order to have them executed on a regular basis. In larger environments, the
freely available Amanda backup suite or a commercial solution may be
implemented to back up multiple machines. Working with tapes, however, is a
system administration task beyond the scope of this document.
2.5. Tools from your distribution
Most Linux distributions offer their own tools for making your life easy. A
short overview:
* SuSE: YaST now includes expanded backup and restore modules.
* RedHat: the File Roller tool provides visual management of (compressed)
archives. They seem to be in favour of the X-CD-Roast tool for moving backups
to an external device.
* Mandrake: X-CD-Roast.
* Most distributions come with the BSD
dump
and
restore
utilities for making backups of ext2 and ext3 file systems. This tool can
write to a variety of devices and literally dumps the file(s) or file system
bit per bit onto the specified device. Like dd, this allows for backing up
special file types such as the ones in /dev.
3. Using rsync
3.1. Introduction
The
rsync
program is a fast and flexible tool for remote backup. It is common on UNIX
and UNIX-like systems, easy to configure and use in scripts. While the r in
rsync stands for “remoteâ€, you do not need to take this all too literally. Your
“remote†device might just as well be a USB storage device or another partition
on your hard disk, you do not need to have two separated machines.
3.2. An example: rsync to a USB storage device
As discussed in Section 1.2.3,_“Mount_pointsâ€, we will first have to mount the
device. Possibly, this should be done as root:
root@theserver# mkdir /mnt/usbstore
root@theserver# mount -t vfat /dev/sda1 /mnt/usbstore
Userfriendly
More and more distributions give access to removable devices for non-prilileged
users and mount USB devices, CD-ROMs and other removable devices automatically.
Note that this guideline requires USB support to be installed on your system.
See the_USB_Guide for help if this does not work. Check with dmesg that /dev/
sda1 is indeed the device to mount.
Then you can start the actual backup, for instance of the /home/karl directory:
karl@theserver:~> rsync -avz /home/karl/ /mnt/usbstore
As usual, refer to the man pages for more.
4. Encryption
4.1. General remarks
4.1.1. Why should you encrypt data?
Encryption is synonym to secrecy. In the context of backups, encryption can be
very useful, for instance if you need to leave your backed up data in a place
where you can not control access, such as the server of your provider.
Apart from that, encryption can be applied to E-mails as well: normally, mail
is not encrypted and it is often sent in the open over the netwerk or the
Internet. If your message contains sensitive information, better encrypt it.
4.1.2. GNU Privacy Guard
On Linux systems you will find GnuPG, the GNU Privacy Guard, which is a suite
of programs that are compatible with the PGP (Pretty Good Privacy) tools that
are commercially available.
In this guide we will only discuss the very simple usage of the encryption
tools and show what you will need in order to generate an encryption key and
use it to encrypt data for yourself, which you can then safely store in a
public place. More advanced usage directions can be found in the man pages of
the various commands.
4.2. Generate a key
Before you can start encrypting your data, you need to create a pair of keys.
The pair consists of a private and a public key. You can send the public key to
correspondents, who can use it to encrypt data for you, which you decrypt with
your private key. You always keep the private key, never share it with somebody
else, or they will be able to decrypt data that is only destined for you. Just
to make sure that no accidents happen, the private key is protected with a
password. The key pair is created using this command:
willy@ubuntu:~$ gpg --key-gen
gpg (GnuPG) 1.4.2.2; Copyright (C) 2005 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions. See the file COPYING for details.
gpg: directory `/home/willy.gnupg' created
gpg: new configuration file `/home/willy/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/willy/.gnupg/gpg.conf' are not yet
active during this run
gpg: keyring `/home/willy/.gnupg/secring.gpg' created
gpg: keyring `/home/willy/.gnupg/pubring.gpg' created
Please select what kind of key you want:
(1) DSA and Elgamal (default)
(2) DSA (sign only)
(5) RSA (sign only)
Your selection? 1
DSA keypair will have 1024 bits.
ELG-E keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
0 = key does not expire
<n> = key expires in n days
<n>w = key expires in n weeks
<n>m = key expires in n month
<n>y = key expires in n years
Key is valid for? (0) 0
Key does not expire at all
Is this correct? (y/N) y
You need a user ID to identify your key; the software constructs the
user ID from the Real Name, Comment and Email Address in this form:
"Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"
Real name: Willy De Wandel
Email address: wdw@mvg.vl
Comment: Willem
You selected this USER-ID:
"Willy De Wandel (Willem) <wdw@mvg.vl>"
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.
Passphrase:
Now enetr your password. This can be a phrase, the longer, the better, the only
condition is that you should be able to remember it at all times. For
verification, you need to enter the same phrase again.
Now the key pair is generated by a program that spawns random numbers and that
is, among other factors, fed with the activity data of the system. So it is a
good idea to start some programs now, to move the mouse cursor or to type some
random characters in a terminal window. That way, the chances to generate a
number that contains lots of different digits will be much bigger and the key
will be more difficult to crack.
4.3. About your key
When your key has been created, you will get a message about the
fingerprint
. This is a sequence of 40 hexadecimal numbers, which is so long that it is
very, very hard to generate the same key twice, on any computer. You can be
rather sure that this is a unique sequence. The short form of this key consists
of your name, followed by the last 8 hexadecimal numbers.
You can get information about your key as follows:
willy@ubuntu:~$ gpg --list-keys
/home/willy/.gnupg/pubring.gpg
------------------------------
pub 1024D/BF5C3DBB 2006-08-08
uid Willy De Wandel (Willem) <wdw@mvg.vl>
sub 4096g/A3449CF7 2006-08-08
The key ID of this key is “BF5C3DBBâ€. You can send your key ID and your name to
a key server, so that other people can get this info about you and use it to
encrypt data for you. Alternatively, you can send your public key directly to
the people who need it. The public part of your key is the long series of
numbers that you see when using the --export option to the gpg command:
gpg --export -a
However, as far is this guide is concerned, we assume that you only need your
key in order to encrypt and decrypt data for yourself. Read the gpg man pages
if you want to know more.
4.4. Encrypt data
Now you can encrypt a .tar archive or a compressed archive, prior to saving it
to a backup medium or transporting it to the backup server. Use the gpg command
like this:
gpg -e -r (part of) uid archive
The -e option tells gpg to encrypt, the -r option indicates who to encrypt for.
Keep in mind that only only the user name(s) following this -r option will be
able to decrypt the data again. An example:
willy@ubuntu:~$ gpg -e -r Willy /var/tmp/home-willy-20060808.tar
4.5. Decrypting files
Using the -d option, you can decrypt files that have been encrypted for you.
The data will scroll over your screen, but an encrypted copy will remain on
disk. So for file formats other than plain text, you will want to save the
decrypted data, so that you can view them with the appropriate program. This is
done using the -o option to the gpg command:
willy@ubuntu:~$ gpg -d -o /var/tmp/home-willy-decrypt.tar /var/tmp/home-
willy-20060808.tar.gpg
You need a passphrase to unlock the secret key for
user: "Willy De Wandel (Willem) <wdw@mvg.vl>"
4096 ELG-E key, ID A3449CF7, created 2006-08-08 (main key ID BF5C3DBB)
gpg: encrypted with 4096-bit ELG-E key, ID A3449CF7, created 2006-08-08
"Willy De Wandel (Willem) <wdw@mvg.vl>"
No password = no data
If you can not remember your password, the data is lost. Not even the system
administrator will be able to decrypt the data. That is why a copy of important
keys is sometimes kept in a sealed vault in a bank.
5. Summary
Here's a list of the commands involving file backup:
Table 9.1. New commands in chapter 9: Backup
____________________________________________________________________________
|Command___|Meaning__________________________________________________________|
|bzip2_____|A_block-sorting_file_compressor._________________________________|
|cdrecord__|Record_audio_or_data_Compact_Disks_from_a_master.________________|
|dd________|Convert_and_copy_a_file__________________________________________|
|fdformat__|Low-level_formats_a_floppy_disk._________________________________|
|gpg_______|Encrypt_and_decrypt_data.________________________________________|
|gzip______|Compress_or_expand_files.________________________________________|
|mcopy_____|Copy_MSDOS_files_to/from_UNIX.___________________________________|
|mdir______|Display_an_MSDOS_directory.______________________________________|
|mformat___|Add_an_MSDOS_file_system_to_a_low-level_formatted_floppy_disk.___|
|mkbootdisk|Creates_a_stand-alone_boot_floppy_for_the_running_system.________|
|mount |Mount a file system (integrate it with the current file system by|
|__________|connecting_it_to_a_mount_point)._________________________________|
|rsync_____|Synchronize_directories._________________________________________|
|tar |Tape archiving utility, also used for making archives on disk |
|__________|instead_of_on_tape.______________________________________________|
|umount____|Unmount_file_systems.____________________________________________|
6. Exercises
* Make a backup copy of your home directory in /var/tmp using the tar command.
Then further compress the file using gzip or bzip2. Make it a clean tarred
file, one that doesn't make a mess when unpacking.
* Format a floppy and put some files from your home directory on it. Switch
floppies with another trainee and recover his/her floppy in your home
directory.
* DOS format the floppy. Use the mtools to put and delete files on it.
* What happens to an unformatted floppy when you want to mount it into the file
system?
* If you have any USB storage, try to put a file on it.
* Using rsync, make a copy of your home directory to another local or remote
file system.
* When leaving files on a network server, it's best to encrypt them. Make a tar
archive of your home directory and encrypt it.
Chapter 10. Networking
Table of Contents
1._Networking_Overview
1.1._The_OSI_Model
1.2._Some_popular_networking_protocols
2._Network_configuration_and_information
2.1._Configuration_of_network_interfaces
2.2._Network_configuration_files
2.3._Network_configuration_commands
2.4._Network_interface_names
2.5._Checking_the_host_configuration_with_netstat
2.6._Other_hosts
3._Internet/Intranet_applications
3.1._Server_types
3.2._Mail
3.3._Web
3.4._File_Transfer_Protocol
3.5._Chatting_and_conferencing
3.6._News_services
3.7._The_Domain_Name_System
3.8._DHCP
3.9._Authentication_services
4._Remote_execution_of_applications
4.1._Introduction
4.2._Rsh,_rlogin_and_telnet
4.3._The_X_Window_System
4.4._The_SSH_suite
4.5._VNC
4.6._The_rdesktop_protocol
4.7._Cygwin
5._Security
5.1._Introduction
5.2._Services
5.3._Update_regularly
5.4._Firewalls_and_access_policies
5.5._Intrusion_detection
5.6._More_tips
5.7._Have_I_been_hacked?
5.8._Recovering_from_intrusion
6._Summary
7._Exercises
7.1._General_networking
7.2._Remote_connections
7.3._Security
Abstract
When it comes to networking, Linux is your operating system of choice, not only
because networking is tightly integrated with the OS itself and a wide variety
of free tools and applications are available, but for the robustness under
heavy loads that can only be achieved after years of debugging and testing in
an Open Source project.
Bookshelves full of information have been written about Linux and networking,
but we will try to give an overview in this chapter. After completing this, you
will know more about
* Supported networking protocols
* Network configuration files
* Commands for configuring and probing the network
* Daemons and client programs enabling different network applications
* File sharing and printing
* Remote execution of commands and applications
* Basic network interconnection
* Secure execution of remote applications
* Firewalls and intrusion detection
1. Networking Overview
1.1. The OSI Model
A protocol is, simply put, a set of rules for communication.
In order to get data over the network, for instance an E-mail from your
computer to some computer at the other end of the world, lots of different
hard- and software needs to work together.
All these pieces of hardware and the different software programs speak
different languages. Imagine your E-mail program: it is able to talk to the
computer operating system, through a specific protocol, but it is not able to
talk to the computer hardware. We need a special program in the operating
system that performs this function. In turn, the computer needs to be able to
communicate with the telephone line or other Internet hookup method. And behind
the scenes, network connection hardware needs to be able to communicate in
order to pass your E-mail from one appliance to the other, all the way to the
destination computer.
All these different types of communication protocols are classified in 7
layers, which are known as the
Open Systems Interconnection Reference Model
, the
OSI Model
for short. For easy understanding, this model is reduced to a 4-layer protocol
description, as described in the table below:
Table 10.1. The simplified OSI Model
______________________________________________
|Layer_name__________|Layer_Protocols__________|
|Application_layer___|HTTP,_DNS,_SMTP,_POP,_...|
|Transport_layer_____|TCP,_UDP_________________|
|Network_layer_______|IP,_IPv6_________________|
|Network_access_layer|PPP,_PPPoE,_Ethernet_____|
Each layer can only use the functionality of the layer below; each layer can
only export functionality to the layer above. In other words: layers
communicate only with adjacent layers. Let's take the example of your E-mail
message again: you enter it through the application layer. In your computer, it
travels down the transport and network layer. Your computer puts it on the
network through the network access layer. That is also the layer that will move
the message around the world. At the destination, the receiving computer will
accept the message through it's own network layer, and will display it to the
recepient using the transport and application layer.
It's really much more complicated
The above and following sections are included because you will come across some
networking terms sooner or later; they will give you some starting points,
should you want to find out about the details.
1.2. Some popular networking protocols
Linux supports many different networking protocols. We list only the most
important:
1.2.1. TCP/IP
The
Transport Control Protocol
and the
Internet Protocol
are the two most popular ways of communicating on the Internet. A lot of
applications, such as your browser and E-mail program, are built on top of this
protocol suite.
Very simply put, IP provides a solution for sending packets of information from
one machine to another, while TCP ensures that the packets are arranged in
streams, so that packets from different applications don't get mixed up, and
that the packets are sent and received in the correct order.
A good starting point for learning more about TCP and IP is in the following
documents:
* man 7 ip: Describes the IPv4 protocol implementation on Linux (version 4
currently being the most wide-spread edition of the IP protocol).
* man 7 tcp: Implementation of the TCP protocol.
* RFC793, RFC1122, RFC2001 for TCP, and RFC791, RFC1122 and RFC1112 for IP.
The Request_For_Comments documents contain the descriptions of networking
standards, protocols, applications and implementation. These documents are
managed by the Internet Engineering Task Force, an international community
concerned with the smooth operation of the Internet and the evolution and
development of the Internet architecture.
Your ISP usually has an RFC archive available, or you can browse the RFCs via
http://www.ietf.org/rfc.html.
1.2.2. TCP/IPv6
Nobody expected the Internet to grow as fast as it does. IP proved to have
quite some disadvantages when a really large number of computers is in a
network, the most important being the availability of unique addresses to
assign to each machine participating. Thus, IP version 6 was deviced to meet
the needs of today's Internet.
Unfortunately, not all applications and services support IPv6, yet. A migration
is currently being set in motion in many environments that can benefit from an
upgrade to IPv6. For some applications, the old protocol is still used, for
applications that have been reworked the new version is already active. So when
checking your network configuration, sometimes it might be a bit confusing
since all kinds of measures can be taken to hide one protocol from the other so
as the two don't mix up connections.
More information can be found in the following documents:
* man 7 ipv6: the Linux IPv6 protocol implementation.
* RFC1883 describing the IPv6 protocol.
1.2.3. PPP, SLIP, PLIP, PPPOE
The Linux kernel has built-in support for PPP (Point-to-Point-Protocol), SLIP
(Serial Line IP), PLIP (Parallel Line IP) and PPPP Over EThernet. PPP is the
most popular way individual users access their ISP (Internet Service Provider),
although in densely populated areas it is often being replaced by PPPOE, the
protocol used for ADSL (Asymmetric Digital Subscriber Line) connections.
Most Linux distributions provide easy-to-use tools for setting up an Internet
connection. The only thing you basically need is a username and password to
connect to your Internet Service Provider (ISP), and a telephone number in the
case of PPP. These data are entered in the graphical configuration tool, which
will likely also allow for starting and stopping the connection to your
provider.
1.2.4. ISDN
The Linux kernel has built-in ISDN capabilities. Isdn4linux controls ISDN PC
cards and can emulate a modem with the Hayes command set (“AT†commands). The
possibilities range from simply using a terminal program to full connection to
the Internet.
Check your system documentation.
1.2.5. AppleTalk
Appletalk is the name of Apple's internetworking stack. It allows a peer-to-
peer network model which provides basic functionality such as file and printer
sharing. Each machine can simultaneously act as a client and a server, and the
software and hardware necessary are included with every Apple computer.
Linux provides full AppleTalk networking. Netatalk is a kernel-level
implementation of the AppleTalk Protocol Suite, originally for BSD-derived
systems. It includes support for routing AppleTalk, serving UNIX and AFS file
systems using AppleShare and serving UNIX printers and accessing AppleTalk
printers.
1.2.6. SMB/NMB
For compatibility with MS Windows environments, the Samba suite, including
support for the NMB and SMB protocols, can be installed on any UNIX-like
system. The Server Message Block protocol (also called Session Message Block,
NetBIOS or LanManager protocol) is used on MS Windows 3.11, NT, 95/98, 2K and
XP to share disks and printers.
The basic functions of the Samba suite are: sharing Linux drives with Windows
machines, accessing SMB shares from Linux machines, sharing Linux printers with
Windows machines and sharing Windows printers with Linux machines.
Most Linux distributions provide a samba package, which does most of the server
setup and starts up smbd, the Samba server, and nmbd, the netbios name server,
at boot time by default. Samba can be configured graphically, via a web
interface or via the command line and text configuration files. The daemons
make a Linux machine appear as an MS Windows host in an MS Windows My Network
Places/Network Neighbourhood window; a share from a Linux machine will be
indistinguishable from a share on any other host in an MS Windows environment.
More information can be found at the following locations:
* man
smb.conf
: describes the format of the main Samba configuration file.
* The Samba_Project_Documentation (or check your local samba.org mirror)
contains an easy to read installation and testing guide, which also explains
how to configure your Samba server as a Primary Domain Controller. All the
man pages are also available here.
1.2.7. Miscellaneous protocols
Linux also has support for Amateur Radio, WAN internetworking (X25, Frame
Relay, ATM), InfraRed and other wireless connections, but since these protocols
usually require special hardware, we won't discuss them in this document.
2. Network configuration and information
2.1. Configuration of network interfaces
All the big, userfriendly Linux distributions come with various graphical
tools, allowing for easy setup of the computer in a local network, for
connecting it to an Internet Service Provider or for wireless access. These
tools can be started up from the command line or from a menu:
* Ubuntu configuration is done selecting System → Administration → Networking.
* RedHat Linux comes with redhat-config-network, which has both a graphical and
a text mode interface.
* Suse's YAST or YAST2 is an all-in-one configuration tool.
* Mandrake/Mandriva comes with a Network and Internet Configuration Wizard,
which is preferably started up from Mandrake's Control Center.
* On Gnome systems: gnome-network-preferences.
* On KDE systems: knetworkconf.
Your system documentation provides plenty of advice and information about
availability and use of tools.
Information that you will need to provide:
* For connecting to the local network, for instance with your home computers,
or at work: hostname, domainname and IP address. If you want to set up your
own network, best do some more reading first. At work, this information is
likely to be given to your computer automatically when you boot it up. When
in doubt, it is better not to specify any information than making it up.
* For connecting to the Internet: username and password for your ISP, telephone
number when using a modem. Your ISP usually automatically assigns you an IP
address and all the other things necessary for your Internet applications to
work.
2.2. Network configuration files
The graphical helper tools edit a specific set of network configuration files,
using a couple of basic commands. The exact names of the configuration files
and their location in the file system is largely dependent on your Linux
distribution and version. However, a couple of network configuration files are
common on all UNIX systems:
2.2.1. /etc/hosts
The
/etc/hosts
file always contains the
localhost
IP address, 127.0.0.1, which is used for interprocess communication. Never
remove this line! Sometimes contains addresses of additional hosts, which can
be contacted without using an external naming service such as DNS (the Domain
Name Server).
A sample hosts file for a small home network:
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 localhost.localdomain localhost
192.168.52.10 tux.mylan.com tux
192.168.52.11 winxp.mylan.com winxp
Read more in man hosts.
