Linux

1. Chapter 3
Writing 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)

2. 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

3. 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”

4. 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 file
vi /etc/myenv
To execute shell script :
 First you need to set file permission
chmod +x myenv

5. 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 bin
mv /etc/myenv bin
myenv

6. 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.

7. Learn about variable (Cont.)
Positional parameter
if [ $# -eq 0 ]
then
echo "Tell me your name please"
else
echo "Welcome "$1
fi
 $# is a positional parameter and $1 is to get first
parameter. You can use $2 to get second parameter.

8. 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

9. Learn about variable (Cont.)
Using backtick ( ` ) to execute Linux command
ls -l >> myfile.txt
mycount=`wc -l myfile.txt`
echo "File size: "$mycount
 The output is:
File size: 33 myfile.txt

10. 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 comparison
string1="abc"
string2="Abc"
if [ $string1 = $string2 ]
then
echo "string1 equal to string2"
else
echo "string1 not equal to string2"
fi

11. Learn about Operator (Cont.)
#String comparison
string1="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

12. 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

13. 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

14. 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

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

16. Learn about Operator (Cont.)
Logical operator
! To negate logical expression
-a Logical AND
-o Logical OR
if [ -x $filename1 -a -x $filename2 ]; then
echo "$filename1 and $filename2 is executable"
else
echo "$filename1 and $filename2 is not executable"
fi
if [ ! -w $file1 ]; then
echo “$file1 is not writable”
else
echo “$file1 is writable”
fi

17. if and case statement
if statement syntax
if [ expression ]; then
Statements
elif [ expression ]; then
Statements
else
Statements
fi
Example:
if [ $var = “Yes” ]; then
echo “Value is Yes”
elif [ $var = “No” ]; then
echo “Value is No”
else
echo “Invalid value”
fi

18. if and case statement (Cont.)
case statement syntax
case str in
str1 | str2)
Statements;;
str3 | str4)
Statements;;
*)
Statements;;
esac
Example:
case $1 in
001 |01 | 1) echo "January";;
02 | 2) echo "February";;
3) echo "March";;
*) echo "Incorrect supplied value";;
esac

19. 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”
fi
done
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.

20. for, while and until statement (Cont.)
Example2: for statement
echo "You have passed following parameter:"
i=1
for parlist in $@
do
echo $i" "$parlist
i=`expr $i + 1`
done
If you type myenv domain1 domain2. The result is:
You have passed following parameter:
1 domain1
2 domain2

21. for, while and until statement (Cont.)
Example: while statement
loopcount=0
while [ $loopcount -le 4 ]
do
useradd "user"$loopcount
loopcount=`expr $loopcount + 1`
done
Above statement is used to create user0, user1, user2, user3,
user4.

22. for, while and until statement (Cont.)
Example: until statement
loopcount=0
until [ $loopcount -ge 4 ]
do
echo $loopcount
loopcount=`expr $loopcount + 1`
done
Above statement is used to output 0, 1, 2, 3 to display.

23. Create function in shell script
You can create function by using below example.
myfunc()
{
case $1 in
1) 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 function
myfunc 3
Output is
March