Chapter 3: Using the Shell

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Chapter 3Using the Shell

• The Complete Guide to Linux System Administration

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Objectives

• Use common features of the shell to work at the
  command line
• Manipulate variables in the shell to control your
  working environment
• Redirect data at the command line
• Edit text using the vi editor
• Print basic files from the command line

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Exploring the Bash Shell

• Shell
• Command program
• Accepts input from keyboard
• Uses input to run commands or otherwise control
  computer
• Started when terminal window opened

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Exploring the Bash Shell (continued)

• Flexible
• Interact with any file that you have permission
  to access
• Start any program on system
• Use special features of shell to work more
  efficiently

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The Shell Prompt

• Set of words or characters indicating that shell
  is ready to accept commands
• Default prompt example
• Nwells_at_inverness nwells
• When logged in as root, prompt character changes
  to hash mark,

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The Functions of a Shell

• Shell purpose
• Make it easy for users to run programs and work
  with files in Linux
• Run programs
• Pass parameters to program being started
• Behavior
• If command succeeds, no response
• If command fails, error message printed

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The Functions of a Shell (continued)

• Shell has many built-in features
• Help you work with files and commands
• Script
• Program
• List of commands stored in text file

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Different Types of Shells

• Shell has no special relationship to Linux kernel
• Can use any shell
• Bourne shell
• Original shell for UNIX
• Called sh
• Old program with limited functionality
• Bash
• Default shell for all Linux systems

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Different Types of Shells (continued)

• Shells fall into two groups
• Bourne shell programming style
• C shell programming style
• Shell started for each user is determined by
  settings in user account
• Each user on system can select preferred shell
• Immediately run different shell
• Enter name of shell program

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Entering Commands

• Features designed to simplify process of entering
  commands
• Tab completion
• Enter part of file or directory name
• Press tab key
• Shell fills in remainder of name
• Command history
• Records each command

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Entering Commands (continued)

• Command history
• Quickly call up and repeat any command from list
• Press the up arrow key at command prompt
• Press enter to execute command
• History command
• Displays entire history list
• Bang
• !

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Entering Commands (continued)

• Locate previously executed command
• Use history number
• Use beginning of command
• Search history list

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The Shell Start-up Process

• Scripts executed when you log in to Linux
• Initialize various parts of environment
• Script /etc/profile executed on first log in
• Contains configuration information that applies
  to every user on system
• Profile script
• Found in users home directory
• Specific to single user

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The Shell Start-up Process (continued)

• Some systems
• Additional scripts are executed when user logs in
• When user starts shell
• Additional scripts executed
• bashrc script
• Executed each time user starts bash shell
• User can add configuration information

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The Shell Start-up Process (continued)
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Using Aliases

• Alias
• String substituted for another string at shell
  prompt
• Alias command
• Define how shell will substitute one string for
  another
• Format
• alias ltstring entered by usergtltstring
  substituted by the shellgt
• Enter without arguments to see list of current
  aliases

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Command-line Expansion

• Shell can make many different substitutions,
  including
• Aliases
• Variables
• Command substitution
• Special characters
• Place commands in single quotes
• Executed first by shell

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Command-line Expansion (continued)

• Special characters
• Place in single quotes
• Precede with
• Example \n
• Other special characters
• ?

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Command-line Expansion (continued)

• Avoid command-line expansion
• Escape individual characters
• Using \
• Single quote
• Double quote

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Shell Variables

• Shell variable
• Name that can have value assigned to it
• Can be number or string of text
• Typically created using all uppercase letters
• Environment variable
• Typically defined during initialization of
  operating system or users shell
• Can be accessed by any program

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Shell Variables (continued)

• env command
• Start program with environment variable setting
  that is not part of current environment
• Only used for started program
• Set command
• Displays list of all environment variables
• Echo command
• View value of single environment variable

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Shell Variables (continued)

• Export command
• Makes newly created environment variable
  available to other programs
• PATH environment variable
• Contains list of directories that shell searches
  each time command is executed
• PS1 environment variable
• Defines shell prompt for bash

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Data Redirection

• Standard input channel
• STDIN
• Keyboard
• Standard output channel
• STDOUT
• Monitor
• Standard error
• STDERR

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Data Redirection (continued)

• Data can be redirected from standard channels
• Pipe
• Connects output channel of one command to the
  input channel of another command
• Example ls /etc sort

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Data Redirection (continued)
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Data Redirection (continued)

• Mail command
• Basic e-mail client
• Use from command line to create e-mail message
• Format mail -s "subject line" e-mail_address

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Editing Text with vi

• Text editor
• Single most important utility for system
  administrator
• Many different text editors are available for
  Linux
• Text-mode editors
• vi
• emacs
• pico
• Joe

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Editing Text with vi (continued)

• Very powerful program
• Available on virtually every Linux system
• Not easy to use
• Must learn to use at least basic features in
  order to work as Linux system administrator
• Memorize keystroke sequences
• Work without any onscreen prompts

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Editing Text with vi (continued)

• To start
• vi command
• Can be followed by name of file to edit
• Modal editor
• Keystrokes are interpreted differently depending
  on mode you are working in

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Editing Text with vi (continued)

• Modes
• Command mode
• Insert mode
• Replace mode
• Begin in command mode
• Return to command mode by pressing Esc key

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Editing Text with vi (continued)
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Editing Text with vi (continued)
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Editing Text with vi (continued)
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Editing Text with vi (continued)
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Editing Text with vi (continued)
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Printing from the Command Line

• Quickly configure printer attached to parallel
  port
• Print files directly from command line

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Setting Up a Printer

• Print queue
• Printer definition
• Associate physical printer with named printer
  definition
• Set up printer via GNOME desktop interface
• Choose system settings
• Print
• Follow steps in dialog

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Printing From a Command Line

• lpr command
• Print from command line
• Uses series of filters based on type of file to
  print
• lpoptions command
• Save commonly used list of options

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Summary

• Shell
• Command interpreter used to start programs
• Many shells are available
• Tab completion
• Helps to quickly enter long file and directory
  names
• Command history
• Quickly reenter previously entered command

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Summary (continued)

• Scripts executed when
• User logs in
• New shell opened
• Aliases
• Shell replaces text on command-line entry with
  different text
• Shell expands text on command line in several ways

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Summary (continued)

• Environment variables store values that programs
  can access
• Flow of data between programs can be changed
  using redirection operators on command line
• Linux supports numerous text editors
• After setting up printer, print files from
  command line using lpr command