The Complete Guide for Linux System Administration CH03 powerpoint

Chapter 3:Using the Shell

The Complete Guide to Linux System Administration

The Complete Guide to Linux System Administration 2

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


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


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


The Shell Prompt

• Set of words or characters indicating that shell is ready to accept commands

• Default prompt example:– [Nwells@inverness nwells]$

• When logged in as root, prompt character changes to hash mark, #


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


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


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


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


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


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– !


Entering Commands (continued)

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


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 user’s home directory– Specific to single user


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


The Shell Start-up Process (continued)


Using Aliases

• Alias – String substituted for another string at shell prompt

• Alias command – Define how shell will substitute one string for

another – Format:

• alias <string entered by user>=<string substituted by the shell>

– Enter without arguments to see list of current aliases


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


Command-line Expansion (continued)

• Special characters– Place in single quotes– Precede with $

• Example: $’\n’

– Other special characters:• ;

• *

• ?


Command-line Expansion (continued)

• Avoid command-line expansion– Escape individual characters

• Using \

– Single quote– Double quote


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 user’s shell– Can be accessed by any program


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


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


Data Redirection

• Standard input channel– STDIN– Keyboard

• Standard output channel– STDOUT– Monitor

• Standard error– STDERR


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





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


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


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



• 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



• Modes:– Command mode– Insert mode– Replace mode

• Begin in command mode– Return to command mode by pressing Esc key










Printing from the Command Line

• Quickly configure printer attached to parallel port

• Print files directly from command line


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


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


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


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


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

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