2.2.2. /etc/resolv.conf
The
/etc/resolv.conf
file configures access to a DNS server, see Section 3.7,_“The_Domain_Name
Systemâ€. This file contains your domain name and the name server(s) to contact:
search mylan.com
nameserver 193.134.20.4
Read more in the resolv.conf man page.
2.2.3. /etc/nsswitch.conf
The
/etc/nsswitch.conf
file defines the order in which to contact different name services. For
Internet use, it is important that dns shows up in the “hosts†line:
[bob@tux ~] grep hosts /etc/nsswitch.conf
hosts: files dns
This instructs your computer to look up hostnames and IP addresses first in the
/etc/hosts file, and to contact the DNS server if a given host does not occur
in the local hosts file. Other possible name services to contact are LDAP, NIS
and NIS+.
More in man nsswitch.conf.
2.3. Network configuration commands
2.3.1. The ip command
The distribution-specific scripts and graphical tools are front-ends to
ip
(or
ifconfig
and
route
on older systems) to display and configure the kernel's networking
configuration.
The ip command is used for assigning IP addresses to interfaces, for setting up
routes to the Internet and to other networks, for displaying TCP/IP
configurations etcetera.
The following commands show IP address and routing information:
benny@home benny> ip addr show
1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
inet6 ::1/128 scope host
2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
link/ether 00:50:bf:7e:54:9a brd ff:ff:ff:ff:ff:ff
inet 192.168.42.15/24 brd 192.168.42.255 scope global eth0
inet6 fe80::250:bfff:fe7e:549a/10 scope link
benny@home benny> ip route show
192.168.42.0/24 dev eth0 scope link
127.0.0.0/8 dev lo scope link
default via 192.168.42.1 dev eth0
Things to note:
* two network interfaces, even on a system that has only one network interface
card: “lo†is the local loop, used for internal network communication; “eth0â€
is a common name for a real interface. Do not ever change the local loop
configuration, or your machine will start mallfunctioning! Wireless
interfaces are usually defined as “wlan0â€; modem interfaces as “ppp0â€, but
there might be other names as well.
* IP addresses, marked with “inetâ€: the local loop always has 127.0.0.1, the
physical interface can have any other combination.
* The hardware address of your interface, which might be required as part of
the authentication procedure to connect to a network, is marked with “etherâ€.
The local loop has 6 pairs of all zeros, the physical loop has 6 pairs of
hexadecimal characters, of which the first 3 pairs are vendor-specific.
2.3.2. The ifconfig command
While ip is the most novel way to configure a Linux system, ifconfig is still
very popular. Use it without option for displaying network interface
information:
els@asus:~$ /sbin/ifconfig
eth0 Link encap:Ethernet HWaddr 00:50:70:31:2C:14
inet addr:60.138.67.31 Bcast:66.255.255.255 Mask:255.255.255.192
inet6 addr: fe80::250:70ff:fe31:2c14/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:31977764 errors:0 dropped:0 overruns:0 frame:0
TX packets:51896866 errors:0 dropped:0 overruns:0 carrier:0
collisions:802207 txqueuelen:1000
RX bytes:2806974916 (2.6 GiB) TX bytes:2874632613 (2.6 GiB)
Interrupt:11 Base address:0xec00
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:765762 errors:0 dropped:0 overruns:0 frame:0
TX packets:765762 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:624214573 (595.2 MiB) TX bytes:624214573 (595.2 MiB)
Here, too, we note the most important aspects of the interface configuration:
* The IP address is marked with “inet addrâ€.
* The hardware address follows the “HWaddr†tag.
Both ifconfig and ip display more detailed configuration information and a
number of statistics about each interface and, maybe most important, whether it
is “UP†and “RUNNINGâ€.
2.3.3. PCMCIA commands
On your laptop which you usually connect to the company network using the
onboard Ethernet connection, but which you are now to configure for dial-in at
home or in a hotel, you might need to activate the PCMCIA card. This is done
using the
cardctl
control utility, or the
pccardctl
on newer distributions.
A usage example:
cardctl insert
Now the card can be configured, either using the graphical or the command line
interface. Prior to taking the card out, use this command:
cardctl eject
However, a good distribution should provide PCMCIA support in the network
configuration tools, preventing users from having to execute PCMCIA commands
manually.
2.3.4. More information
Further discussion of network configuration is out of the scope of this
document. Your primary source for extra information is the man pages for the
services you want to set up. Additional reading:
* The_Modem-HOWTO: Help with selecting, connecting, configuring, trouble-
shooting, and understanding analog modems for a PC.
* LDP_HOWTO_Index,_section_4.4: categorized list of HOWTOs about general
networking, protocols, dial-up, DNS, VPNs, bridging, routinfg, security and
more.
* Most systems have a version of the ip-cref file (locate it using the locate
command); the PS format of this file is viewable with for instance gv.
2.4. Network interface names
On a Linux machine, the device name lo or the local loop is linked with the
internal 127.0.0.1 address. The computer will have a hard time making your
applications work if this device is not present; it is always there, even on
computers which are not networked.
The first ethernet device, eth0 in the case of a standard network interface
card, points to your local LAN IP address. Normal client machines only have one
network interface card. Routers, connecting networks together, have one network
device for each network they serve.
If you use a modem to connect to the Internet, your network device will
probably be named ppp0.
There are many more names, for instance for Virtual Private Network interfaces
(VPNs), and multiple interfaces can be active simultaneously, so that the
output of the ifconfig or ip commands might become quite extensive when no
options are used. Even multiple interfaces of the same type can be active. In
that case, they are numbered sequentially: the first will get the number 0, the
second will get a suffix of 1, the third will get 2, and so on. This is the
case on many application servers, on machines which have a failover
configuration, on routers, firewalls and many more.
2.5. Checking the host configuration with netstat
Apart from the ip command for displaying the network configuration, there's the
common netstat command which has a lot of options and is generally useful on
any UNIX system.
Routing information can be displayed with the -nr option to the netstat
command:
bob:~> netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.42.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 192.168.42.1 0.0.0.0 UG 40 0 0 eth0
This is a typical client machine in an IP network. It only has one network
device, eth0. The lo interface is the local loop.
The modern way
The novel way to get this info from your system is by using the ip command:
ip route show
When this machine tries to contact a host that is on another network than its
own, indicated by the line starting with 0.0.0.0, it will send the connection
requests to the machine (router) with IP address 192.168.42.1, and it will use
its primary interface, eth0, to do this.
Hosts that are on the same network, the line starting with 192.168.42.0, will
also be contacted through the primary network interface, but no router is
necessary, the data are just put on the network.
Machines can have much more complicated routing tables than this one, with lots
of different “Destination-Gateway†pairs to connect to different networks. If
you have the occasion to connect to an application server, for instance at
work, it is most educating to check the routing information.
2.6. Other hosts
An impressive amount of tools is focused on network management and remote
administration of Linux machines. Your local Linux software mirror will offer
plenty of those. It would lead us too far to discuss them in this document, so
please refer to the program-specific documentation.
We will only discuss some common UNIX/Linux text tools in this section.
2.6.1. The host command
To display information on hosts or domains, use the host command:
[emmy@pc10 emmy]$ host www.eunet.be
www.eunet.be. has address 193.74.208.177
[emmy@pc10 emmy]$ host -t any eunet.be
eunet.be. SOA dns.eunet.be. hostmaster.Belgium.EU.net.
2002021300 28800 7200 604800 86400
eunet.be. mail is handled by 50 pophost.eunet.be.
eunet.be. name server ns.EU.net.
eunet.be. name server dns.eunet.be.
Similar information can be displayed using the
dig
command, which gives additional information about how records are stored in
the name server.
2.6.2. The ping command
To check if a host is alive, use
ping
. If your system is configured to send more than one packet, interrupt ping
with the Ctrl+C key combination:
[emmy@pc10 emmy]$ ping a.host.be
PING a.host.be (1.2.8.3) from 80.20.84.26: 56(84) bytes of data.
64 bytes from a.host.be(1.2.8.3):icmp_seq=0 ttl=244 time=99.977msec
--- a.host.be ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/mdev = 99.977/99.977/99.977/0.000 ms
2.6.3. The traceroute command
To check the route that packets follow to a network host, use the
traceroute
command:
[emmy@pc10 emmy]$ /usr/sbin/traceroute www.eunet.be
traceroute to www.eunet.be(193.74.208.177),30 hops max,38b packets
1 blob (10.0.0.1)
0.297ms 0.257ms 0.174ms
2 adsl-65.myprovider.be (217.136.111.1)
12.120ms 13.058ms 13.009ms
3 194.78.255.177 (194.78.255.177)
13.845ms 14.308ms 12.756ms
4 gigabitethernet2-2.intl2.gam.brussels.skynet.be (195.238.2.226)
13.123ms 13.164ms 12.527ms
5 pecbru2.car.belbone.be (194.78.255.118)
16.336ms 13.889ms 13.028ms
6 ser-2-1-110-ias-be-vil-ar01.kpnbelgium.be (194.119.224.9)
14.602ms 15.546ms 15.959ms
7 unknown-195-207-939.eunet.be (195.207.93.49)
16.514ms 17.661ms 18.889ms
8 S0-1-0.Leuven.Belgium.EU.net (195.207.129.1)
22.714ms 19.193ms 18.432ms
9 dukat.Belgium.EU.net (193.74.208.178) 22.758ms * 25.263ms
On some systems, traceroute has been renamed to tracepath.
2.6.4. The whois command
Specific domain name information can be queried using the
whois
command, as is explained by many whois servers, like the one below:
[emmy@pc10 emmy]$ whois cnn.com
[whois.crsnic.net]
Whois Server Version 1.3
$<--snap server message-->
Domain Name: CNN.COM
Registrar: NETWORK SOLUTIONS, INC.
Whois Server: whois.networksolutions.com
Referral URL: http://www.networksolutions.com
Name Server: TWDNS-01.NS.AOL.COM
Name Server: TWDNS-02.NS.AOL.COM
Name Server: TWDNS-03.NS.AOL.COM
Name Server: TWDNS-04.NS.AOL.COM
Updated Date: 12-mar-2002
>>> Last update of whois database: Fri, 5 Apr 2002 05:04:55 EST <<<
The Registry database contains ONLY .COM, .NET, .ORG, .EDU domains
and Registrars.
[whois.networksolutions.com]
$<--snap server message-->
Registrant:
Turner Broadcasting (CNN-DOM)
1 CNN Center
Atlanta, GA 30303
Domain Name: CNN.COM
Administrative Contact:
$<--snap contactinfo-->
Technical Contact:
$<--snap contactinfo-->
Billing Contact:
$<--snap contactinfo-->
Record last updated on 12-Mar-2002.
Record expires on 23-Sep-2009.
Record created on 22-Sep-1993.
Database last updated on 4-Apr-2002 20:10:00 EST.
Domain servers in listed order:
TWDNS-01.NS.AOL.COM 149.174.213.151
TWDNS-02.NS.AOL.COM 152.163.239.216
TWDNS-03.NS.AOL.COM 205.188.146.88
TWDNS-04.NS.AOL.COM 64.12.147.120
For other domain names than .com, .net, .org and .edu, you might need to
specify the whois server, such as this one for .be domains:
whois domain.be@whois.dns.be
3. Internet/Intranet applications
The Linux system is a great platform for offering networking services. In this
section, we will try to give an overview of most common network servers and
applications.
3.1. Server types
3.1.1. Standalone server
Offering a service to users can be approached in two ways. A daemon or service
can run in standalone mode, or it can be dependent on another service to be
activated.
Network services that are heavily and/or continuously used, usually run in the
standalone mode: they are independent program daemons that are always running.
They are most likely started up at system boot time, and they wait for requests
on the specific connection points or ports for which they are set up to listen.
When a request comes, it is processed, and the listening continues until the
next request. A web server is a typical example: you want it to be available 24
hours a day, and if it is too busy it should create more listening instances to
serve simultaneous users. Other examples are the large software archives such
as Sourceforge or your_Tucows_mirror, which must handle thousands of FTP
requests per day.
An example of a standalone network service on your home computer might be the
named (name daemon), a caching name server. Standalone services have their own
processes running, you can check any time using ps:
bob:~> ps auxw | grep named
named 908 0.0 1.0 14876 5108 ? S Mar14 0:07 named -u named
However, there are some services that you can use on your PC, even if there is
no server process running for that services. Examples could be the FTP service,
the secure copy service or the finger service. Those services have the Internet
Daemon (inetd) listening in their place.
3.1.2. (x)inetd
On your home PC, things are usually a bit calmer. You may have a small network,
for instance, and you may have to transfer files from one PC to another from
time to time, using FTP or Samba (for connectivity with MS Windows machines).
In those cases, starting all the services which you only need occasionally and
having them run all the time would be a waste of resources. So in smaller
setups, you will find the necessary daemons dependent on a central program,
that listen on all the ports of the services for which it is responsible.
This super-server, the Internet services daemon, is started up at system
initialization time. There are two common implementations:
inetd
and
xinetd
(the extended Internet services daemon). One or the other is usually running
on every Linux system:
bob:~> ps -ef | grep inet
root 926 1 0 Mar14 ? 00:00:00 xinetd-ipv6 -stayalive -reuse \
-pidfile /var/run/xinetd.pid
The services for which the Internet daemon is responsible, are listed in its
configuration file,
/etc/inetd.conf
, for inetd, and in the directory
/etc/xinetd.d
for xinetd. Commonly managed services include file share and print services,
SSH, FTP, telnet, the Samba configuration daemon, talk and time services.
As soon as a connection request is received, the central server will start an
instance of the required server. Thus, in the example below, when user bob
starts an FTP session to the local host, an FTP daemon is running as long as
the session is active:
bob:~> ps auxw | grep ftp
bob 793 0.1 0.2 3960 1076 pts/6 S 16:44 0:00 ncftp localhost
ftp 794 0.7 0.5 5588 2608 ? SN 16:44 0:00 ftpd:
localhost.localdomain: anonymous/bob@his.server.com: IDLE
Of course, the same happens when you open connections to remote hosts: either a
daemon answers directly, or a remote (x)inetd starts the service you need and
stops it when you quit.
3.2. Mail
3.2.1. Servers
Sendmail is the standard mail server program or Mail Transport Agent for UNIX
platforms. It is robust, scalable, and when properly configured with
appropriate hardware, handles thousands of users without blinking. More
information about how to configure Sendmail is included with the sendmail and
sendmail-cf packages, you may want to read the README and README.cf files in /
usr/share/doc/sendmail. The man sendmail and man aliases are also useful.
Qmail is another mail server, gaining popularity because it claims to be more
secure than Sendmail. While Sendmail is a monolithic program, Qmail consists of
smaller interacting program parts that can be better secured. Postfix is
another mail server which is gaining popularity.
These servers handle mailing lists, filtering, virus scanning and much more.
Free and commercial scanners are available for use with Linux. Examples of
mailing list software are Mailman, Listserv, Majordomo and EZmlm. See the web
page of your favorite virus scanner for information on Linux client and server
support. Amavis and Spamassassin are free implementations of a virus scanner
and a spam scanner.
3.2.2. Remote mail servers
The most popular protocols to access mail remotely are
POP3
and
IMAP4
. IMAP and POP both allow offline operation, remote access to new mail and they
both rely on an SMTP server to send mail.
While POP is a simple protocol, easy to implement and supported by almost any
mail client, IMAP is to be preferred because:
* It can manipulate persistent message status flags.
* It can store as well as fetch mail messages.
* It can access and manage multiple mailboxes.
* It supports concurrent updates and shared mailboxes.
* It is also suitable for accessing Usenet messages and other documents.
* IMAP works both on-line and off-line.
* it is optimized for on-line performance, especially over low-speed links.
3.2.3. Mail user-agents
There are plenty of both text and graphical E-mail clients, we'll just name a
few of the common ones. Pick your favorite.
The UNIX mail command has been around for years, even before networking
existed. It is a simple interface to send messages and small files to other
users, who can then save the message, redirect it, reply to it etcetera.
While it is not commonly used as a client anymore, the mail program is still
useful, for example to mail the output of a command to somebody:
mail <future.employer@whereIwant2work.com> < cv.txt
The elm mail reader is a much needed improvement to mail, and so is pine (Pine
Is Not ELM). The mutt mail reader is even more recent and offers features like
threading.
For those users who prefer a graphical interface to their mail (and a tennis
elbow or a mouse arm), there are hundreds of options. The most popular for new
users are Mozilla Mail/Thunderbird, which has easy anti-spam configuring
options, and Evolution, the MS Outlook clone. Kmail is popular among KDE users.
Figure 10.1. Evolution mail and news reader
Evolution E-mail client looks just like MS Outlook.
There are also tens of web mail applications available, such as Squirrelmail,
Yahoo! mail, gmail from Google and Hotmail.
An overview is available via the Linux_Mail_User_HOWTO.
Most Linux distributions include
fetchmail
, a mail-retrieval and forwarding utility. It fetches mail from remote mail
servers (POP, IMAP and some others) and forwards it to your local delivery
system. You can then handle the retrieved mail using normal mail clients. It
can be run in daemon mode to repeatedly poll one or more systems at a specified
interval. Information and usage examples can be found in the Info pages; the
directory /usr/share/doc/fetchmail[-<version>] contains a full list of features
and a FAQ for beginners.
The
procmail
filter can be used for filtering incoming mail, to create mailing lists, to
pre-process mail, to selectively forward mail and more. The accompanying
formail
program, among others, enables generation of auto-replies and splitting up
mailboxes. Procmail has been around for years on UNIX and Linux machines and is
a very robust system, designed to work even in the worst circumstances. More
information may be found in the /usr/share/doc/procmail[-<version>] directory
and in the man pages.
A note on E-mail Etiquette
Some people these days seem to think that an E-mail message shouldn't be too
formal. That depends, of course. If you are writing to someone you don't know,
best to keep some distance, just like you would do in a traditional letter. And
don't forget: people you don't know might be male or female...
3.3. Web
3.3.1. The Apache Web Server
Apache is by far the most popular web server, used on more than half of all
Internet web servers. Most Linux distributions include Apache. Apache's
advantages include its modular design, SSL support, stability and speed. Given
the appropriate hardware and configuration it can support the highest loads.
On Linux systems, the server configuration is usually done in the /etc/httpd
directory. The most important configuration file is httpd.conf; it is rather
self-explanatory. Should you need help, you can find it in the httpd man page
or on the Apache_website.
3.3.2. Web browsers
A number of web browsers, both free and commercial, exist for the Linux
platform. Netscape Navigator as the only decent option has long been a thing of
the past, as Mozilla/Firefox offers a competitive alternative running on many
other operating systems, like MS Windows and MacOS X as well.
Amaya is the W3C browser. Opera is a commercial browser, compact and fast. Many
desktop managers offer web browsing features in their file manager, like
nautilus.
Among the popular text based browsers are lynx and links. You may need to
define proxy servers in your shell, by setting the appropriate variables. Text
browsers are fast and handy when no graphical environment is available, such as
when used in scripts.
3.3.3. Proxy servers
3.3.3.1. What is a proxy server?
Companies and organizations often want their users to use a proxy server.
Especially in environments with lots of users, a proxy server can enable faster
downloads of web pages. The proxy server stores web pages. When a user asks for
a web page that has already been requested previously, the proxy server will
give that page to the user directly, so that s/he does not need to get it from
the Internet, which would take longer. Of course, measures can be taken so that
the proxy server does a quick check and always serves the most recent version
of a page. In some environments, usage of the proxy server is compulsory, in
other environments you may have the choice whether or not to use it.
3.3.3.2. Proxy configuration
If you have the proxy server name and port, it should be rather obvious to feed
that information into your browser. However, many (command line) applications
depend on the variables
http_proxy
and
ftp_proxy
for correct functioning. For your convenience, you might want to add a line
like the following to your
~/.bashrc
:
export http_proxy=http://username:password@proxy_server_name:port_number
For instance:
export http_proxy=http://willy:Appelsi3ntj3@proxy:80
If you do not need to give a username and password, simply leave out everything
before the “@†sign, this sign included.
