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Certification

bash Shell Scripting

Copyright © 2003 Red Hat, Inc.

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UNIT 16

bash Shell Scripting


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UNIT 16: Objectives

� Learn why shell scripting is useful.� Learn how to create a basic shell script.� Learn how to generate output and read

input.� Learn how to use flow control to write

more powerful shell scripts.


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UNIT 16: Agenda

� Shell scripting


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Scripting Basics

� Shell scripts are text files that contain a series of commands or statements to be executed.

� Shell scripts are useful for:� Automating commonly used commands� Performing system administration and

troubleshooting� Creating simple applications� Manipulation of text or files


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Creating Shell Scripts

� Step 1: Use a text editor such as vi to create a text file containing commands

� First line contains the magic “shbang”sequence: #!#!/bin/bash

� Comment your scripts!� Comments start with a #


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Creating Shell Scripts cont.

� Step 2: Make the script executable:$ chmod a+x myscript.sh

� To execute the new script:� Place the script file in a directory in the

executable path -OR-� Specify the absolute or relative path to the

script on the command line


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Generating Output

� Use echo to generate simple outputecho 'Welcome to Red Hat Linux paradise!'

echo -n "Please enter the file name: "

� Use printf to generate formatted outputprintf "The result is %0.2f\n" $RESULT

� Syntax similar to C printf()function� Does not automatically put a newline at the end of the

output.


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Handling Input

� Use read to assign an input value to a shell variable:echo -n "Enter the filename: "

read FILENAME

� read reads from standard input and assigns one word to each variable

� Any leftover words are assigned to the last variable


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Using Positional Parameters

� Positional parameters are special variables that hold the command-line arguments to the script.

� The positional parameters available are $1, $2, $3, etc. . These are normally assigned to more meaningful variable names to improve clarity.

� $* holds all command-line arguments


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Using functions in shell scripts

� Shell scripts may include shell functions.� Shell functions may improve program

readability. They also help to remove repetitious code from the scripts.

� Shell functions must be declared before they are used.


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Using functions, continued

� Arguments may be passed to a shell function by using their own set of positional parameters ( $1, $2 etc. )

myFunction $filename

The value of $filename will be available as $1 inside the body of myFunction

� Functions may return values by using the 'return' keyword which sets the value of the special variable $?


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Exit Status

� Commands exit with an exit status� 0 for success, 1 to 255 for failure� Exit status of most recently executed

command is kept in the $? variable just like return values from shell functions

� Shell scripts may set an exit status with the exit command:

exit 1 # Indicates an error


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Control Structures

� The three types in shell programming :� Sequential structures - the program flows

one line after another� Selection structures - code execution based

on a logical decision � if, if/else , if/elif/else and

conditional operators

� Repetition structures - code execution is repeated based on a logical decision

� for, while and until


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Conditional Execution

� Commands may be executed conditionally, based on the exit status of the previous command && logical AND || logical OR

� Examples:• $ grep joe passwd || echo 'No joe!'

• $ cp -a /tmp/*.o . && echo 'Done!'


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Selection Structures:Using the if Statement

� if selection structures execute the body of the structure only if the condition tested is true

if [ condition ]; then

do something

fi


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File Tests

� File tests: -f tests to see if file exists and is a

regular file -d tests to see if a file exists and is a

directory -x tests to see if a file exists and is

executableif [ -f $HOME/lib/functions ];then

source ~/lib/functions

fi


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String Tests

� Strings may be tested as well -z returns true if the string is empty -n returns true if the string is not empty� operators such as =, !=, < and > may be

used to compare strings as well

if [ $(id -u) = "0" ]; then

echo "You are logged in as root"

fi


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Selection StructuresUsing if/else Statements

� if/else selection structures execute the body of the if structure only if the condition tested is true, otherwise the else is executed

if [ condition ]; then

do something

else

do something else

fi


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Selection Structures:Using the case Statement

� The case statement provides an alternative method for performing selections that may be cleaner than multiple if/elif/else tests• case variable in

• pattern1)

• do something ;;

• pattern2)

• do another thing ;;

• esac


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Repetition Structures:The for-loop

� The for repetition structure provides a method for iterating, or looping, through a list of values and executing commands on each of these values.

for variable in list-of-values

docommands...

done


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Selection StructuresThe while-loop

� The while loop structure provides a useful method for performing a set of commands while a condition remains true. The syntax is:while condition

do

commands...

done


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continue and break

� while and until loops can be disrupted during execution

� continue stops the current execution of the loop and reexamines the initial condition, possibly restarting the loop

� break stops processing the loop entirely, jumping past the done statement

� exit exits from the shell script entirely� You may provide an exit status


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Shell script debugging

� In order to debug a shell script invoke the shell interpreter with debug options or change the shebang to include the debug options

bash -x scriptname

bash -v scriptname

#!/bin/bash -x

#!/bin/bash -v


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End of Unit 16

� Questions and answers� Summary