Chapter 3Writing and Executing a shell script

Advanced Linux administration using Fedora v. 9

Lecturer: Mr. Kao Sereyrath, MScIT (SMU, India)Director of Technology and Consulting Service (DCD Organization)ICT Manager (CHC Microfinance Limited)

Contents

Advantage of using shell script1

3

4

5

2 Creating and executing shell script

Learn about variable

Learn about operator

if and case statement

6 for, while and until statement

7 Create function in shell script

Advantage of using shell script Hundreds of commands included with Fedora are actually

shell scripts. For example startx command.

Shell script can save your time and typing, if you routinely use the same command lines multiple times every day.

A shell program can be smaller in size than a compiled program.

The process of creating and testing shell scripts is also generally simpler and faster than the development process.

You can learn more with “Sams Teach Yourself Shell Programming in 24 Hours”

Creating and Executing shell script

To create shell script : You can use vi command. Because it won't wrap text.

Or you can use nano -w to disable line wrap

For example, to create myenv filevi /etc/myenv

To execute shell script : First you need to set file permission

chmod +x myenv

Creating and Executing shell script (Cont.)

To execute myenv file in /etc directory ./myenv

To enable shell script as your Linux command, first you need to create bin directory in user home directory. Then put shell script file into that directory.

mkdir binmv /etc/myenv binmyenv

Learn about variable

There are 3 types of variable:– Environment variables: Part of the system

environment, you can use and modify them in your shell program. For example PATH variable.

– Built-in variables: Unlike environment variables, you cannot modify them. For example $#, $0

– User variable: Defined by you when you write a shell script.

Learn about variable (Cont.)

Positional parameter

if [ $# -eq 0 ]thenecho "Tell me your name please"elseecho "Welcome "$1fi

$# is a positional parameter and $1 is to get first parameter. You can use $2 to get second parameter.

Learn about variable (Cont.)

Using single quote to escape variable

var="Welcome to Linux"echo 'The value of $var is '$var

The output is:

The value of $var is Welcome to Linux

Learn about variable (Cont.)

Using backtick ( ` ) to execute Linux command

ls -l >> myfile.txtmycount=`wc -l myfile.txt`echo "File size: "$mycount

The output is:

File size: 33 myfile.txt

Learn about Operator

Comparison operator

= To compare whether two strings are equal!= To compare whether two strings are not equal-n To evaluate whether the string length is greater than zero-z To evaluate whether the string length is equal to zero

#String comparisonstring1="abc"string2="Abc"if [ $string1 = $string2 ]then echo "string1 equal to string2"else echo "string1 not equal to string2"fi

Learn about Operator (Cont.)#String comparisonstring1="abc"string2="Abc"if [ $string1 != $string2 ]; then echo "string1 not equal to string2"else echo "string1 equal to string2"fi

if [ $string1 ]; then echo “string1 is not empty”else echo “string1 is empty”fi

Learn about Operator (Cont.)if [ -n $string2 ]; then echo “string2 has a length greater than zero”else echo “string2 has length equal to zero”fi

if [ -z $string1 ]; then echo “string1 has a length equal to zero”else echo “string1 has a length greater than zero”fi

Learn about Operator (Cont.)

Number comparison

-eq To compare Equal-ge To compare Greater than or equal to -le To compare Less than or equal to-ne To compare Not equal-gt To compare Greater than-lt To compare Less than

if [ $number1 -gt $number2 ]; then echo “number1 is greater than number2”else echo “number1 is not greater than number2”fi

Learn about Operator (Cont.)

File operator

-d Check if it is a directory

-f Check if it is a file

-r Check if the file has Read permission

-w Check if the file has Write permission

-x Check if the file has Execute permission

-s Check if file exist and has length greater than zero

Learn about Operator (Cont.)filename="/root/bin/myfile.txt"if [ -f $filename ]; thenecho "Yes $filename is a file"elseecho "No $filename is not a file"fiif [ -x $filename ]; thenecho "$filename has Execute permission"elseecho "$filename has no Execute permission"fiif [ -d "/root/bin/dir1" ]; thenecho "It is a directory"elseecho "No it is not a directory"fi

Learn about Operator (Cont.)

Logical operator

! To negate logical expression-a Logical AND-o Logical OR

if [ -x $filename1 -a -x $filename2 ]; thenecho "$filename1 and $filename2 is executable"elseecho "$filename1 and $filename2 is not executable"fiif [ ! -w $file1 ]; then echo “$file1 is not writable”else echo “$file1 is writable”fi

if and case statement

if statement syntaxif [ expression ]; then Statementselif [ expression ]; then Statementselse Statementsfi

Example:if [ $var = “Yes” ]; then echo “Value is Yes”elif [ $var = “No” ]; then echo “Value is No”else echo “Invalid value”fi

if and case statement (Cont.)

case statement syntaxcase str in str1 | str2) Statements;; str3 | str4) Statements;; *) Statements;;esac

Example:case $1 in001 |01 | 1) echo "January";;02 | 2) echo "February";;3) echo "March";;*) echo "Incorrect supplied value";;esac

for, while and until statement

Example1: for statement

for filename in *do cp $filename backup/$filename if [ $? -ne 0 ]; then echo “copy for $filename failed” fidone

Above example is used to copy all files from current directory to backup directory. if [ $? -ne 0 ] statement is used to check status of execution.

for, while and until statement (Cont.)

Example2: for statement

echo "You have passed following parameter:"i=1for parlist in $@doecho $i" "$parlisti=`expr $i + 1`done

If you type myenv domain1 domain2. The result is:

You have passed following parameter:1 domain12 domain2

for, while and until statement (Cont.)

Example: while statement

loopcount=0while [ $loopcount -le 4 ]douseradd "user"$loopcountloopcount=`expr $loopcount + 1`done

Above statement is used to create user0, user1, user2, user3,

user4.

for, while and until statement (Cont.)

Example: until statement

loopcount=0until [ $loopcount -ge 4 ]doecho $loopcountloopcount=`expr $loopcount + 1`done

Above statement is used to output 0, 1, 2, 3 to display.

Create function in shell scriptYou can create function by using below example.myfunc(){case $1 in1) echo "January";;2) echo "February";;3) echo "March";;4) echo "April";;5) echo "May";;6) echo "June";;7) echo "July";;8) echo "August";;9) echo "September";;10) echo "October";;11) echo "November";;12) echo "December";;*) echo "Invalid input";;esac }

Calling functionmyfunc 3

Output isMarch