3.4. File Transfer Protocol
3.4.1. FTP servers
On a Linux system, an FTP server is typically run from xinetd, using the WU-
ftpd server, although the FTP server may be configured as a stand-alone server
on systems with heavy FTP traffic. See the exercises.
Other FTP servers include among others vsftpd, Ncftpd and Proftpd.
Most Linux distributions contain the anonftp package, which sets up an
anonymous FTP server tree and accompanying configuration files.
3.4.2. FTP clients
Most Linux distributions include
ncftp
, an improved version of the common UNIX
ftp
command, which you may also know from the Windows command line. The ncftp
program offers extra features such as a nicer and more comprehensible user
interface, file name completion, append and resume functions, bookmarking,
session management and more:
thomas:~> ncftp blob
NcFTP 3.0.3 (April 15, 2001) by Mike Gleason (ncftp@ncftp.com).
Connecting to blob...
blob.some.net FTP server (Version wu-2.6.1-20) ready.
Logging in...
Guest login ok, access restrictions apply.
Logged in to blob.
ncftp / > help
Commands may be abbreviated. 'help showall' shows hidden and
unsupported commands.
'help <command>' gives a brief description of <command>.
ascii cat help lpage open quote site
bgget cd jobs lpwd page rename type
bgput chmod lcd lrename pdir rhelp umask
bgstart close lchmod lrm pls rm version
binary debug lls lrmdir put rmdir
bookmark dir lmkdir ls pwd set
bookmarks get lookup mkdir quit show
ncftp / >
Excellent help with lot of examples can be found in the man pages. And again, a
number of GUI applications are available.
FTP is insecure!
Don't use the File Transfer Protocol for non-anonymous login unless you know
what you are doing. Your user name and password might be captured by malevolent
fellow network users! Use secure FTP instead; the
sftp
program comes with the Secure SHell suite, see Section 4.4.4,_“Secure_remote
copyingâ€.
3.5. Chatting and conferencing
Various clients and systems are available in each distribution, replacing the
old-style IRC text-based chat. A short and incomplete list of the most popular
programs:
* gaim: multi-protocol instant messaging client for Linux, Windows and Mac,
compatible with MSN Messenger, ICQ, IRC and much more; see the Info pages or
the_Gaim_site for more.
* xchat: IRC client for the X window system:
Figure 10.2. X-Chat
X-Chat
The home page is at SourceForge.
* aMSN: an MSN clone.
* Konversation, kopete, KVIrc and many other K-tools from the KDE suite.
* gnomemeeting: videoconferencing program for UNIX (now Ekiga).
* jabber: Open Source Instant Messenging platform, compatible with ICQ, AIM,
Yahoo, MSN, IRC, SMTP and much more.
* psi: jabber client, see the_PSI_Jabber_Client_Homepage.
* skype: program for making telephone-like calls over the Internet to other
Skype users, see http://www.skype.com for more info. Skype is free but not
open.
* Gizmo: a free (but not open) phone for your computer, see http://
www.gizmoproject.com.
3.6. News services
Running a Usenet server involves a lot of expertise and fine-tuning, so refer
to the INN_homepage for more information.
There are a couple of interesting newsgroups in the comp.* hierarchy, which can
be accessed using a variety of text and graphical clients. A lot of mail
clients support newsgroup browsing as well, check your program or see your
local Open Source software mirror for text clients such as tin, slrnn and mutt,
or download Mozilla or one of a number of other graphical clients.
Deja.com keeps a searchable archive of all newsgroups, powered by Google. This
is a very powerful instrument for getting help: chances are very high that
somebody has encountered your problem, found a solution and posted it in one of
the newsgroups.
3.7. The Domain Name System
All these applications need DNS services to match IP addresses to host names
and vice versa. A DNS server does not know all the IP addresses in the world,
but networks with other DNS servers which it can query to find an unknown
address. Most UNIX systems can run
named
, which is part of the BIND (Berkeley Internet Name Domain) package distributed
by the Internet Software Consortium. It can run as a stand-alone caching
nameserver
, which is often done on Linux systems in order to speed up network access.
Your main client configuration file is
/etc/resolv.conf
, which determines the order in which Domain Name Servers are contacted:
search somewhere.org
nameserver 192.168.42.1
nameserver 193.74.208.137
More information can be found in the Info pages on named, in the /usr/share/
doc/bind[-<version>] files and on the Bind_project homepage. The DNS_HOWTO
covers the use of BIND as a DNS server.
3.8. DHCP
DHCP is the Dynamic Host Configuration Protocol, which is gradually replacing
good old bootp in larger environments. It is used to control vital networking
parameters such as IP addresses and name servers of hosts. DHCP is backward
compatible with bootp. For configuring the server, you will need to read the
HOWTO.
DHCP client machines will usually be configured using a GUI that configures the
dhcpcd
, the DHCP client daemon. Check your system documentation if you need to
configure your machine as a DHCP client.
3.9. Authentication services
3.9.1. Traditional
Traditionally, users are authenticated locally, using the information stored in
/etc/passwd
and
/etc/shadow
on each system. But even when using a network service for authenticating, the
local files will always be present to configure system accounts for
administrative use, such as the root account, the daemon accounts and often
accounts for additional programs and purposes.
These files are often the first candidates for being examined by hackers, so
make sure the permissions and ownerships are strictly set as should be:
bob:~> ls -l /etc/passwd /etc/shadow
-rw-r--r-- 1 root root 1803 Mar 10 13:08 /etc/passwd
-r-------- 1 root root 1116 Mar 10 13:08 /etc/shadow
3.9.2. PAM
Linux can use PAM, the Pluggable Authentication Module, a flexible method of
UNIX authentication. Advantages of PAM:
* A common authentication scheme that can be used with a wide variety of
applications.
* PAM can be implemented with various applications without having to recompile
the applications to specifically support PAM.
* Great flexibility and control over authentication for the administrator and
application developer.
* Application developers do not need to develop their program to use a
particular authentication scheme. Instead, they can focus purely on the
details of their program.
The directory /etc/pam.d contains the PAM configuration files (used to be /etc/
pam.conf). Each application or service has its own file. Each line in the file
has four elements:
* Module:
o auth: provides the actual authentication (perhaps asking for and checking a
password) and sets credentials, such as group membership or Kerberos
tickets.
o account: checks to make sure that access is allowed for the user (the
account has not expired, the user is allowed to log in at this time of day,
and so on).
o password: used to set passwords.
o session: used after a user has been authenticated. This module performs
additional tasks which are needed to allow access (for example, mounting
the user's home directory or making their mailbox available).
The order in which modules are stacked, so that multiple modules can be used,
is very important.
* Control Flags: tell PAM which actions to take upon failure or success. Values
can be required, requisite, sufficient or optional.
* Module Path: path to the pluggable module to be used, usually in /lib/
security.
* Arguments: information for the modules
Shadow password files are automatically detected by PAM.
More information can be found in the pam man pages or at the_Linux-PAM_project
homepage.
3.9.3. LDAP
The Lightweight Directory Access Protocol is a client-server system for
accessing global or local directory services over a network. On Linux, the
OpenLDAP implementation is used. It includes
slapd
, a stand-alone server; slurpd, a stand-alone LDAP replication server;
libraries implementing the LDAP protocol and a series of utilities, tools and
sample clients.
The main benefit of using LDAP is the consolidation of certain types of
information within your organization. For example, all of the different lists
of users within your organization can be merged into one LDAP directory. This
directory can be queried by any LDAP-enabled applications that need this
information. It can also be accessed by users who need directory information.
Other LDAP or X.500 Lite benefits include its ease of implementation (compared
to X.500) and its well-defined Application Programming Interface (API), which
means that the number of LDAP-enabled applications and LDAP gateways should
increase in the future.
On the negative side, if you want to use LDAP, you will need LDAP-enabled
applications or the ability to use LDAP gateways. While LDAP usage should only
increase, currently there are not very many LDAP-enabled applications available
for Linux. Also, while LDAP does support some access control, it does not
possess as many security features as X.500.
Since LDAP is an open and configurable protocol, it can be used to store almost
any type of information relating to a particular organizational structure.
Common examples are mail address lookups, central authentication in combination
with PAM, telephone directories and machine configuration databases.
See your system specific information and the man pages for related commands
such as ldapmodify and ldapsearch for details. More information can be found in
the LDAP_Linux_HOWTO, which discusses installation, configuration, running and
maintenance of an LDAP server on Linux. The author of this Introduction to
Linux document also wrote an LDAP_Operations_HOWTO, describing the basics
everyone should know about when dealing with LDAP management, operations and
integration of services.
4. Remote execution of applications
4.1. Introduction
There are a couple of different ways to execute commands or run programs on a
remote machine and have the output, be it text or graphics, sent to your
workstation. The connections can be secure or insecure. While it is of course
advised to use secure connections instead of transporting your password over
the network unencrypted, we will discuss some practical applications of the
older (unsafe) mechanisms, as they are still useful in a modern networked
environment, such as for troubleshooting or running exotic programs.
4.2. Rsh, rlogin and telnet
The
rlogin
and
rsh
commands for remote login and remote execution of commands are inherited from
UNIX. While seldom used because they are blatantly insecure, they still come
with almost every Linux distribution for backward compatibility with UNIX
programs.
Telnet, on the other hand, is still commonly used, often by system and network
administrators. Telnet is one of the most powerful tools for remote access to
files and remote administration, allowing connections from anywhere on the
Internet. Combined with an X server, remote graphical applications can be
displayed locally. There is no difference between working on the local machine
and using the remote machine.
Because the entire connection is unencrypted, allowing
telnet
connections involves taking high security risks. For normal remote execution
of programs, Secure SHell or
ssh
is advised. We will discuss the secure method later in this section.
However, telnet is still used in many cases. Below are some examples in which a
mail server and a web server are tested for replies:
Checking that a mail server works:
[jimmy@blob ~] telnet mailserver 25
Trying 192.168.42.1...
Connected to mailserver.
Escape character is '^]'.
220 m1.some.net ESMTP Sendmail 8.11.6/8.11.6; 200302281626
ehlo some.net
250-m1.some.net Hello blob.some.net [10.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE
250-DSN
250-ONEX
250-ETRN
250-XUSR
250 HELP
mail from: jimmy@some.net
250 2.1.0 jimmy@some.net... Sender ok
rcpt to: davy@some.net
250 2.1.5 davy@some.net... Recipient ok
data
354 Enter mail, end with "." on a line by itself
test
.
250 2.0.0 g2MA1R619237 Message accepted for delivery
quit
221 2.0.0 m1.some.net closing connection
Connection closed by foreign host.
Checking that a web server answers to basic requests:
[jimmy@blob ~] telnet www.some.net 80
Trying 64.39.151.23...
Connected to www.some.net.
Escape character is '^]'.
HEAD / ;HTTP/1.1
HTTP/1.1 200 OK
Date: Fri, 22 Mar 2002 10:05:14 GMT
Server: Apache/1.3.22 (UNIX) (Red-Hat/Linux)
mod_ssl/2.8.5 OpenSSL/0.9.6
DAV/1.0.2 PHP/4.0.6 mod_perl/1.24_01
Last-Modified: Fri, 04 Jan 2002 08:21:00 GMT
ETag: "70061-68-3c3565ec"
Accept-Ranges: bytes
Content-Length: 104
Connection: close
Content-Type: text/html
Connection closed by foreign host.
[jimmy@blob ~]
This is perfectly safe, because you never have to give a username and/or
password for getting the data you want, so nobody can snoop that important
information off the cable.
4.3. The X Window System
4.3.1. X features
As we already explained in Chapter 7 (see Section 3.3,_“X_server
configurationâ€), the X Window system comes with an X server which serves
graphics to clients that need a display.
It is important to realize the distinction between the X server and the X
client application(s). The X server controls the display directly and is
responsible for all input and output via keyboard, mouse and display. The X
client, on the other hand, does not access the input and output devices
directly. It communicates with the X server which handles input and output. It
is the X client which does the real work, like computing values, running
applications and so forth. The X server only opens windows to handle input and
output for the specified client.
In normal operation (graphical mode), every Linux workstation is an X server to
itself, even if it only runs client applications. All the applications you are
running (for example, Gimp, a terminal window, your browser, your office
application, your CD playing tool, and so on) are clients to your X server.
Server and client are running on the same machine in this case.
This client/server nature of the X system makes it an ideal environment for
remote execution of applications and programs. Because the process is actually
being executed on the remote machine, very little CPU power is needed on the
local host. Such machines, purely acting as servers for X, are called X
terminals and were once very popular. More information may be found in the
Remote_X_applications_mini-HOWTO.
4.3.2. Telnet and X
If you would want to use telnet to display graphical applications running on a
remote machine, you first need to give the remote machine access to your
display (to your X server!) using the
xhost
command, by typing a command similar to the one below in a terminal window on
your local machine:
davy:~> xhost +remote.machine.com
After that, connect to the remote host and tell it to display graphics on the
local machine by setting the environment variable
DISPLAY
:
[davy@remote ~] export DISPLAY="local.host.com:0.0"
After completing this step, any application started in this terminal window
will be displayed on your local desktop, using remote resources for computing,
but your local graphical resources (your X server) for displaying the
application.
This procedure assumes that you have some sort of X server (XFree86, X.org,
Exceed, Cygwin) already set up on the machine where you want to display images.
The architecture and operating system of the client machine are not important
as long as they allow you to run an X server on it.
Mind that displaying a terminal window from the remote machine is also
considered to be a display of an image.
4.4. The SSH suite
4.4.1. Introduction
Most UNIX and Linux systems now run Secure SHell in order to leave out the
security risks that came with telnet. Most Linux systems will run a version of
OpenSSH, an Open Source implementation of the SSH protocol, providing secure
encrypted communications between untrusted hosts over an untrusted network. In
the standard setup X connections are automatically forwarded, but arbitrary
TCP/IP ports may also be forwarded using a secure channel.
The
ssh
client connects and logs into the specified host name. The user must provide
his identity to the remote machine as specified in the
sshd_config
file, which can usually be found in /etc/ssh. The configuration file is rather
self-explanatory and by defaults enables most common features. Should you need
help, you can find it in the sshd man pages.
When the user's identity has been accepted by the server, the server either
executes the given command, or logs into the machine and gives the user a
normal shell on the remote machine. All communication with the remote command
or shell will be automatically encrypted.
The session terminates when the command or shell on the remote machine exits
and all X11 and TCP/IP connections have been closed.
When connecting to a host for the first time, using any of the programs that
are included in the SSH collection, you need to establish the authenticity of
that host and acknowledge that you want to connect:
lenny ~> ssh blob
The authenticity of host 'blob (10.0.0.1)' can't be established.
RSA fingerprint is 18:30:50:46:ac:98:3c:93:1a:56:35:09:8d:97:e3:1d.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'blob,192.168.30.2' (RSA) to the list of
known hosts.
Last login: Sat Dec 28 13:29:19 2002 from octarine
This space for rent.
lenny is in ~
It is important that you type “yesâ€, in three characters, not just “yâ€. This
edits your ~/.ssh/known_hosts file, see Section 4.4.3,_“Server_authenticationâ€.
If you just want to check something on a remote machine and then get your
prompt back on the local host, you can give the commands that you want to
execute remotely as arguments to ssh:
lenny ~> ssh blob who
jenny@blob's password:
root tty2 Jul 24 07:19
lena tty3 Jul 23 22:24
lena 0: Jul 25 22:03
lenny ~> uname -n
magrat.example.com
4.4.2. X11 and TCP forwarding
If the
X11Forwarding
entry is set to yes on the target machine and the user is using X
applications, the
DISPLAY
environment variable is set, the connection to the X11 display is
automatically forwarded to the remote side in such a way that any X11 programs
started from the shell will go through the encrypted channel, and the
connection to the real X server will be made from the local machine. The user
should not manually set DISPLAY. Forwarding of X11 connections can be
configured on the command line or in the sshd configuration file.
The value for DISPLAY set by ssh will point to the server machine, but with a
display number greater than zero. This is normal, and happens because ssh
creates a proxy X server on the server machine (that runs the X client
application) for forwarding the connections over the encrypted channel.
This is all done automatically, so when you type in the name of a graphical
application, it is displayed on your local machine and not on the remote host.
We use xclock in the example, since it is a small program which is generally
installed and ideal for testing:
Figure 10.3. SSH X11 forwarding
SSH X11 forwarding
SSH will also automatically set up Xauthority data on the server machine. For
this purpose, it will generate a random authorization cookie, store it in
Xauthority
on the server, and verify that any forwarded connections carry this cookie and
replace it by the real cookie when the connection is opened. The real
authentication cookie is never sent to the server machine (and no cookies are
sent in the plain).
Forwarding of arbitrary TCP/IP connections over the secure channel can be
specified either on the command line or in a configuration file.
The X server
This procedure assumes that you have a running X server on the client where you
want to display the application from the remote host. The client may be of
different architecture and operating system than the remote host, as long as it
can run an X server, such as Cygwin (which implements an X.org server for MS
Windows clients and others) or Exceed, it should be possible to set up a remote
connection with any Linux or UNIX machine.
4.4.3. Server authentication
The ssh client/server system automatically maintains and checks a database
containing identifications for all hosts it has ever been used with. Host keys
are stored in $HOME/.ssh/known_hosts in the user's home directory.
Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for
known hosts. Any new hosts are automatically added to the user's file. If a
host's identification ever changes, ssh warns about this and disables password
authentication to prevent a Trojan horse from getting the user's password.
Another purpose of this mechanism is to prevent man-in-the-middle attacks which
could otherwise be used to circumvent the encryption. In environments where
high security is needed, sshd can even be configured to prevent logins to
machines whose host keys have changed or are unknown.
4.4.4. Secure remote copying
The SSH suite provides
scp
as a secure alternative to the rcp command that used to be popular when only
rsh existed.
scp
uses ssh for data transfer, uses the same authentication and provides the same
security as ssh. Unlike rcp, scp will ask for passwords or passphrases if they
are needed for authentication:
lenny /var/tmp> scp Schedule.sdc.gz blob:/var/tmp/
lenny@blob's password:
Schedule.sdc.gz 100% |*****************************| 100 KB 00:00
lenny /var/tmp>
Any file name may contain a host and user specification to indicate that the
file is to be copied to/from that host. Copies between two remote hosts are
permitted. See the Info pages for more information.
If you would rather use an FTP-like interface, use sftp:
lenny /var/tmp> sftp blob
Connecting to blob...
lenny@blob's password:
sftp> cd /var/tmp
sftp> get Sch*
Fetching /var/tmp/Schedule.sdc.gz to Schedule.sdc.gz
sftp> bye
lenny /var/tmp>
Secure copy or FTP GUIs
Don't feel comfortable with the command line yet? Try Konqueror's capabilities
for secure remote copy, or install Putty.
4.4.5. Authentication keys
The
ssh-keygen
command generates, manages and converts authentication keys for ssh. It can
create RSA keys for use by SSH protocol version 1 and RSA or DSA keys for use
by SSH protocol version 2.
Normally each user wishing to use SSH with RSA or DSA authentication runs this
once to create the authentication key in $HOME/.ssh/identity, id_dsa or id_rsa.
Additionally, the system administrator may use this to generate host keys for
the system.
Normally this program generates the key and asks for a file in which to store
the private key. The public key is stored in a file with the same name but .pub
appended. The program also asks for a passphrase. The passphrase may be empty
to indicate no passphrase (host keys must have an empty passphrase), or it may
be a string of arbitrary length.
There is no way to recover a lost passphrase. If the passphrase is lost or
forgotten, a new key must be generated and copied to the corresponding public
keys.
We will study SSH keys in the exercises. All information can be found in the
man or Info pages.
4.5. VNC
VNC or Virtual Network Computing is in fact a remote display system which
allows viewing a desktop environment not only on the local machine on which it
is running, but from anywhere on the Internet and from a wide variety of
machines and architectures, including MS Windows and several UNIX
distributions. You could, for example, run MS Word on a Windows NT machine and
display the output on your Linux desktop. VNC provides servers as well as
clients, so the opposite also works and it may thus be used to display Linux
programs on Windows clients. VNC is probably the easiest way to have X
connections on a PC. The following features make VNC different from a normal X
server or commercial implementations:
* No state is stored at the viewer side: you can leave your desk and resume
from another machine, continuing where you left. When you are running a PC X
server, and the PC crashes or is restarted, all remote applications that you
were running will die. With VNC, they keep on running.
* It is small and simple, no installation needed, can be run from a floppy if
needed.
* Platform independent with the Java client, runs on virtually everything that
supports X.
* Sharable: one desktop may be displayed on multiple viewers.
* Free.
More information can be found in the VNC client man pages (man vncviewer) or on
the VNC_website.
4.6. The rdesktop protocol
In order to ease management of MS Windows hosts, recent Linux distributions
support the Remote Desktop Protocol (RDP), which is implemented in the
rdesktop
client. The protocol is used in a number of Microsoft products, including
Windows NT Terminal Server, Windows 2000 Server, Windows XP and Windows 2003
Server.
Surprise your friends (or management) with the fullscreen mode, multiple types
of keyboard layouts and single application mode, just like the real thing. The
man rdesktop manual provides more information. The project's homepage is at
http://www.rdesktop.org/.
4.7. Cygwin
Cygwin provides substantial UNIX functionality on MS Windows systems. Apart
from providing UNIX command line tools and graphical applications, it can also
be used to display a Linux desktop on an MS Windows machine, using remote X.
From a Cygwin Bash shell, type the command
/usr/X11R6/bin/XWin.exe -query your_linux_machine_name_or_IP
The connection is by default denied. You need to change the X Display Manager
(XDM) configuration and possibly the X Font Server (XFS) configuration to
enable this type of connection, where you get a login screen on the remote
machine. Depending on your desktop manager (Gnome, KDE, other), you might have
to change some configurations there, too.
If you do not need to display the entire desktop, you can use SSH in Cygwin,
just like explained in Section 4.4,_“The_SSH_suiteâ€. without all the fuss of
editing configuration files.
5. Security
5.1. Introduction
As soon as a computer is connected to the network, all kinds of abuse becomes
possible, be it a UNIX-based or any other system. Admittedly, mountains of
papers have been spilled on this subject and it would lead us too far to
discuss the subject of security in detail. There are, however, a couple of
fairly logical things even a novice user can do to obtain a very secure system,
because most break-ins are the result of ignorant or careless users.
Maybe you are asking yourself if this all applies to you, using your computer
at home or working at your office on a desktop in a fairly protected
environment. The questions you should be asking yourself, however, are more on
the lines of:
* Do you want to be in control of your own system?
* Do you want to (unwittingly) participate in criminal activities?
* Do you want your equipment to be used by someone else?
* Do you want to take risks on losing your Internet connection?
* Do you want to rebuild your system every time it has been hacked?
* Do you want to risk personal or other data loss?
Presuming you don't, we will quickly list the steps you can take to secure your
machine. Extended information can be found in the Linux_Security_HOWTO.
5.2. Services
The goal is to run as few services as possible. If the number of ports that are
open for the outside world are kept to a minimum, this is all the better to
keep an overview. If services can't be turned off for the local network, try to
at least disable them for outside connections.
A rule of thumb is that if you don't recognize a particular service, you
probably won't need it anyway. Also keep in mind that some services are not
really meant to be used over the Internet. Don't rely on what should be
running, check which services are listening on what TCP ports using the
netstat
command:
[elly@mars ~] netstat -l | grep tcp
tcp 0 0 *:32769 *:* LISTEN
tcp 0 0 *:32771 *:* LISTEN
tcp 0 0 *:printer *:* LISTEN
tcp 0 0 *:kerberos_master *:* LISTEN
tcp 0 0 *:sunrpc *:* LISTEN
tcp 0 0 *:6001 *:* LISTEN
tcp 0 0 *:785 *:* LISTEN
tcp 0 0 localhost.localdom:smtp *:* LISTEN
tcp 0 0 *:ftp *:* LISTEN
tcp 0 0 *:ssh *:* LISTEN
tcp 0 0 ::1:x11-ssh-offset *:* LISTEN
Things to avoid:
* exec
,
rlogin
and
rsh
, and
telnet
just to be on the safe side.
* X11 on server machines.
* No lp if no printer is physically attached.
* No MS Windows hosts in the network, no Samba required.
* Don't allow FTP unless an FTP server is required.
* Don't allow NFS and NIS over the Internet, disable all related services on a
stand-alone installation.
* Don't run an MTA if you're not actually on a mail server.
* ...
Stop running services using the chkconfig command, the initscripts or by
editing the (x)inetd configuration files.
5.3. Update regularly
Its ability to adapt quickly in an ever changing environment is what makes
Linux thrive. But it also creates a possibility that security updates have been
released even while you are installing a brand new version, so the first thing
you should do (and this goes for about any OS you can think of) after
installing is getting the updates as soon as possible. After that, update all
the packages you use regularly.
Some updates may require new configuration files, and old files may be
replaced. Check the documentation, and ensure that everything runs normal after
updating.
Most Linux distributions provide mailing list services for security update
announcements, and tools for applying updates to the system. General Linux only
security issues are reported among others at Linuxsecurity.com.
Updating is an ongoing process, so it should be an almost daily habit.
5.4. Firewalls and access policies
5.4.1. What is a firewall?
In the previous section we already mentioned firewall capabilities in Linux.
While firewall administration is one of the tasks of your network admin, you
should know a couple of things about firewalls.
Firewall is a vague term that can mean anything that acts as a protective
barrier between us and the outside world, generally the Internet. A firewall
can be a dedicated system or a specific application that provides this
functionality. Or it can be a combination of components, including various
combinations of hardware and software. Firewalls are built from “rules†that
are used to define what is allowed to enter and/or exit a given system or
network.
After disabling unnecessary services, we now want to restrict accepted services
as to allow only the minimum required connections. A fine example is working
from home: only the specific connection between your office and your home
should be allowed, connections from other machines on the Internet should be
blocked.
5.4.2. Packet filters
The first line of defense is a
packet filter
, which can look inside IP packets and make decisions based on the content.
Most common is the Netfilter package, providing the iptables command, a next
generation packet filter for Linux.
One of the most noteworthy enhancements in the newer kernels is the stateful
inspection feature, which not only tells what is inside a packet, but also
detects if a packet belongs or is related to a new or existing connection.
The Shoreline Firewall or Shorewall for short is a front-end for the standard
firewall functionality in Linux.
More information can be found at the_Netfilter/iptables_project_page.
5.4.3. TCP wrappers
TCP wrapping provides much the same results as the packet filters, but works
differently. The wrapper actually accepts the connection attempt, then examines
configuration files and decides whether to accept or reject the connection
request. It controls connections at the application level rather than at the
network level.
TCP wrappers are typically used with xinetd to provide host name and IP-
address-based access control. In addition, these tools include logging and
utilization management capabilities that are easy to configure.
The advantages of TCP wrappers are that the connecting client is unaware that
wrappers are used, and that they operate separately from the applications they
protect.
The host based access is controlled in the hosts.allow and hosts.deny files.
More information can be found in the TCP wrapper documentation files in /usr/
share/doc/tcp_wrappers[-<version>/] or /usr/share/doc/tcp and in the man pages
for the host based access control files, which contain examples.
5.4.4. Proxies
Proxies can perform various duties, not all of which have much to do with
security. But the fact that they are an intermediary make proxies a good place
to enforce access control policies, limit direct connections through a
firewall, and control how the network behind the proxy looks to the Internet.
Usually in combination with a packet filter, but sometimes all by themselves,
proxies provide an extra level of control. More information can be found in the
Firewall_HOWTO or on the Squid website.
5.4.5. Access to individual applications
Some servers may have their own access control features. Common examples
include Samba, X Window, Bind, Apache and CUPS. For every service you want to
offer check which configuration files apply.
5.4.6. Log files
If anything, the UNIX way of logging all kinds of activities into all kinds of
files confirms that “it is doing something.†Of course, log files should be
checked regularly, manually or automatically. Firewalls and other means of
access control tend to create huge amounts of log files, so the trick is to try
and only log abnormal activities.
5.5. Intrusion detection
Intrusion Detection Systems are designed to catch what might have gotten past
the firewall. They can either be designed to catch an active break-in attempt
in progress, or to detect a successful break-in after the fact. In the latter
case, it is too late to prevent any damage, but at least we have early
awareness of a problem. There are two basic types of IDS: those protecting
networks, and those protecting individual hosts.
For host based IDS, this is done with utilities that monitor the file system
for changes. System files that have changed in some way, but should not change,
are a dead give-away that something is amiss. Anyone who gets in and gets root
access will presumably make changes to the system somewhere. This is usually
the very first thing done, either so he can get back in through a backdoor, or
to launch an attack against someone else, in which case, he has to change or
add files to the system. Some systems come with the tripwire monitoring system,
which is documented at the Tripwire_Open_Source_Project website.
Network intrusion detection is handled by a system that sees all the traffic
that passes the firewall (not by portscanners, which advertise usable ports).
Snort is an Open Source example of such a program. Whitehats.com features an
open Intrusion detection database, arachNIDS.
5.6. More tips
Some general things you should keep in mind:
* Do not allow root logins. UNIX developers came up with the su over two
decades ago for extra security.
* Direct root access is always dangerous and susceptible to human errors, be it
by allowing root login or by using the su - command. Rather than using su, it
is even better to use sudo to only execute the command that you need extra
permissions for, and to return afterwards to your own environment.
* Take passwords seriously. Use shadow passwords. Change your passwords
regularly.
* Try to always use SSH or SSL. Avoid telnet, FTP and E-mail clients and other
client programs which send unencrypted passwords over the network. Security
is not only about securing your computer, it is also about securing your
passwords.
* Limit resources using quota and/or ulimit.
* The mail for root should be delivered to, or at least read by, an actual
person.
* The SANS_institute has more tips and tricks, sorted per distribution, with
mailing list service.
* Check the origin of new software, get it from a trusted place/site. Verify
new packages before installing.
* When using a non-permanent Internet connection, shut it down as soon as you
don't need it anymore.
* Run private services on odd ports instead of the ones expected by possible
hackers.
* Know your system. After a while, you can almost feel when something is
happening.
5.7. Have I been hacked?
How can you tell? This is a checklist of suspicious events:
* Mysterious open ports, strange processes.
* System utilities (common commands) behaving strange.
* Login problems.
* Unexplained bandwidth usage.
* Damaged or missing log files, syslog daemon behaving strange.
* Interfaces in unusual modes.
* Unexpectedly modified configuration files.
* Strange entries in shell history files.
* Unidentified temporary files.
5.8. Recovering from intrusion
In short, stay calm. Then take the following actions in this order:
* Disconnect the machine from the network.
* Try to find out as much as you can about how your security was breached.
* Backup important non-system data. If possible, check these data against
existing backups, made before the system was compromised, to ensure data
integrity.
* Re-install the system.
* Use new passwords.
* Restore from system and data backups.
* Apply all available updates.
* Re-examine the system: block off unnecessary services, check firewall rules
and other access policies.
* Reconnect.
6. Summary
Linux and networking go hand in hand. The Linux kernel has support for all
common and most uncommon network protocols. The standard UNIX networking tools
are provided in each distribution. Next to those, most distributions offer
tools for easy network installation and management.
Linux is well known as a stable platform for running various Internet services,
the amount of Internet software is endless. Like UNIX, Linux can be just as
well used and administered from a remote location, using one of several
solutions for remote execution of programs.
We briefly touched the subject of security. Linux is an ideal firewall system,
light and cheap, but can be used in several other network functions such as
routers and proxy servers.
Increasing network security is mainly done by applying frequent updates and
common sense.
Here is an overview of network related commands:
Table 10.2. New commands in chapter 10: Networking
_________________________________________________________________________
|Command_____________|Meaning_____________________________________________|
|ftp_________________|Transfer_files_to_another_host_(insecure).__________|
|host________________|Get_information_about_networked_hosts.______________|
|ifconfig____________|Display_IP_address_information._____________________|
|ip__________________|Display_IP_address_information._____________________|
|netstat_____________|Display_routing_information_and_network_statistics._|
|ping________________|Send_answer_requests_to_other_hosts.________________|
|rdesktop____________|Display_and_MS_Windows_desktop_on_your_Linux_system.|
|route_______________|Show_routing_information.___________________________|
|scp_________________|Secure_copy_files_to_and_from_other_hosts.__________|
|sftp________________|Secure_FTP_files_to_and_from_other_hosts.___________|
|ssh_________________|Make_an_encrypted_connection_to_another_host._______|
|ssh-keygen__________|Generate_authentication_keys_for_Secure_SHell.______|
|telnet______________|Make_an_insecure_connection_to_another_hosts._______|
|tracepath/traceroute|Print_the_route_that_packets_follow_to_another_host.|
|whois_______________|Get_information_abotu_a_domain_name.________________|
|xclock |X Window clock application, handy for testing remote|
|____________________|display.____________________________________________|
|xhost_______________|X_Window_access_control_tool._______________________|
7. Exercises
7.1. General networking
* Display network information for your workstation: IP address, routes, name
servers.
* Suppose no DNS is available. What would you do to reach your neighbour's
machine without typing the IP address all the time?
* How would you permanently store proxy information for a text mode browser
such as links?
* Which name servers handle the redhat.com domain?
* Send an E-mail to your local account. Try two different ways to send and read
it. How can you check that it really arrived?
* Does your machine accept anonymous FTP connections? How do you use the ncftp
program to authenticate with your user name and password?
* Does your machine run a web server? If not, make it do so. Check the log
files!
7.2. Remote connections
* From your local workstation, display a graphical application, such as xclock
on your neighbour's screen. The necessary accounts will have to be set up.
Use a secure connection!
* Set up SSH keys so you can connect to your neighbour's machine without having
to enter a password.
* Make a backup copy of your home directory in /var/tmp on your neighbour's
“backup server,†using scp. Archive and compress before starting the data
transfer! Connect to the remote host using ssh, unpack the backup, and put
one file back on the original machine using sftp.
7.3. Security
* Make a list of open (listening) ports on your machine.
* Supposing you want to run a web server. Which services would you deactivate?
How would you do that?
* Install available updates.
* How can you see who connected to your system?
* Make a repetitive job that reminds you to change your password every month,
and preferably the root password as well.
Chapter 11. Sound and Video
Table of Contents
1._Audio_Basics
1.1._Installation
1.2._Drivers_and_Architecture
2._Sound_and_video_playing
2.1._CD_playing_and_copying
2.2._Playing_music_files
2.3._Recording
3._Video_playing,_streams_and_television_watching
4._Internet_Telephony
4.1._What_is_it?
4.2._What_do_you_need?
5._Summary
6._Exercises
Abstract
This chapter addresses the following tasks (briefly, as the field of sound and
video is very wide):
* Sound card configuration
* Playing CDs, copying CDs,
* Playing music files
* Volume control
* Video and television
* Recording sound
1. Audio Basics
1.1. Installation
Most likely, your system is already installed with audio drivers and the
configuration was done at installation time. Likewise, should you ever need to
replace your audio hardware, most systems provide tools that allow easy setup
and configuration of the device. Most currently available plug-and-play sound
cards should be recognized automatically. If you can hear the samples that are
played during configuration, just click OK and everything will be set up for
you.
If your card is not detected automatically, you may be presented with a list of
sound cards and/or of sound card properties from which to choose. After that,
you will have to provide the correct I/O port, IRQ and DMA settings.
Information about these settings can be found in your sound card documentation.
If you are on a dual boot system with MS Windows, this information can be found
in the Windows Control Panel as well.
If automatic sound card detection fails
If your soundcard is not supported by default, you will need to apply other
techniques. These are described in the Linux_Sound_HOWTO.
1.2. Drivers and Architecture
There are generally two types of sound architecture: the older Open Sound
System or OSS, which works with every UNIX-like system, and the newer Advanced
Linux Sound Architecture or ALSA, that has better support for Linux, as the
name indicates. ALSA also has more features and allows for faster driver
development. We will focus here on the ALSA system.
Today, almost all mainstream audio chipsets are supported. Only some high-end
professional solutions and some cards developed by manufacturers refusing to
document their chipset specifications are unsupported. An overview of supported
devices can be found on the ALSA site at http://www.alsa-project.org/alsa-doc/
index.php?vendor=All#matrix.
Configuring systems installed with ALSA is done using the alsaconf tool.
Additionally, distributions usually provide their own tools for configuring the
sound card; these tools might even integrate the old and the new way of
handling sound devices.
2. Sound and video playing
2.1. CD playing and copying
The cdp package comes with most distributions and provides cdp or
cdplay
, a text-based CD player. Desktop managers usually include a graphical tool,
such as the
gnome-cd
player in Gnome, that can be started from a menu.
Be sure to understand the difference between an audio CD and a data CD. You do
not have to mount an audio CD into the file system in order to listen to it.
This is because the data on such a CD are not Linux file system data; they are
accessed and sent to the audio output channel directly, using a CD player
program. If your CD is a data CD containing .mp3 files, you will first need to
mount it into the file system, and then use one of the programs that we discuss
below in order to play the music. How to mount CDs into the file system is
explained in Section 5.5,_“Installing_extra_packages_from_the_installation
CDsâ€.
The
cdparanoia
tool from the package with the same name reads audio directly as data from the
CD, without analog conversions, and writes data to a file or pipe in different
formats, of which .wav is probably the most popular. Various tools for
conversion to other formats, formats, such as .mp3, come with most
distributions or are downloadable as separate packages. The GNU project
provides several CD playing, ripping and encoding tools, database managers; see
the Free_Software_Directory,_Audio_section for detailed information.
Audio-CD creation is eased, among many others, with the
kaudiocreator
tool from the KDE suite. It comes with clear information from the KDE Help
Center.
CD burning is covered in general in Section 2.2,_“Making_a_copy_with_a_CD-
writerâ€.
2.2. Playing music files
2.2.1. mp3 files
The popular .mp3 format is widely supported on Linux machines. Most
distributions include multiple programs that can play these files. Among many
other applications, XMMS, which is presented in the screenshot below, is one of
the most wide-spread, partially because it has the same look and feel as the
Windows tool.
Figure 11.1. XMMS mp3 player
XMMS looks very familiar to MS Windows users, showing a hi-fi installation
frontpanel.
Also very popular for playing music are AmaroK, a KDE application that is
steadily gaining popularity, and MPlayer, which can also play movies.
Restrictions
Some distributions don't allow you to play MP3's without modifying your
configuration, this is due to license restrictions on the MP3 tools. You might
need to install extra software to be able to play your music.
In text mode, you can use the
mplayer
command:
[tille@octarine ~]$ mplayer /opt/mp3/oriental/*.mp3
MPlayer 1.0pre7-RPM-3.4.2 (C) 2000-2005 MPlayer Team
CPU: Advanced Micro Devices Duron Spitfire (Family: 6, Stepping: 1)
Detected cache-line size is 64 bytes
CPUflags: MMX: 1 MMX2: 1 3DNow: 1 3DNow2: 1 SSE: 0 SSE2: 0
Playing /opt/oldopt/mp3/oriental/Mazika_Diana-Krozon_Super-Star_Ensani-Ma-
Bansak.mp3.
Cache fill: 1.17% (98304 bytes) Audio file detected.
Clip info:
Title: Ensani-Ma-Bansak.mp3
Artist: Diana-Krozon
Album: Super-Star
Year:
Comment:
Genre: Unknown
==========================================================================
Opening audio decoder: [mp3lib] MPEG layer-2, layer-3
mpg123: Can't rewind stream by 450 bits!
AUDIO: 44100 Hz, 2 ch, s16le, 160.0 kbit/11.34% (ratio: 20000->176400)
Selected audio codec: [mp3] afm:mp3lib (mp3lib MPEG layer-2, layer-3)
==========================================================================
Checking audio filter chain for 44100Hz/2ch/s16le -> 44100Hz/2ch/s16le...
AF_pre: 44100Hz/2ch/s16le
AO: [oss] 44100Hz 2ch s16le (2 bps)
Building audio filter chain for 44100Hz/2ch/s16le -> 44100Hz/2ch/s16le...
Video: no video
Starting playback...
A: 227.8 (03:23:.1) 1.8% 12%
2.2.2. Other formats
It would lead us too far to discuss all possible audio formats and ways to play
them. An (incomplete) overview of other common sound playing and manipulating
software:
* Ogg Vorbis: Free audio format: see the_GNU_audio_directory for tools - they
might be included in your distribution as well. The format was developed
because MP3 is patented.
* Real audio and video: realplay from RealNetworks.
* SoX or Sound eXchange: actually a sound converter, comes with th e play
program. Plays .wav, . ogg and various other formats, including raw binary
formats.
* Playmidi: a MIDI player, see the GNU directory.
* AlsaPlayer: from the Advanced Linux Sound Architecture project, see the
AlsaPlayer_web_site.
* mplayer: plays just about anything, including mp3 files. More info on the
MPlayerHQ_website.
* hxplay: supports RealAudio and RealVideo, mp3, mp4 audio, Flash, wav and
more, see HelixDNA (not all components of this software are completely free).
* rhythmbox: based on the GStreamer framework, can play everything supported in
GStreamer, which claims to be able to play everything, see the Rhythmbox and
GStreamer sites.
Check your system documentation and man pages for particular tools and detailed
explanations on how to use them.
I don't have these applications on my system!
A lot of the tools and applications discussed in the above sections are
optional software. It is possible that such applications are not installed on
your system by default, but that you can find them in your distribution as
additional packages. It might also very well be that the application that you
are looking for is not in your distribution at all. In that case, you need to
download it from the application's web site.
2.2.3. Volume control
aumix
and
alsamixer
are two common text tools for adjusting audio controls. Use the arrow keys to
toggle settings. The alsamixer has a graphical interface when started from the
Gnome menu or as
gnome-alsamixer
from the command line. The
kmix
tool does the same in KDE.
Regardless of how you choose to listen to music or other sounds, remember that
there may be other people who may not be interested in hearing you or your
computer. Try to be courteous, especially in office environments. Use a quality
head-set, rather than the ones with the small ear pieces. This is better for
your ears and causes less distraction for your colleagues.
2.3. Recording
Various tools are again available that allow you to record voice and music. For
recording voice you can use
arecord
on the command line:
alexey@russia:~> arecord /var/tmp/myvoice.wav
Recording WAVE '/var/tmp/myvoice.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
Aborted by signal Interrups...
“Interrupt†means that the application has caught a Ctrl+C. Play the sample
using the simple play command.
This is a good test that you can execute prior to testing applications that
need voice input, like Voice over IP (VoIP). Keep in mind that the microphone
input should be activated. If you don't hear your own voice, check your sound
settings. It often happens that the microphone is muted or on verry low volume.
This can be easily adjusted using alsamixer or your distribution-specific
graphical interface to the sound system.
In KDE you can start the
krec
utility, Gnome provides the
gnome-sound-recorder
.
3. Video playing, streams and television watching
Various players are available:
* xine: a free video player
* ogle: DVD player.
* okle: KDE version of ogle
* mplayer: Movie Player for Linux
* totem: plays both audio and video files, audio CDs, VCD and DVD.
* realplay: from RealNetworks.
* hxplay: a Real alternative, see HelixDNA.
* kaffeine: media player for KDE3.
Most likely, you will find one of these in your graphical menus.
Keep in mind that all codecs necessary for viewing different types of video
might not be on your system by default. You can get a long way downloading
W32codecs and libdvdcss.
The LDP released a document that is very appropriate for this section. It is
entitled DVD_Playback_HOWTO and describes the different tools available for
viewing movies on a system that has a DVD drive. It is a fine addition to the
DVD_HOWTO that explains installation of the drive.
For watching TV there is choice of the following tools, among many others for
watching and capturing TV, video and other streams:
* tvtime: great program with station management, interaction with teletext,
film mode and much_more.
* zapping: Gnome-specific TV viewer.
* xawtv: X11 TV viewer.
4. Internet Telephony
4.1. What is it?
Internet telephony, or more common, Voice over IP (VoIP) or digital telephony
allows parties to exchange voice data flows over the network. The big
difference is that the data flows over a general purpose network, the Internet,
contrary to conventional telephony, that uses a dedicated network of voice
transmission lines. The two networks can be connected, however, under special
circumstances, but for now this is certainly not a standard. In other words: it
is very likely that you will not be able to call people who are using a
conventional telephone. If it is possible at all, it is likely that you will
need to pay for a subscription.
While there are currently various applications available for free download,
both free and proprietary, there are some major drawbacks to telephony over the
Internet. Most noticably, the system is unreliable, it can be slow or there can
be a lot of noise on the connection, and it can thus certainly not be used to
replace conventional telephony - think about emergency calls. While some
providers take their precautions, there is no guarantee that you can reach the
party that you want to call.
Most applications currently do not use encryption, so be aware that it is
potentially easy for someone to eavesdrop on your conversations. If security is
a concern for you, read the documentation that comes with your VoIP client.
Additionally, if you are using a firewall, it should be configured to allow
incoming connections from anywhere, so using VoIP also includes taking risks on
the level of site security.
4.2. What do you need?
4.2.1. Server Side
First of all, you need a provider offering the service. This service might
integrate traditional telephony and it might or might not be free. Among others
are SIPphone, Vonage, Lingo, AOL_TotalTalk and many locally accessible
providers offering the so-called “full phone serviceâ€. Internet phone service
only is offered by Skype, SIP_Broker, Google and many others.
If you want to set up a server of your own, you might want to look into
Asterisk.
4.2.2. Client Side
On the client side, the applications that you can use depend on your network
configuration. If you have a direct Internet connection, there won't be any
problems, provided that you know on what server you can connect, and usually
that you also have a username and password to authenticate to the service.
If you are behind a firewall that does Network Address Translation (NAT),
however, some services might not work, as they will only see the IP address of
the firewall and not the address of your computer, which might well be
unroutable over the Internet, for instance when you are in a company network
and your IP address starts with 10., 192.168. or another non-routable subnet
prefix. This depends on the protocol that is used by the application.
Also, available bandwidth might be a blocking factor: some applications are
optimized for low bandwidth consumption, while others might require high
bandwidth connections. This depends on the codec that is used by the
application.
Among the most common applications are the Skype client, which has an interface
that reminds of instant messaging, and X-Lite, the free version of the XTen
softphone, which looks like a mobile telephone. However, while these programs
are available for free download and very popular, they are not free as in free
speech: they use proprietary protocols and/or are only available in binary
packages, not in source format.
Free and open VoIP clients are for instance Gizmo, Linphone, GnomeMeeting and
KPhone.
Client hardware
While your computer, especially if it is a laptop PC, might have a built-in
microphone, the result will be far better if you connect a headset. If you have
the choice, opt for a USB headset, as it functions independently from existing
audio hardware. Use alsamixer to configure input and output sound levels to
your taste.
VoIP applications are definitely a booming market. Volunteers try to document
the current status at http://www.voip-info.org/.
5. Summary
The GNU/Linux platform is fully multi-media enabled. A wide variety of devices
like sound cards, tv-cards, headsets, microphones, CD and DVD players is
supported. The list of applications is sheer endless, that is why we needed to
shortenthe list of new commands below and limit ourselves to general audio
commands.
Table 11.1. New commands in chapter 11: Audio
_________________________________________________________________
|Command_____________|Meaning_____________________________________|
|alsaconf____________|Configure_the_ALSA_sound_system.____________|
|alsamixer___________|Tune_output_levels_of_ALSA_driver.__________|
|arecord_____________|Record_a_sound_sample.______________________|
|aumix_______________|Audio_mixer_tool.___________________________|
|cdp_________________|Play_an_audio_CD.___________________________|
|cdparanoia__________|Rip_an_audio_CD.____________________________|
|cdplay______________|Play_an_audio_CD.___________________________|
|gnome-alsamixer_____|Gnome_ALSA_front-end._______________________|
|gnome-cd____________|Gnome_front-end_for_playing_audio_CDs.______|
|gnome-sound-recorder|Gnome_front-end_for_recording_sound_samples.|
|kaudiocreator_______|KDE_front-end_for_creating_audio_CDs._______|
|kmix________________|KDE_front-end_for_sound_settings.___________|
|krec________________|KDE_front-end_for_recording_sound_samples.__|
|mplayer_____________|Multi-media_player._________________________|
|play________________|Command_line_tool_for_playing_sound_samples.|
6. Exercises
1. From the Gnome or KDE menu, open your sound configuration panel. Make sure
audio boxes or headset are connected to your system and find an output
level that is comfortable for you. Make sure, when your system is ALSA-
compatible, that you use the appropriate panel.
2. If you have a microphone, try recording a sample of your own voice. Make
sure the input volume is not too high, as this will result in high-pitched
tones when you communicate with others, or in transfering background noise
to the other party. On the command line, you might even try to use arecord
and aplay for recording and playing sound.
3. Locate sound files on your system and try to play them.
4. Insert an audio CD and try to play it.
5. Find a chat partner and configure a VoIP program. (You might need to
install one first.)
6. Can you listen to Internet radio?
7. If you have a DVD player and a movie on a DVD disk, try to play it.
Appendix A. Where to go from here?
Table of Contents
1._Useful_Books
1.1._General_Linux
1.2._Editors
1.3._Shells
1.4._X_Window
1.5._Networking
2._Useful_sites
2.1._General_information
2.2._Architecture_Specific_References
2.3._Distributions
2.4._Software
Abstract
This document gives an overview of useful books and sites.
1. Useful Books
1.1. General Linux
* "Linux in a Nutshell" by Ellen Siever, Jessica P. Hackman, Stephen Spainhour,
Stephen Figgins, O'Reilly UK, ISBN 0596000251
* "Running Linux" by Matt Welsh, Matthias Kalle Dalheimer, Lar Kaufman,
O'Reilly UK, ISBN 156592469X
* "Linux Unleashed" by Tim Parker, Bill Ball, David Pitts, Sams, ISBN
0672316889
* "When You Can't Find Your System Administrator" by Linda Mui, O'Reilly UK,
ISBN 1565921046
* When you actually buy a distribution, it will contain a very decent user
manual.
1.2. Editors
* "Learning the Vi Editor" by Linda Lamb and Arnold Robbins, O'Reilly UK, ISBN
1565924266
* "GNU Emacs Manual" by Richard M.Stallman, iUniverse.Com Inc., ISBN 0595100333
* "Learning GNU Emacs" by Debra Cameron, Bill Rosenblatt and Eric Raymond,
O'Reilly UK, ISBN 1565921526
* "Perl Cookbook" by Tom Christiansen and Nathan Torkington, O'Reilly UK, ISBN
1565922433
1.3. Shells
* "Unix Shell Programming" by Stephen G.Kochan and Patrick H.Wood, Sams
Publishing, ISBN 067248448X
* "Learning the Bash Shell" by Cameron Newham and Bill Rosenblatt, O'Reilly UK,
ISBN 1565923472
* "The Complete Linux Shell Programming Training Course" by Ellie Quigley and
Scott Hawkins, Prentice Hall PTR, ISBN 0130406767
* "Linux and Unix Shell Programming" by David Tansley, Addison Wesley
Publishing Company, ISBN 0201674726
* "Unix C Shell Field Guide" by Gail and Paul Anderson, Prentice Hall, ISBN
013937468X
1.4. X Window
* "Gnome User's Guide" by the Gnome Community, iUniverse.Com Inc., ISBN
0595132251
* "KDE Bible" by Dave Nash, Hungry Minds Inc., ISBN 0764546929
* "The Concise Guide to XFree86 for Linux" by Aron HSiao, Que, ISBN 0789721821
* "The New XFree86" by Bill Ball, Prima Publishing, ISBN 0761531521
* "Beginning GTK+ and Gnome" by Peter Wright, Wrox Press, ISBN 1861003811
* "KDE 2.0 Development" by David Sweet and Matthias Ettrich, Sams Publishing,
ISBN 0672318911
* "GTK+/Gnome Application Development" by Havoc Pennington, New Riders
Publishing, ISBN 0735700788
1.5. Networking
* "TCP/IP Illustrated, Volume I: The Protocols" by W. Richard Stevens, Addison-
Wesley Professional Computing Series, ISBN 0-201-63346-9
* "DNS and BIND" by Paul Albitz, Cricket Liu, Mike Loukides and Deborah
Russell, O'Reilly & Associates, ISBN 0596001584
* "The Concise Guide to DNS and BIND" by Nicolai Langfeldt, Que, ISBN
0789722739
* "Implementing LDAP" by Mark Wilcox, Wrox Press, ISBN 1861002211
* "Understanding and deploying LDAP directory services" by Tim Howes and co.,
Sams, ISBN 0672323168
* "Sendmail" by Brian Costales and Eric Allman, O'Reilly UK, ISBN 1565922220
* "Removing the Spam : Email Processing and Filtering" by Geoff Mulligan,
Addison Wesley Publishing Company, ISBN 0201379570
* "Managing IMAP" by Dianna & Kevin Mullet, O'Reilly UK, ISBN 059600012X
2. Useful sites
2.1. General information
* The_Linux_documentation_project: all docs, manpages, HOWTOs, FAQs
* LinuxQuestions.org: forum, downloads, docs and much more
* Google_for_Linux: the specialized search engine
* Google_Groups: an archive of all newsgroup postings, including the
comp.os.linux hierarchy
* Slashdot: daily news
* http://www.oreilly.com: books on Linux System and Network administration,
Perl, Java, ...
* POSIX: the standard
* Linux_HQ: Maintains a complete database of source, patches and documentation
for various versions of the Linux kernel.
2.2. Architecture Specific References
* AlphaLinux: Linux on Alpha architecture (e.g. Digital Workstation)
* Linux-MIPS: Linux on MIPS (e.g. SGI Indy)
* Linux_on_the_Road: Specific guidelines for installing and running Linux on
laptops, PDAs, mobile phones and so on. Configuration files for various
models.
* MkLinux: Linux on Apple
2.3. Distributions
* The_Fedora_Project: RedHat-sponsored community effort OS.
* Mandriva
* Ubuntu
* Debian
* TurboLinux
* Slackware
* SuSE
* LinuxISO.org: CD images for all distributions.
* Knoppix: distribution that runs from a CD, you don't need to install anything
for this one.
* DistroWatch.com: find a Linux that goes with your style.
* ...
2.4. Software
* Freshmeat: new software, software archives
* OpenSSH: Secure SHell site
* OpenOffice: MS compatible Office Suite
* KDE: K Desktop site
* GNU: GNU and GNU software
* Gnome: The official Gnome site
* RPM_Find: all RPM packages
* Samba: MS Windows file and print services
* Home_of_the_OpenLDAP_Project: OpenLDAP server/clients/utilities, FAQ and
other documentation.
* Sendmail_Homepage: A thorough technical discussion of Sendmail features,
includes configuration examples.
* Netfilter: contains assorted information about iptables: HOWTO, FAQ, guides,
...
* Official_GIMP_website: All information about the GNU Image Manipulation
Program.
* SourceForge.net: Open SOurce software development site.
* vIm_homepage
Appendix B. DOS versus Linux commands
Abstract
In this appendix, we matched DOS commands with their Linux equivalent.
As an extra means of orientation for new users with a Windows background, the
table below lists MS-DOS commands with their Linux counterparts. Keep in mind
that Linux commands usually have a number of options. Read the Info or man
pages on the command to find out more.
Table B.1. Overview of DOS/Linux commands
____________________________________________
|DOS_commands|Linux_command__________________|
|<command>_/?|man_<command>_or_command_--help|
|cd__________|cd_____________________________|
|chdir_______|pwd____________________________|
|cls_________|clear__________________________|
|copy________|cp_____________________________|
|date________|date___________________________|
|del_________|rm_____________________________|
|dir_________|ls_____________________________|
|echo________|echo___________________________|
|edit________|vim_(or_other_editor)__________|
|exit________|exit___________________________|
|fc__________|diff___________________________|
|find________|grep___________________________|
|format______|mke2fs_or_mformat______________|
|mem_________|free___________________________|
|mkdir_______|mkdir__________________________|
|more________|more_or_even_less______________|
|move________|mv_____________________________|
|ren_________|mv_____________________________|
|time________|date___________________________|
Appendix C. Shell Features
Table of Contents
1._Common_features
2._Differing_features
Abstract
This document gives an overview of common shell features (the same in every
shell flavour) and differing shell features (shell specific features).
1. Common features
The following features are standard in every shell. Note that the stop,
suspend, jobs, bg and fg commands are only available on systems that support
job control.
Table C.1. Common Shell Features
______________________________________________________________
|Command_|Meaning______________________________________________|
|>_______|Redirect_output______________________________________|
|>>______|Append_to_file_______________________________________|
|<_______|Redirect_input_______________________________________|
|<<______|"Here"_document_(redirect_input)_____________________|
||_______|Pipe_output__________________________________________|
|&___|Run_process_in_background.___________________________|
|;_______|Separate_commands_on_same_line_______________________|
|*_______|Match_any_character(s)_in_filename___________________|
|?_______|Match_single_character_in_filename___________________|
|[_]_____|Match_any_characters_enclosed________________________|
|(_)_____|Execute_in_subshell__________________________________|
|`_`_____|Substitute_output_of_enclosed_command________________|
|"_"_____|Partial_quote_(allows_variable_and_command_expansion)|
|'_'_____|Full_quote_(no_expansion)____________________________|
|\_______|Quote_following_character____________________________|
|$var____|Use_value_for_variable_______________________________|
|$$______|Process_id___________________________________________|
|$0______|Command_name_________________________________________|
|$n______|nth_argument_(n_from_0_to_9)_________________________|
|$*______|All_arguments_as_a_simple_word_______________________|
|#_______|Begin_comment________________________________________|
|bg______|Background_execution_________________________________|
|break___|Break_from_loop_statements___________________________|
|cd______|Change_directories___________________________________|
|continue|Resume_a_program_loop________________________________|
|echo____|Display_output_______________________________________|
|eval____|Evaluate_arguments___________________________________|
|exec____|Execute_a_new_shell__________________________________|
|fg______|Foreground_execution_________________________________|
|jobs____|Show_active_jobs_____________________________________|
|kill____|Terminate_running_jobs_______________________________|
|newgrp__|Change_to_a_new_group________________________________|
|shift___|Shift_positional_parameters__________________________|
|stop____|Suspend_a_background_job_____________________________|
|suspend_|Suspend_a_foreground_job_____________________________|
|time____|Time_a_command_______________________________________|
|umask___|Set_or_list_file_permissions_________________________|
|unset___|Erase_variable_or_function_definitions_______________|
|wait____|Wait_for_a_background_job_to_finish__________________|
2. Differing features
The table below shows major differences between the standard shell (sh), Bourne
Again SHell (bash), Korn shell (ksh) and the C shell (csh).
Shell compatibility
Since the Bourne Again SHell is a superset of sh, all sh commands will also
work in bash - but not vice versa. bash has many more features of its own, and,
as the table below demonstrates, many features incorporated from other shells.
Since the Turbo C shell is a superset of csh, all csh commands will work in
tcsh, but not the other way round.
Table C.2. Differing Shell Features
__________________________________________________________________________________________________________________________________________________
|sh_______|bash______________________________|ksh______|csh______|Meaning/Action___________________________________________________________________|
|$________|$_________________________________|$________|%________|Default_user_prompt______________________________________________________________|
|Â _______|>|________________________________|>|_______|>!_______|Force_redirection________________________________________________________________|
|> file |&> file or > file 2>&1 |> file |>& |Redirect stdout and stderr to file |
|2>&1_|__________________________________|2>&1_|file_____|_________________________________________________________________________________|
|Â _______|{_}_______________________________|Â _______|{_}______|Expand_elements_in_list__________________________________________________________|
|`command`|`command` or $(command) |$ |`command`|Substitute output of enclosed command |
|_________|__________________________________|(command)|_________|_________________________________________________________________________________|
|$HOME____|$HOME_____________________________|$HOME____|$home____|Home_directory___________________________________________________________________|
|Â _______|~_________________________________|~________|~________|Home_directory_symbol____________________________________________________________|
|Â _______|~+,_~-,_dirs______________________|~+,_~-___|=-,_=N___|Access_directory_stack___________________________________________________________|
|var=value|VAR=value |var=value|set |Variable assignment |
|_________|__________________________________|_________|var=value|_________________________________________________________________________________|
|export |export VAR=value |export |setenv |Set environment variable |
|var______|__________________________________|var=val__|var_val__|_________________________________________________________________________________|
|Â _______|${nnnn}___________________________|${nn}____|Â _______|More_than_9_arguments_can_be_referenced__________________________________________|
|"$@"_____|"$@"______________________________|"$@"_____|Â _______|All_arguments_as_separate_words__________________________________________________|
|$#_______|$#________________________________|$#_______|$#argv___|Number_of_arguments______________________________________________________________|
|$?_______|$?________________________________|$?_______|$status__|Exit_status_of_the_most_recently_executed_command________________________________|
|$!_______|$!________________________________|$!_______|Â _______|PID_of_most_recently_backgrounded_process________________________________________|
|$-_______|$-________________________________|$-_______|Â _______|Current_options__________________________________________________________________|
|. file |source file or . file |. file |source |Read commands in file |
|_________|__________________________________|_________|file_____|_________________________________________________________________________________|
|Â _______|alias_x='y'_______________________|alias_x=y|alias_x_y|Name_x_stands_for_command_y______________________________________________________|
|case |case |case |switch or|Choose alternatives |
|_________|__________________________________|_________|case_____|_________________________________________________________________________________|
|done_____|done______________________________|done_____|end______|End_a_loop_statement_____________________________________________________________|
|esac_____|esac______________________________|esac_____|endsw____|End_case_or_switch_______________________________________________________________|
|exit n |exit n |exit n |exit |Exit with a status |
|_________|__________________________________|_________|(expr)___|_________________________________________________________________________________|
|for/do___|for/do____________________________|for/do___|foreach__|Loop_through_variables___________________________________________________________|
| |set -f, set - | | | |
|Â |o |Â |noglob |Ignore substitution characters for filename generation |
|_________|nullglob|dotglob|nocaseglob|noglob|_________|_________|_________________________________________________________________________________|
|hash_____|hash______________________________|alias_-t_|hashstat_|Display_hashed_commands_(tracked_aliases)________________________________________|
|hash cmds|hash cmds |alias - |rehash |Remember command locations |
|_________|__________________________________|t_cmds___|_________|_________________________________________________________________________________|
|hash_-r__|hash_-r___________________________|Â _______|unhash___|Forget_command_locations_________________________________________________________|
|Â _______|history___________________________|history__|history__|List_previous_commands___________________________________________________________|
|Â _______|ArrowUp+Enter_or_!!_______________|r________|!!_______|Redo_previous_command____________________________________________________________|
| _______|!str______________________________|r_str____|!str_____|Redo_last_command_that_starts_with_“strâ€_____________________________________|
| |!cmd:s/x/y/ |r x=y cmd|!cmd:s/x/|Replace “x†with “y†in most recent command starting with “cmdâ€, then|execute.
|_________|__________________________________|_________|y/_______|_________________________________________________________________________________|
|if [ $i -|if [ $i -eq 5 ] |if ( |if |Sample condition test |
|eq_5_]___|__________________________________|(i==5))__|($i==5)__|_________________________________________________________________________________|
|fi_______|fi________________________________|fi_______|endif____|End_if_statement_________________________________________________________________|
|ulimit___|ulimit____________________________|ulimit___|limit____|Set_resource_limits______________________________________________________________|
|pwd______|pwd_______________________________|pwd______|dirs_____|Print_working_directory__________________________________________________________|
|read_____|read______________________________|read_____|$<_______|Read_from_terminal_______________________________________________________________|
|trap_2___|trap_2____________________________|trap_2___|onintr___|Ignore_interrupts________________________________________________________________|
|Â _______|unalias___________________________|unalias__|unalias__|Remove_aliases___________________________________________________________________|
|until____|until_____________________________|until____|Â _______|Begin_until_loop_________________________________________________________________|
|while/do_|while/do__________________________|while/do_|while____|Begin_while_loop_________________________________________________________________|
The Bourne Again SHell has many more features not listed here. This table is
just to give you an idea of how this shell incorporates all useful ideas from
other shells: there are no blanks in the column for bash. More information on
features found only in Bash can be retrieved from the Bash info pages, in the
“Bash Features†section.
More information:
You should at least read one manual, being the manual of your shell. The
preferred choice would be info bash, bash being the GNU shell and easiest for
beginners. Print it out and take it home, study it whenever you have 5 minutes.
See Appendix B,_DOS_versus_Linux_commands if you are having difficulties to
assimilate shell commands.
Glossary
Abstract
This section contains an alphabetical overview of commands discussed in this
document.
A
a2ps
Format files for printing on a PostScript printer, see Section 1.2,
“Formattingâ€.
acroread
PDF viewer, see Section 1.2.2,_“Previewing_formatted_filesâ€.
adduser
Create a new user or update default new user information.
alias
Create a shell alias for a command.
alsaconf
Configure sound card using the ALSA driver, see Section 1.2,_“Drivers_and
Architectureâ€.
alsamixer
Tune ALSA sound device output, see Section 2.2.3,_“Volume_controlâ€.
anacron
Execute commands periodically, does not assume continuously running
machine.
apropos
Search the whatis database for strings, see Section 3.3.2,_“The_whatis
and_apropos_commandsâ€.
apt-get
APT package handling utility, see Section 5.3.2,_“APTâ€.
arecord
Record a sound sample, see Section 2.3,_“Recordingâ€.
aspell
Spell checker.
at, atq, atrm
Queue, examine or delete jobs for later execution, see Section 1.2.2,
“Automatic_processes†and Section 4.3,_“The_at_commandâ€.
aumix
Adjust audio mixer, see Section 2.2.3,_“Volume_controlâ€.
(g)awk
Pattern scanning and processing language.
B
bash
Bourne Again SHell, see Section 2.3.2,_“The_shell†and Section 2.5,
“Shell_scriptsâ€.
batch
Queue, examine or delete jobs for later execution, see Section 1.2.2,
“Automatic_processesâ€.
bg
Run a job in the background, see Section 1.2.1,_“Interactive_processesâ€.
bitmap
Bitmap editor and converter utilities for the X window System.
bzip2
A block-sorting file compressor, see Section 1.1.3,_“Compressing_and
unpacking_with_gzip_or_bzip2â€.
C
cardctl
Manage PCMCIA cards, see Section 2.3.3,_“PCMCIA_commandsâ€.
cat
Concatenate files and print to standard output, see Section 2,_“Absolute
basics†and Section 2.4,_“The_most_important_configuration_filesâ€.
cd
Change directory, see Section 2,_“Absolute_basicsâ€.
cdp/cdplay
An interactive text-mode program for controlling and playing audio CD
Roms under Linux, see Section 2.1,_“CD_playing_and_copyingâ€.
cdparanoia
An audio CD reading utility which includes extra data verification
features, see Section 2.1,_“CD_playing_and_copyingâ€.
cdrecord
Record a CD-R, see Section 2.2,_“Making_a_copy_with_a_CD-writerâ€.
chattr
Change file attributes.
chgrp
Change group ownership, see Section 4.2.3,_“The_file_maskâ€.
chkconfig
Update or query run level information for system services, see
Section 2.5.1,_“Toolsâ€.
chmod
Change file access permissions, see Section 4.1,_“Access_rights:_Linux's
first_line_of_defenseâ€, Section 4.2.1,_“The_chmod_command†and
Section 4.2.4,_“Changing_user_and_group_ownershipâ€.
chown
Change file owner and group, see Section 4.2.3,_“The_file_maskâ€.
compress
Compress files.
cp
Copy files and directories, see Section 3.2,_“Creating_and_deleting_files
and_directoriesâ€.
crontab
Maintain crontab files, see Section 4.4,_“Cron_and_crontabâ€.
csh
Open a C shell, see Section 2.3.2,_“The_shellâ€.
cut
Remove sections from each line of file(s), see Section 2.5.2,_“Some
simple_examplesâ€.
D
date
Print or set system date and time.
dd
Convert and copy a file (disk dump), see Section 2.1.2,_“Using_the_dd
command_to_dump_dataâ€.
df
Report file system disk usage, see Section 1.2.3,_“Mount_pointsâ€.
dhcpcd
DHCP client daemon, see Section 3.8,_“DHCPâ€.
diff
Find differences between two files.
dig
Send domain name query packets to name servers, see Section 2.6.1,_“The
host_commandâ€.
dmesg
Print or control the kernel ring buffer.
du
Estimate file space usage.
dump
Backup file system, see Section 2.5,_“Tools_from_your_distributionâ€.
E
echo
Display a line of text, see Section 2.1,_“The_pathâ€.
ediff
Diff to English translator.
egrep
Extended grep.
eject
Unmount and eject removable media, see Section 5.5.2,_“Using_the_CDâ€.
emacs
Start the Emacs editor, see Section 1.2.1,_“GNU_Emacsâ€.
exec
Invoke subprocess(es), see Section 1.5.1,_“Process_creationâ€.
exit
Exit current shell, see Section 2,_“Absolute_basicsâ€.
export
Add function(s) to the shell environment, see Section 2.1,_“The_pathâ€,
Section 2.1.2,_“Exporting_variables†and Section 2.4.2,_“Some_examplesâ€.
F
fax2ps
Convert a TIFF facsimile to PostScript, see Section 1.2,_“Formattingâ€.
fdformat
Format floppy disk, see Section 2.1.1,_“Formatting_the_floppyâ€.
fdisk
Partition table manipulator for Linux, see Section 1.2.2,_“Partition
layout_and_typesâ€.
fetchmail
Fetch mail from a POP, IMAP, ETRN or ODMR-capable server, see
Section 3.2.3,_“Mail_user-agentsâ€.
fg
Bring a job in the foreground, see Section 1.2.1,_“Interactive
processesâ€.
file
Determine file type, see Section 3.1.2,_“More_toolsâ€.
find
Find files, see Section 3.3.3,_“Find_and_locateâ€.
firefox
Web browser, see Section 3.3.2,_“Web_browsersâ€.
fork
Create a new process, see Section 1.5.1,_“Process_creationâ€.
formail
Mail (re)formatter, see Section 3.2.3,_“Mail_user-agentsâ€.
fortune
Print a random, hopefully interesting adage.
ftp
Transfer files (unsafe unless anonymous account is used!)services, see
Section 3.4.2,_“FTP_clientsâ€.
G
galeon
Graphical web browser.
gdm
Gnome Display Manager, see Section 2.4,_“Initâ€.
gedit
GUI editor, see Section 3.3.3,_“But_I_want_a_graphical_text_editor!â€.
(min/a)getty
Control console devices.
gimp
Image manipulation program.
gpg
Encrypt, check and decrypt files, see Section 4.1.2,_“GNU_Privacy_Guardâ€.
grep
Print lines matching a pattern, see Section 3.3.4,_“The_grep_command†and
Section 3.1,_“More_about_grepâ€.
groff
Emulate nroff command with groff, see Section 1.2,_“Formattingâ€.
grub
The grub shell, see Section 2.3,_“GRUB_features†and Section 5.4,
“Upgrading_your_kernelâ€.
gv
A PostScript and PDF viewer, see Section 1.2.2,_“Previewing_formatted
filesâ€.
gvim
Graphical version of the vIm editor, see Section 3.3.3,_“But_I_want_a
graphical_text_editor!â€.
gzip
Compress or expand files, see Section 1.1.3,_“Compressing_and_unpacking
with_gzip_or_bzip2â€.
H
halt
Stop the system, see Section 2.6,_“Shutdownâ€.
head
Output the first part of files, see Section 3.4.3,_“The_head_and_tail
commandsâ€.
help
Display help on a shell built-in command.
host
DNS lookup utility, see Section 2.6.1,_“The_host_commandâ€.
httpd
Apache hypertext transfer protocol server, see Section 2.3.1,_“The_ip
commandâ€.
I
id
Print real and effective UIDs and GIDs, see Section 4.1,_“Access_rights:
Linux's_first_line_of_defenseâ€.
ifconfig
Configure network interface or show configuration, see Section 1.2.3,
“PPP,_SLIP,_PLIP,_PPPOEâ€.
info
Read Info documents, see Section 3.3.1,_“The_Info_pagesâ€.
init
Process control initialization, see Section 1.5.1,_“Process_creationâ€,
Section 2.4,_“Init†and Section 2.5,_“Init_run_levelsâ€.
insserv
Manage init scripts, see Section 2.5.1,_“Toolsâ€.
iostat
Display I/O statistics, see Section 3.5.4,_“I/O_resourcesâ€.
ip
Display/change network interface status, see Section 1.2.3,_“PPP,_SLIP,
PLIP,_PPPOEâ€.
ipchains
IP firewall administration, see Section 4.4.2,_“X11_and_TCP_forwardingâ€.
iptables
IP packet filter administration, see Section 4.4.2,_“X11_and_TCP
forwardingâ€.
J
jar
Java archive tool, see Section 1.1.4,_“Java_archivesâ€.
jobs
List backgrounded tasks.
K
kdm
Desktop manager for KDE, see Section 2.4,_“Initâ€.
kedit
KDE graphical editor, see Section 3.3.3,_“But_I_want_a_graphical_text
editor!â€.
kill(all)
Terminate process(es), see Section 1.2.1,_“Interactive_processesâ€.
konqueror
File manager, (help) browser, see Section 3.2.1,_“Making_a_mess...â€.
ksh
Open a Korn shell, see Section 2.3.2,_“The_shellâ€.
kwrite
KDE graphical editor, see Section 3.3.3,_“But_I_want_a_graphical_text
editor!â€.
L
less
more with features, see Section 3.4.2,_““less_is_moreâ€â€.
lilo
Linux boot loader, see Section 2,_“Boot_process,_Init_and_shutdownâ€.
links
Text mode WWW browser, see Section 2.3.2,_“The_ifconfig_commandâ€.
ln
Make links between files, see Section 3.5,_“Linking_filesâ€.
loadkeys
Load keyboard translation tables, see Section 4.1,_“Keyboard_setupâ€.
locate
Find files, see Section 3.3.3,_“Find_and_locate†and Section 4.4,_“Cron
and_crontabâ€.
logout
Close current shell, see Section 1.3,_“Text_modeâ€.
lp
Send requests to the LP print service, see Section 1,_“Printing_filesâ€.
lpc
Line printer control program, see Section 1,_“Printing_filesâ€.
lpq
Print spool queue examination program, see Section 1,_“Printing_filesâ€.
lpr
Offline print, see Section 1,_“Printing_filesâ€.
lprm
Remove print requests, see Section 1,_“Printing_filesâ€.
ls
List directory content, see Section 2,_“Absolute_basicsâ€, Section 1.1.2,
“Sorts_of_files†and Section 3.1.1,_“More_about_lsâ€.
lynx
Text mode WWW browser, see Section 2.3.2,_“The_ifconfig_commandâ€.
M
mail
Send and receive mail, see Section 3.2.3,_“Mail_user-agentsâ€.
man
Read man pages, see Section 3.2,_“The_man_pagesâ€.
mc
Midnight COmmander, file manager, see Section 3.2.1,_“Making_a_mess...â€.
mcopy
Copy MSDOS files to/from Unix.
mdir
Display an MSDOS directory.
memusage
Display memory usage, see Section 3.5.3,_“Memory_resourcesâ€.
memusagestat
Display memory usage statistics, see Section 3.5.3,_“Memory_resourcesâ€.
mesg
Control write access to your terminal, see Section 1.6,_“SUID_and_SGIDâ€.
mformat
Add an MSDOS file system to a low-level formatted floppy disk, see
Section 2.1.1,_“Formatting_the_floppyâ€.
mkbootdisk
Creates a stand-alone boot floppy for the running system.
mkdir
Create directory, see Section 3.2,_“Creating_and_deleting_files_and
directoriesâ€.
mkisofs
Create a hybrid ISO9660 filesystem, see Section 2.2,_“Making_a_copy_with
a_CD-writerâ€.
mplayer
Movie player/encoder for Linux, see Section 2.2,_“Playing_music_filesâ€
and Section 3,_“Video_playing,_streams_and_television_watchingâ€.
more
Filter for displaying text one screen at the time, see Section 3.4.2,
““less_is_moreâ€â€.
mount
Mount a file system or display information about mounted file systems,
see Section 5.5.1,_“Mounting_a_CDâ€.
mozilla
Web browser, see Section 2.3.2,_“The_ifconfig_commandâ€.
mt
Control magnetic tape drive operation.
mtr
Network diagnostic tool.
mv
Rename files, Section 3.2,_“Creating_and_deleting_files_and_directoriesâ€.
N
named
Internet domain name server, see Section 3.7,_“The_Domain_Name_Systemâ€.
nautilus
File manager, see Section 3.2.1,_“Making_a_mess...â€.
ncftp
Browser program for ftp services (insecure!), see Section 3.4.2,_“FTP
clientsâ€.
netstat
Print network connections, routing tables, interface statistics,
masquerade connections, and multi-cast memberships, see Section 1.2.5,
“AppleTalk†and Section 4.2,_“Rsh,_rlogin_and_telnetâ€.
newgrp
Log in to another group, see Section 4.2.2,_“Logging_on_to_another
groupâ€.
nfsstat
Print statistics about networked file systems.
nice
Run a program with modified scheduling priority, see Section 3.5.1,
“Priorityâ€.
nmap
Network exploration tool and security scanner.
ntpd
Network Time Protocol Daemon, see Section 4.3,_“Date_and_time_zoneâ€.
ntpdate
Set the date and time via an NTP server, see Section 4.3,_“Date_and_time
zoneâ€.
ntsysv
Simple interface for configuring run levels, see Section 2.5.1,_“Toolsâ€.
O
ogle
DVD player with support for DVD menus, see Section 3,_“Video_playing,
streams_and_television_watchingâ€.
P
passwd
Change password, see Section 2,_“Absolute_basics†and Section 1.6,_“SUID
and_SGIDâ€.
pccardctl
Manage PCMCIA cards, see Section 2.3.3,_“PCMCIA_commandsâ€.
pdf2ps
Ghostscript PDF to PostScript translator, see Section 1.2,_“Formattingâ€.
perl
Practical Extraction and Report Language.
pg
Page through text output, see Section 3.4.2,_““less_is_moreâ€â€.
pgrep
Look up processes based on name and other attributes, see Section 1.4,
“Displaying_process_informationâ€.
ping
Send echo request to a host, see Section 2.6.2,_“The_ping_commandâ€.
play
Play a sound sample, see Section 2.3,_“Recordingâ€.
pr
Convert text files for printing.
printenv
Print all or part of environment, see Section 2.1,_“Environment
variablesâ€.
procmail
Autonomous mail processor, see Section 3.2.3,_“Mail_user-agentsâ€.
ps
Report process status, see Section 1.4,_“Displaying_process_informationâ€
and Section 3.5.4,_“I/O_resourcesâ€.
pstree
Display a tree of processes, see Section 1.4,_“Displaying_process
informationâ€.
pwd
Print present working directory, see Section 2,_“Absolute_basicsâ€.
Q
quota
Display disk usage and limits, see Section 2.3.3,_“Your_home_directoryâ€.
R
rcp
Remote copy (unsafe!)
rdesktop
Remote Desktop Protocol client, see Section 4.6,_“The_rdesktop_protocolâ€.
reboot
Stop the system, see Section 2.6,_“Shutdownâ€.
recode
Convert files to another character set, see Section 4.4,_“Languageâ€.
renice
Alter priority of a running process, see Section 3.5.1,_“Priorityâ€.
restore
Restore backups made with dump, see Section 2.5,_“Tools_from_your
distributionâ€.
rlogin
Remote login (telnet, insecure!), see Section 4.2,_“Rsh,_rlogin_and
telnet†and Section 5.2,_“Servicesâ€.
rm
Remove a file, see Section 3.2,_“Creating_and_deleting_files_and
directoriesâ€.
rmdir
Remove a directory, see Section 3.2.2,_“The_toolsâ€.
roff
A survey of the roff typesetting system, see Section 1.2,_“Formattingâ€.
rpm
RPM Package Manager, see Section 5.2.1,_“RPM_packagesâ€.
rsh
Remote shell (insecure!), see Section 4.2,_“Rsh,_rlogin_and_telnetâ€.
rsync
Synchronize two directories, see Section 3,_“Using_rsyncâ€.
S
scp
Secure remote copy, see Section 4.4.1,_“Introductionâ€.
screen
Screen manager with VT100 emulation, see Section 1.2.1,_“Interactive
processesâ€.
set
Display, set or change variable.
setterm
Set terminal attributes.
sftp
Secure (encrypted) ftp, see and Section 4.4.1,_“Introductionâ€.
sh
Open a standard shell, see Section 2.3.2,_“The_shellâ€.
shutdown
Bring the system down, see Section 2.6,_“Shutdownâ€.
sleep
Wait for a given period, see Section 4.1,_“Use_that_idle_time!â€.
slocate
Security Enhanced version of the GNU Locate, see Section 3.3.3,_“Find_and
locateâ€.
slrnn
text mode Usenet client, see Section 2.6,_“Other_hostsâ€.
snort
Network intrusion detection tool.
sort
Sort lines of text files, see Section 3.2,_“Filtering_outputâ€.
spell
Spell checker, see Section 1.2.3,_“Combining_redirectionsâ€.
ssh
Secure shell, see Section 4.4.1,_“Introductionâ€.
ssh-keygen
Authentication key generation, management and conversion, see
Section 4.4.5,_“Authentication_keysâ€.
stty
Change and print terminal line settings.
su
Switch user, see Section 2.1,_“The_pathâ€, Section 5.3.2,_“APT†and
Section 4.6,_“The_rdesktop_protocolâ€.
T
tac
Concatenate and print files in reverse, see cat.
tail
Output the last part of files, see Section 3.4.3,_“The_head_and_tail
commandsâ€.
talk
Talk to a user.
tar
Archiving utility, see Section 1.1.1,_“Archiving_with_tarâ€.
tcsh
Open a Turbo C shell, see Section 2.3.2,_“The_shellâ€.
telinit
Process control initialization, see Section 2.5,_“Init_run_levelsâ€.
telnet
User interface to the TELNET protocol (insecure!), see Section 4.2,_“Rsh,
rlogin_and_telnetâ€.
tex
Text formatting and typesetting, see Section 1.2,_“Formattingâ€.
time
Time a simple command or give resource usage, see Section 3.2,_“How_long
does_it_take?â€.
tin
News reading program, see Section 2.6,_“Other_hostsâ€.
top
Display top CPU processes, see Section 1.4,_“Displaying_process
informationâ€, Section 3.5.3,_“Memory_resources†and Section 3.5.4,_“I/
O_resourcesâ€.
touch
Change file timestamps, see Section 1.2,_“Make_spaceâ€.
traceroute
Print the route packets take to network host, see Section 2.6.3,_“The
traceroute_commandâ€.
tripwire
A file integrity checker for UNIX systems, see Section 4.5,_“VNCâ€.
troff
Format documents, see Section 1.2,_“Formattingâ€.
tvime
A high quality television application.
twm
Tab Window Manager for the X Window System.
U
ulimit
Controll resources, see Section 1.2.5,_“Limit_file_sizesâ€.
umask
Set user file creation mask, see Section 4.2.2,_“Logging_on_to_another
groupâ€.
umount
Unmount a file system.
uncompress
Decompress compressed files.
uniq
Remove duplicate lines from a sorted file, see Section 3.2,_“Filtering
outputâ€.
up2date
Update RPM packages, see Section 5.3.3,_“Systems_using_RPM_packagesâ€.
update
Kernel daemon to flush dirty buffers back to disk.
update-rc.d
Configure init scripts, see Section 2.5.1,_“Toolsâ€.
uptime
Display system uptime and average load, see Section 1.4,_“Displaying
process_information†and Section 3.5.2,_“CPU_resourcesâ€.
urpmi
Update RPM packages, see Section 5.3.3,_“Systems_using_RPM_packagesâ€.
userdel
Delete a user account and related files.
V
vi(m)
Start the vi (improved) editor, see Section 1.2.2,_“Vi(m)â€.
vimtutor
The Vim tutor.
vmstat
Report virtual memory statistics, see Section 3.5.4,_“I/O_resourcesâ€.
W
w
Show who is logged on and what they are doing.
wall
Send a message to everybody's terminal, see Section 1.6,_“SUID_and_SGIDâ€.
wc
Print the number of bytes, words and lines in files, see Section 2.1,
“The_pathâ€.
which
Shows the full path of (shell) commands, see Section 2.1,_“The_path†and
Section 3.3.2,_“Whichâ€.
who
Show who is logged on, see Section 1.6,_“SUID_and_SGIDâ€.
who am i
Print effective user ID.
whois
Query a whois or nicname database, see Section 2.6.4,_“The_whois
commandâ€.
write
Send a message to another user, see Section 1.6,_“SUID_and_SGIDâ€.
X
xargs
Build and execute command lines from standard input, see Section 3.3.3,
“Find_and_locateâ€.
xauth
X authority file utility.
xawtv
An X11 program for watching TV.
xcdroast
Graphical front end to cdrecord, see Section 2.2,_“Making_a_copy_with_a
CD-writerâ€.
xclock
Analog/digital clock for X.
xconsole
Monitor system console messages with X.
xdm
X Display Manager with support for XDMCP, host chooser, see Section 2.4,
“Init†and Section 3.2,_“The_X_Window_Systemâ€.
xdvi
DVI viewer, see Section 1.2.2,_“Previewing_formatted_filesâ€.
xedit
X Window graphical editor, see Section 3.3.3,_“But_I_want_a_graphical
text_editor!â€.
xfs
X font server.
xhost
Server access control program for X, see Section 4.3.2,_“Telnet_and_Xâ€.
xine
A free video player, see Section 3,_“Video_playing,_streams_and
television_watchingâ€.
xinetd
The extended Internet services daemon, see Section 3.1.2,_“(x)inetdâ€.
xload
System load average display for X, see Section 3.5.6,_“Graphical_toolsâ€.
xlsfonts
Server font list displayer for X.
xmms
Audio player for X, see Section 2.2.1,_“mp3_filesâ€.
xpdf
PDF viewer, see Section 1.2.2,_“Previewing_formatted_filesâ€.
xterm
Terminal emulator for X.
Y
yast
System administration tool on Novell SuSE Linux.
yum
Update RPM packages, see Section 5.3.3,_“Systems_using_RPM_packagesâ€.
Z
zapping
A TV viewer for the Gnome environment.
zcat
Compress or expand files.
zgrep
Search possibly compressed files for a regular expression.
zmore
Filter for viewing compressed text.
Index
Symbols
.bashrc, Output_redirection_with_>_and_|, Shell_setup_files, Some_examples,
Proxy_configuration
.bash_login, Shell_setup_files
example, .bash_login_example
.bash_logout, Shell_setup_files
example, .bash_logout_example
.bash_profile
, Shell_setup_files
example, .bash_profile_example
.forward, Mail
.profile, Shell_setup_files
/etc/fstab, Mounting_a_CD
A
absolute paths, Absolute_and_relative_paths
acroread, Previewing_formatted_files
aliases, The_most_important_configuration_files
alsamixer, Volume_control
apache, The_most_important_configuration_files
Apache, The_Apache_Web_Server
apropos, The_commands, The_man_pages
example, The_whatis_and_apropos_commands
apt-get, What_are_Debian_packages?, APT
installing packages, APT
update package list, APT
upgrade system, APT
aptitude, What_are_Debian_packages?
archiving
burning a CD, Making_a_copy_with_a_CD-writer
CD-writer, Making_a_copy_with_a_CD-writer
dd, Using_the_dd_command_to_dump_data
floppies, Formatting_the_floppy
Java archives, Java_archives
tapes, Backing_up_data_using_a_tape_device
tools, Archiving_with_tar
USB devices, Backups_on/from_jazz_drives,_USB_devices_and_other
removables
arecord, Recording
arrows, Using_Bash_features
at, Use_that_idle_time!
example, The_at_command
audio
ALSA, Drivers_and_Architecture
applications, Other_formats
architecture, Drivers_and_Architecture
drivers, Installation
OSS, Drivers_and_Architecture
recording, Recording
volume control, Volume_control
aumix, Volume_control
automount, Mounting_a_CD
B
backslash, The_path, Special_characters
backups
rsync, Introduction
tape device, Archiving_with_tar
tar, Archiving_with_tar
tools, Tools_from_your_distribution
Bash
command name completion, Using_Bash_features
features, Using_Bash_features
file name completion, Using_Bash_features
job control, Interactive_processes
noclobber, Output_redirection_with_>_and_|
bash, Shell_types
bashrc, The_most_important_configuration_files, The_file_mask, Shell_setup
files
example, /etc/bashrc_example
batch, Automatic_processes
bg, Interactive_processes
bin, Visual
BIOS, The_boot_process
boot, Visual, Introduction
BIOS, The_boot_process
boot-loader, The_boot_process
direct loading, GRUB_features
GRUB, Introduction
init, Init
LILO, Introduction
MBR, The_boot_process
boot partition, Partition_layout_and_types
bootstrapping, Process_creation
Bourne Again SHell, Shell_types
Bourne Shell, Shell_types
bzip2, Compressing_and_unpacking_with_gzip_or_bzip2
C
C shell, Shell_types
camera
mounting, Backups_on/from_jazz_drives,_USB_devices_and_other_removables
cancel, Removing_jobs_from_the_print_queue
cardctl, PCMCIA_commands
cat, The_commands, “less_is_moreâ€
syntax, The_most_important_configuration_files
cd, The_commands
CD
audio CD playing, CD_playing_and_copying
audio CD ripping, CD_playing_and_copying
eject, Ejecting_the_CD
using, Using_the_CD
CD burning
command line tools, Making_a_copy_with_a_CD-writer
graphical tools, Making_a_copy_with_a_CD-writer
CD images
mkisofs example, Making_a_copy_with_a_CD-writer
cdp, CD_playing_and_copying
cdparanoia, CD_playing_and_copying
cdplay, CD_playing_and_copying
cdrecord, Making_a_copy_with_a_CD-writer
cdrom, The_most_common_devices
character encoding, Language
chgrp, Changing_user_and_group_ownership
example, Changing_user_and_group_ownership
chkconfig, Tools
chmod, The_chmod_command
example, The_chmod_command
chown, Logging_on_to_another_group, Changing_user_and_group_ownership
example, Changing_user_and_group_ownership
restrictions, Changing_user_and_group_ownership
syntax, Changing_user_and_group_ownership
coloured ls output, Sorts_of_files
command name completion
, Using_Bash_features
commands
aliases, Which
arguments, General_remarks
basics, The_commands
built-ins, Exceptions
entering, General_remarks
finding, Which
multiple man pages, The_man_pages
not found, Which
options, General_remarks
remote execution, Introduction
synopsis, The_man_pages
configuration files
, The_most_important_configuration_files
console, The_most_common_devices
copy interactively, Removing_files
coreutils, More_about_ls
cp, Copying_files
cron, Use_that_idle_time!
crontab syntax, Cron_and_crontab
edit crontab, Cron_and_crontab
run-parts, Cron_and_crontab
Vixie cron, Cron_and_crontab
cron.*, The_most_important_configuration_files
cron.d, Cron_and_crontab
crontab, The_most_important_configuration_files, Cron_and_crontab
example, Cron_and_crontab
csh, Shell_types
Ctrl, Using_Bash_features
cua, The_most_common_devices
CUPS, General
Cygwin, Cygwin
D
daemons
cron, Cron_and_crontab
definition, Daemons
inetd, (x)inetd
dash, General_remarks, Sorts_of_files
data partition, Partition_layout_and_types
data partitions, Partition_layout_and_types
date
configuration, Date_and_time_zone
dd, Using_the_dd_command_to_dump_data
default, The_most_important_configuration_files, Init
delete interactively, Removing_files
desktop
development, Linux_for_non-experienced_users
look and feel, Linux_for_non-experienced_users
menus, Graphical_mode
office, Ten_years_of_experience_at_your_service
types, Which_distribution_should_I_install?
dev, Visual, The_most_common_devices
devices
entries, The_most_common_devices
df, Mount_points, Making_a_copy_with_a_CD-writer
example, Mount_points, Visual
DHCP, DHCP
dhcpcd, DHCP
dict, General_line_filtering
dig, The_host_command
directories
creation, Creating_directories
definition, Sorts_of_files
removing, Removing_files
directory tree, Visual
DISPLAY, Reserved_variables, Display_names, Telnet_and_X, X11_and_TCP
forwarding
distributions
how to chose?, Which_distribution_should_I_install?
ISO images, Which_distribution_should_I_install?
mainstream, Which_distribution_should_I_install?
dmesg, Backups_on/from_jazz_drives,_USB_devices_and_other_removables
DOMAIN, Reserved_variables
dots, General_remarks, More_about_ls
DrakConf, Tools
dselect, What_are_Debian_packages?
dsp, The_most_common_devices
dump, Tools_from_your_distribution
E
echo, The_path, Exporting_variables
example, Your_home_directory
EDITOR, Reserved_variables
editors
emacs, GNU_Emacs
graphical editors, But_I_want_a_graphical_text_editor!
text editor overview, Which_editor_should_I_use?
vim, Vi(m)
why?, Why_should_I_use_an_editor?
EGID, Process_attributes
emacs, GNU_Emacs
encryption
creating encrypted files, Encrypt_data
decrypting, Decrypting_files
export public key, About_your_key
fingerprint, About_your_key
generate keys, Generate_a_key
GnuPG, GNU_Privacy_Guard
key ID, About_your_key
searching keys, About_your_key
why?, Why_should_you_encrypt_data?
environment
graphical, Introduction
organization, Introduction
show variables, General
escape characters, Special_characters
etc, Visual, The_most_important_configuration_files
EUID, Process_attributes
example, Process_attributes
exec, Services
exit, The_commands
exit status, Ending_processes
export, Exporting_variables
F
fd, The_most_common_devices
fdformat, Formatting_the_floppy
fdisk, Partition_layout_and_types
fetchmail, Mail_user-agents
fg, Interactive_processes
file, The_commands, More_tools, General
example, More_tools
file descriptors
examples, Use_of_file_descriptors, Analyzing_errors
overview, Use_of_file_descriptors
file managers
overview, Making_a_mess...
file name completion
, Using_Bash_features
file permissions
codes, Access_rights:_Linux's_first_line_of_defense
display, Access_rights:_Linux's_first_line_of_defense
overview, The_chmod_command
setting, The_chmod_command
SGID, Special_modes, SUID_and_SGID
special modes, Special_modes
SUID, Special_modes, SUID_and_SGID
file system
hierarchy, Visual
layout, Visual
orientation, The_path
file types
colours, More_about_ls
determining, More_tools
suffixes, More_about_ls
files
abstraction, General
change ownership recursively, Changing_user_and_group_ownership
changing ownership, Changing_user_and_group_ownership
compressing, Compressing_and_unpacking_with_gzip_or_bzip2
concept, General
copying, Copying_files
core dumps, Limit_file_sizes
decrypting, Decrypting_files
display permissions, Access_rights:_Linux's_first_line_of_defense
emptying, Emptying_files
encrypting, Encrypt_data
limit size, Limit_file_sizes
linking, Link_types
list of types, Sorts_of_files
logging, Log_files
logs, More_about_log_files
mask, The_file_mask
moving, Moving_files
music files, mp3_files
naming conventions, Creating_directories
properties, More_about_ls
removing, Removing_files
renaming, Moving_files
searching, Find_and_locate
searching content, General_line_filtering
security, Access_rights:_Linux's_first_line_of_defense
sharing in a group, Special_modes
size, Find_and_locate, Filtering_output
structure, General
types, Sorts_of_files
uncompressing, Compressing_and_unpacking_with_gzip_or_bzip2
filesystems, The_most_important_configuration_files
filtering
grep, More_about_grep
sort, Filtering_output
find, Find_and_locate
example, Find_and_locate
finding files
find, Find_and_locate
locate, Find_and_locate
using the shell, Using_shell_features
fingerprint, About_your_key
fonts
configuration, Fonts
fork-and-exec, Process_creation
formail, Mail_user-agents
fstab, Mount_points, The_most_important_configuration_files, Backups_on/from
jazz_drives,_USB_devices_and_other_removables
FTP
clients, FTP_clients
secure FTP, FTP_clients
servers, FTP_servers
ftp, FTP_clients
ftp*, The_most_important_configuration_files
ftp_proxy, Proxy_configuration
G
gdm, Init
getty, Init
gimp, General
Gnome System Monitor, Displaying_process_information, Graphical_tools
gnome-alsamixer, Volume_control
gnome-cd, CD_playing_and_copying
gnome-sound-recorder, Recording
GNU, Linux_and_GNU
examples, Linux_and_GNU
GNU/Linux, GNU/Linux
public license, Linux_Pros
tools, Linux_and_GNU
gpdf, Previewing_formatted_files
gpg, Generate_a_key
GPL, Linux_Pros
greater-than, Output_redirection_with_>_and_|
grep, General_line_filtering
example, General_line_filtering
I/O redirection, More_about_grep
special characters, Special_characters
groff, Tools_and_languages
group, The_most_important_configuration_files, Logging_on_to_another_group
groups
changing ownership, Changing_user_and_group_ownership
display, Logging_on_to_another_group
primary group, Logging_on_to_another_group
switching, Logging_on_to_another_group
user private groups, Logging_on_to_another_group
GRUB
, The_boot_process
features, GRUB_features
grub.conf
example, Upgrading_your_kernel
GUI, Linux_for_non-experienced_users
gv, General, Previewing_formatted_files
gzip, Compressing_and_unpacking_with_gzip_or_bzip2
H
halt, Shutdown
hd, The_most_common_devices
head, The_head_and_tail_commands
help
/usr/share/doc, Exceptions
example, The_--help_option
Gnome Help, Graphical_help
graphical, Graphical_help
option, The_--help_option
HISTSIZE, Reserved_variables
home, Visual, Your_home_directory
HOME, Your_home_directory, Reserved_variables
home directory
definition, Your_home_directory
example, Making_a_mess...
limitations, Your_home_directory
path, Your_home_directory
usage, Your_home_directory
host, The_host_command
HOSTNAME, Reserved_variables
hosts, The_most_important_configuration_files, /etc/hosts
http_proxy, Proxy_configuration
I
I/O redirection
append output, The_>>_operator
combining, Combining_redirections
emptying files, Output_redirection_with_>_and_|
null command, Emptying_files
prevent overwrite, Output_redirection_with_>_and_|
printing, Getting_the_file_to_the_printer
saving output in a file, Output_redirection_with_>_and_|
splitting stdout and stderr, Separating_standard_output_from_standard
error
write to stdout and file, Writing_to_output_and_files_simultaneously
id, Access_rights:_Linux's_first_line_of_defense, Logging_on_to_another_group
ifconfig, The_ip_command
IMAP, Remote_mail_servers
inetd, (x)inetd
inetd.conf, The_most_important_configuration_files, (x)inetd
info, The_commands, The_Info_pages
example, The_Info_pages
info info, The_Info_pages
Info pages, The_Info_pages
info pages
browsing, The_Info_pages
init, Process_creation, Init
kill scripts, Init
run levels, Init_run_levels
start scripts, Init
SysV/BSD, Init
init scripts
administration, Tools
init.d, Tools
initrd, Visual
inittab, The_most_important_configuration_files, Init
inode
definition, The_file_system_in_reality
stored information, The_file_system_in_reality
input
default input, What_are_standard_input_and_standard_output?
redirection, Input_redirection
standard input, What_are_standard_input_and_standard_output?
INPUTRC, Reserved_variables
inputrc, Fonts
insserv, Tools
installing software
package managers, Linux_and_GNU
Internet Protocol, TCP/IP
iostat, I/O_resources
ip, The_ip_command
ir, The_most_common_devices
ISC, The_Domain_Name_System
isdn, The_most_common_devices
issue, The_most_important_configuration_files
J
jar, Java_archives
job control, Interactive_processes
jobs, Interactive_processes
journaled file system, Why_partition?
js, The_most_common_devices
K
kaudiocreator, CD_playing_and_copying
kdm, Init
kdvi, Previewing_formatted_files
kernel
functions, The_kernel
upgrade, Upgrading_your_kernel
key combinations, Using_Bash_features
keyboard
layout, Keyboard_setup
XkbdLayout, Keyboard_setup
KeyBoard
switching layouts, Keyboard_setup
kill, Interactive_processes, Signals
example, Interrupting_your_processes
kmem, SUID_and_SGID
kmix, Volume_control
Konqueror
, Graphical_help
konqueror, Making_a_mess...
Korn shell, Shell_types
kpdf, Previewing_formatted_files
krec, Recording
ksh, Shell_types
L
LANG, Reserved_variables, Language
LANGUAGE, Language
LBA, GRUB_features
ld.so.conf, The_most_important_configuration_files
LDAP, LDAP
LD_LIBRARY_PATH, Reserved_variables
less, “less_is_moreâ€
pager for man, The_man_pages
less-than, Input_redirection
lib, Visual
LILO, Introduction
lilo.conf, The_most_important_configuration_files
links
creating, Creating_symbolic_links
definition, Sorts_of_files
hard links, Link_types
inodes, Link_types
mechanism, Link_types
saving space, Save_space_with_a_link
symbolic links, Link_types
types, Link_types
usage, Creating_symbolic_links
user-space link, Link_types
Linus
creator, Linus_and_Linux
Linux
advantages, Linux_Pros
applications, Current_application_of_Linux_systems
bugs, Linux_Pros
disadvantages, Linux_Cons
distributions, Linux_and_GNU
ease of use, Linux_for_non-experienced_users
free, Linux_Pros
goals, Linus_and_Linux
origin, UNIX
portability, Linux_Pros
properties, Linux_Pros
robustness, Linux_Pros
scalability, Linux_Pros
security, Linux_Pros
user interface, Is_Linux_difficult?
ln, Creating_symbolic_links
load, Load
loadkeys, Keyboard_setup
localhost, /etc/hosts
locate, Find_and_locate
log files
syslog, The_most_common_variable_files
login, Shell_setup_files
authentication, Introduction
graphical, Graphical_mode
language configuration, Language
root, Graphical_mode
text mode, Text_mode
LOGNAME, Reserved_variables
logout, Text_mode
graphical, Graphical_mode
logrotate.*, The_most_important_configuration_files
lost+found, Visual
lp, The_most_common_devices, Getting_the_file_to_the_printer
lpq, Status_of_your_print_jobs
lpr, Getting_the_file_to_the_printer
lprm, Removing_jobs_from_the_print_queue
lpstat, Status_of_your_print_jobs
ls, The_commands, General_remarks, Sorts_of_files, More_about_ls, Access
rights:_Linux's_first_line_of_defense
coloured output, More_about_ls
example, More_about_ls
M
mail, The_most_important_configuration_files
anti-spam/anti-virus, Servers
client applications, Mail_user-agents
mailing lists, Servers
protocols, Remote_mail_servers
servers overview, Servers
MAIL, Reserved_variables
man, The_commands
example, The_man_pages
man pages
browsing, The_man_pages
graphical, Graphical_help
man man, The_man_pages
printing, Tools_and_languages
sections, The_man_pages
structure, The_man_pages
MANPATH, Exporting_variables, Reserved_variables
mask, The_file_mask
MBR, The_boot_process
mc, Making_a_mess...
mem, The_most_common_devices, SUID_and_SGID
messages
language, Language
mformat, Formatting_the_floppy
midi, The_most_common_devices
misc, Visual
mixer, The_most_common_devices
mkbootdisk, Formatting_the_floppy
mkdir, Creating_directories
mkisofs, Making_a_copy_with_a_CD-writer
mnt, Visual
modem, The_most_common_devices
modules.conf, The_most_important_configuration_files, Backups_on/from_jazz
drives,_USB_devices_and_other_removables
monitoring, Graphical_tools
more, “less_is_moreâ€
motd, The_most_important_configuration_files
mount, Mounting_a_CD
example, Mounting_a_CD
mount a CD, Mounting_a_CD
mount point, Mount_points
example, Mount_points
mouse, The_most_common_devices
mplayer, mp3_files
mt, Backing_up_data_using_a_tape_device
mtab, The_most_important_configuration_files
music, The_most_common_devices
mv, Moving_files
N
named, The_Domain_Name_System
named pipes
definition, Sorts_of_files
nameserver, The_Domain_Name_System
nautilus, Graphical_help, Making_a_mess...
ncftp, FTP_clients
net, Visual
netstat, I/O_resources, Services
network configuration
files, Network_configuration_files
graphical tools, Configuration_of_network_interfaces
networking
chatting and conferencing, Chatting_and_conferencing
check other hosts, The_ping_command
device names, The_ip_command, Network_interface_names
display IP address, The_ip_command
display routes, The_ip_command, Checking_the_host_configuration_with
netstat
host lookup, The_host_command
IETF, TCP/IP
info on hosts, The_whois_command
local loop, The_ip_command
OSI Model, The_OSI_Model
PCMCIA, PCMCIA_commands
protocol overview, Some_popular_networking_protocols
protocols, The_OSI_Model
remote execution, Introduction
Samba, SMB/NMB
show IP address, The_ifconfig_command
trace hosts, The_traceroute_command
newgrp, Logging_on_to_another_group
nice, Priority
nice number, Priority
noclobber, Output_redirection_with_>_and_|
nsswitch.conf, The_most_important_configuration_files, /etc/nsswitch.conf
NTP, Date_and_time_zone
ntpdate, Date_and_time_zone
ntsysv, Tools
null, The_most_common_devices
O
office suites
overview, History
office suits
best practice, General_use_of_office_documents
history, History
Open Source
achievements, Open_Source
community, Ten_years_of_experience_at_your_service
other projects, Ten_years_of_experience_at_your_service
reliability, Linux_Cons
what is it?, Open_Source
OpenOffice.org, History
opt, Visual
options
--help, The_--help_option
long options, General_remarks
OS, General, Reserved_variables
OSI Model, The_OSI_Model
OSTYPE, Reserved_variables
output
default output, What_are_standard_input_and_standard_output?
redirection with >, Output_redirection_with_>_and_|
redirection with |, Output_redirection_with_>_and_|
standard output, What_are_standard_input_and_standard_output?
P
packages
APT, APT
DEB, What_are_Debian_packages?
install from CD, Mounting_a_CD
install RPM, RPM_examples
kernel, Upgrading_your_kernel
RPM, What_is_RPM?
source, Source_packages
Update Agent, Systems_using_RPM_packages
updates, General_remarks
URPMI, Systems_using_RPM_packages
YaST, Systems_using_RPM_packages
YOU, Systems_using_RPM_packages
yum, Systems_using_RPM_packages
PAGER, Reserved_variables
PAM, PAM
pam.d, The_most_important_configuration_files
par, The_most_common_devices
partitions
creation, Partition_layout_and_types
file system, The_file_system_in_reality
layout, Partition_layout_and_types
mount points, Mount_points
root partition, Partition_layout_and_types, Mount_points
server layout, Partition_layout_and_types
swap, Partition_layout_and_types
types, Partition_layout_and_types
var partition, The_most_common_variable_files
why?, Why_partition?
workstation layout, Partition_layout_and_types
passwd, The_commands, Shell_types, The_most_important_configuration_files,
Logging_on_to_another_group, SUID_and_SGID, Traditional
PATH, The_path, Reserved_variables
configuration, The_path, Which
example, The_path
paths
examples, Absolute_and_relative_paths
types, Absolute_and_relative_paths
pccardctl, PCMCIA_commands
performance, Performance
pg, “less_is_moreâ€
pgrep, Displaying_process_information
PID, Process_attributes
ping, The_ping_command
pipe, Output_redirection_with_>_and_|
piping
examples, Output_redirection_with_>_and_|
POP3, Remote_mail_servers
Postfix, Servers
power button, Text_mode
power off, Text_mode
PPID, Process_attributes
prefdm, Init
printcap, The_most_important_configuration_files
printenv, General
printers
choosing, Buying_a_printer_for_Linux
PostScript, Buying_a_printer_for_Linux
printer configuration, Graphical_printer_configuration
printing
CUPS, General
formatting tools, Tools_and_languages
languages overview, Tools_and_languages
lp/lpr, Getting_the_file_to_the_printer
man pages, Tools_and_languages
preview, Previewing_formatted_files
remove jobs, Removing_jobs_from_the_print_queue
status, Status_of_your_print_jobs, Status_of_your_printer
troubleshooting, Wrong_file
proc, Visual, Displaying_process_information
processes
automatic, Automatic_processes
background, Interactive_processes
changing priority, Interrupting_your_processes
continuous display, Displaying_process_information
creation, Process_creation
daemonizing, Process_creation
daemons, Daemons
displaying, Process_attributes, Displaying_process_information
ending, Ending_processes
exec, Process_creation
foreground, Interactive_processes
forking, Process_creation
init scripts, Init
interactive, Interactive_processes
job control, Interactive_processes
job control overview, Interactive_processes
manage load, Interrupting_your_processes
managing, Work_for_the_system_admin
nice number, Process_attributes
priority, Priority
properties, Process_attributes
relations, Displaying_process_information
return codes, Ending_processes
scheduling, Use_that_idle_time!
signal, Signals
stopping, Signals
timing, How_long_does_it_take?
zombie, Process_creation
procmail, Mail_user-agents
profile, The_most_important_configuration_files, The_file_mask, Shell_setup
files
example, /etc/profile_example
profile.d, The_profile.d_directory
prompt
backup, Introduction
configuration, Introduction
examples, Some_examples
protocols
AppleTalk, AppleTalk
DHCP, DHCP
IPv6, TCP/IPv6
ISDN, ISDN
LDAP, LDAP
PPP, PPP,_SLIP,_PLIP,_PPPOE
RDP, The_rdesktop_protocol
SMB/NMB, SMB/NMB
TCP/IP, TCP/IP
proxy
configuration, Proxy_configuration
ps, I/O_resources
example with options, Displaying_process_information
simple example, Displaying_process_information
PS1, Reserved_variables
PS2, Reserved_variables
pstree, Displaying_process_information
pty, The_most_common_devices
pwd, The_commands
PWD, Reserved_variables
Q
Qmail, Servers
quota, Your_home_directory
R
radio, The_most_common_devices
ram, The_most_common_devices
ranges
substitution, Using_shell_features
rc*, The_most_important_configuration_files
rc*.d, Init
rdesktop, The_rdesktop_protocol
README, General, Compressing_and_unpacking_with_gzip_or_bzip2
reboot, Shutdown
recode, Language
regular expressions
getting started, More_about_grep
regular files
, Sorts_of_files
relative paths, Absolute_and_relative_paths
remote execution
Cygwin, Cygwin
RDP, The_rdesktop_protocol
security, Rsh,_rlogin_and_telnet
VNC, VNC
X Window, Telnet_and_X
remove interactively, Removing_files
rename interactively, Removing_files
renice, Priority
resolv.conf, The_most_important_configuration_files, /etc/resolv.conf, The
Domain_Name_System
restore, Tools_from_your_distribution
RGID, Process_attributes
rlogin, Rsh,_rlogin_and_telnet, Services
rm, Removing_files
rmdir
, Removing_files
root, Visual
file creation mask, The_file_mask
login, Text_mode
root directory, Visual
example, Visual
root login, General_remarks
root partition, Partition_layout_and_types
route, The_ip_command
RPM
installation, RPM_examples
remove package, RPM_examples
search files, RPM_examples
search packages, RPM_examples
rpm, RPM_examples
rsh, Rsh,_rlogin_and_telnet, Services
rsync, Introduction
RUID, Process_attributes
run level, Init
display, Init
rc files, Init
run levels
switching, Init_run_levels
run-parts, Cron_and_crontab
S
sbin, Visual
scheduling
at, The_at_command
batch, The_at_command
cron, Cron_and_crontab
execution time, The_at_command
sleep, The_sleep_command
types, Use_that_idle_time!
scp, Secure_remote_copying
example, Secure_remote_copying
screen, Interactive_processes
script
example, Some_simple_examples
first line, What_are_scripts?
init script example, Some_simple_examples
scripts
chmod, What_are_scripts?
sd, The_most_common_devices
Secure SHell
, Introduction
authentication keys, Authentication_keys
example, Introduction
secure copy, Secure_remote_copying
server authentication, Server_authentication
X11Forwarding, X11_and_TCP_forwarding
security
check open ports, Services
considerations, Introduction
disable services, Services
file permissions, Access_rights:_Linux's_first_line_of_defense
firewall, What_is_a_firewall?
intrusion detection, Intrusion_detection, Have_I_been_hacked?
log files, The_most_common_variable_files
logs, Log_files
packet filters, Packet_filters
proxies, Proxies
recovery, Recovering_from_intrusion
remote commands, Rsh,_rlogin_and_telnet
Secure SHell, Introduction
TCP wrappers, TCP_wrappers
tips, More_tips
updates, Update_regularly
Sendmail, Servers
sendmail.cf, The_most_important_configuration_files
sequencer, The_most_common_devices
server types
managed services, (x)inetd
standalone, Standalone_server
servers
authentication, Traditional
DHCP, DHCP
DNS, The_Domain_Name_System
FTP, FTP_servers
mail, Servers
news, News_services
proxy, What_is_a_proxy_server?, Proxies
web, The_Apache_Web_Server
services, The_most_important_configuration_files
setfont, Fonts
sftp, FTP_clients
example, Secure_remote_copying
SGID, Special_modes, SUID_and_SGID
SGML, Tools_and_languages
sh, Shell_types
shadow, Traditional
shell
configuration files, Shell_setup_files
definition, What_is_a_shell?
history, General_line_filtering
scripts, What_are_scripts?
substitution, Using_shell_features
switching, Shell_types
types, Shell_types
SHELL, Which_shell_am_I_using?, General, Reserved_variables
shells, Shell_types
Shift, Using_Bash_features
shutdown, Introduction, Init, Shutdown
SIGHUP, Signals
SIGINT, Signals, Interrupting_your_processes
SIGKILL, Signals, Interrupting_your_processes
signals
overview, Signals
SIGTERM, Signals, Interrupting_your_processes
single user mode, Init
slapd, LDAP
slash, Visual
backslash, The_path
forward, General_remarks, Partition_layout_and_types
sleep, Use_that_idle_time!
examples, The_sleep_command
smb.conf, SMB/NMB
sndconfig, The_most_important_configuration_files
sockets
definition, Sorts_of_files
software
installing, General
sort, Filtering_output
file size, Filtering_output
unique, Filtering_output
sound, The_most_important_configuration_files
source, .bash_profile_example
special files
definition, Sorts_of_files
spool, The_most_common_variable_files
square braces, The_man_pages, Using_shell_features
ssh, The_most_important_configuration_files, Rsh,_rlogin_and_telnet,
Introduction
ssh-keygen, Authentication_keys
sshd_config, Introduction
standard error, Use_of_file_descriptors
stdin, What_are_standard_input_and_standard_output?
stdout, What_are_standard_input_and_standard_output?
sticky bit, Special_modes
su, The_path
sudo, Text_mode
SUID, Special_modes, SUID_and_SGID
swap partition, Partition_layout_and_types
example, Partition_layout_and_types
swapping, Memory_resources
symbolic links, Creating_symbolic_links
sysconfig, The_most_important_configuration_files, Init
system
CPU resources, CPU_resources
I/O resources, I/O_resources
load, Load
memory resources, Memory_resources
monitoring, Graphical_tools
performance, Performance
updates, General_remarks
T
tab, Using_Bash_features
tail, The_head_and_tail_commands, Emptying_files
tar, Archiving_with_tar
compressed files, Compressing_and_unpacking_with_gzip_or_bzip2
example, Archiving_with_tar
GNU tar, Archiving_with_tar
incremental backups, Incremental_backups_with_tar
options, Archiving_with_tar
tarballs, Archiving_with_tar
tcsh, Shell_types
tee, Writing_to_output_and_files_simultaneously
telinit, Init_run_levels
telnet, Rsh,_rlogin_and_telnet, Services
examples, Rsh,_rlogin_and_telnet
temporary files, The_most_common_variable_files
TERM, General, Reserved_variables
terminal
send a message, SUID_and_SGID
starting up, Graphical_mode
view, Graphical_mode
TeX, Tools_and_languages
text mode
logout, Text_mode
view, Text_mode
tilde, Your_home_directory
time, How_long_does_it_take?
time zone
configuration, Date_and_time_zone
timezone, Date_and_time_zone
tmp, Visual, The_most_common_variable_files
to chmod, The_chmod_command
top, Displaying_process_information, Memory_resources, I/O_resources
changing process priority, Interrupting_your_processes
example, Displaying_process_information
touch, Emptying_files
traceroute, The_traceroute_command
Transport Control Protocol, TCP/IP
troff, Tools_and_languages
truncating, Output_redirection_with_>_and_|, Emptying_files
tty, The_most_common_devices
TTY, Process_attributes
tzconfig, Date_and_time_zone
U
UID, Reserved_variables
ulimit, Limit_file_sizes
umask, The_file_mask
UNIX
origin, UNIX
up2date, Systems_using_RPM_packages
update-rc.d, Tools
uptime, Displaying_process_information, CPU_resources
usb, The_most_common_devices
USB
dmesg, Backups_on/from_jazz_drives,_USB_devices_and_other_removables
mount device, An_example:_rsync_to_a_USB_storage_device
user name
display, Access_rights:_Linux's_first_line_of_defense
USER(NAME), Reserved_variables
users
classification, Users
usr, Visual
UTF-8, Language
V
var, Visual, The_most_common_variable_files
variable files
mail, The_most_common_variable_files
overview, The_most_common_variable_files
spool directories, The_most_common_variable_files
web server data, The_most_common_variable_files
variables
export, Exporting_variables
reserved variables, Reserved_variables
show content, Exporting_variables
vi, Vi(m)
insert mode, Commands_that_switch_the_editor_to_insert_mode
modes, Two_modes
navigation, Moving_through_the_text
video, The_most_common_devices
applications, Video_playing,_streams_and_television_watching
vim, Vi(m)
gvim, Vi(m)
tutor, The_easy_way
vimtutor, The_easy_way
VISUAL, Reserved_variables
vmstat, I/O_resources
VNC, VNC
VoIP
clients, Client_Side
service providers, Server_Side
what?, What_is_it?
W
wall, SUID_and_SGID
wc, The_path
web browsers
overview, Web_browsers
whatis
example, The_whatis_and_apropos_commands
which, The_path, Which
who, Init
example, SUID_and_SGID
whois, The_whois_command
window manager, Window_and_desktop_managers
write
4dwx, SUID_and_SGID
X
X Window
application overview, The_X_Window_System
clients, The_X_Window_System
configuration, X_server_configuration
definition, The_X_Window_System
display names, Display_names
display number, Display_names
features, X_features
hostname, Display_names
screen number, Display_names
X.Org, X_server_configuration
X11, The_most_important_configuration_files
xargs, Find_and_locate
Xauthority, X11_and_TCP_forwarding
xcdroast, Making_a_copy_with_a_CD-writer
xdvi, Previewing_formatted_files
XENVIRONMENT, Reserved_variables
XF86Config, X_server_configuration
KeyBoard, Keyboard_setup
XFILESEARCHPATH, Reserved_variables
xhost, Telnet_and_X
xinetd, (x)inetd
xinetd.*, The_most_important_configuration_files
xinetd.d, (x)inetd
xkill, Interrupting_your_processes
xload, Graphical_tools
XML, Tools_and_languages
xorg.conf, X_server_configuration
xpdf, General, Previewing_formatted_files
Y
yast, Tools
yum, Systems_using_RPM_packages