Shell scripting for system administrators: the basics

We also can use various arithmetic operators inside a shell script. The following table explains these various arithmetic comparison operators:

The following example shows how one can use these operators in a shell script:

#!/bin/sh

# declare integers

NUM1=2
NUM2=2
if [ $NUM1 -eq $NUM2 ]; then
echo    “Both Values are equal”
else
echo     “Values are NOT equal”
fi

We can also compare two strings using various string comparison operators. The following table lists some of the string comparison operators…

Here’s an example to help with our understanding:

#!/bin/sh

#Declare string S1
S1=”User”
#Declare string S2
S2=”Developer”

if [ $S1 = $S2 ]; then
 echo “Both Strings are equal”
else
 echo “Strings are NOT equal”
fi

We can also write a shell script to check whether a file exists or not. The option ‘-e file’ is used to check for file existence. The following shell script checks for the existence of a test file (in this case, my_file)

#!/bin/sh
testfile=”./my_file”
if [ -e $my_file ]; then
 echo “The specified file exists”
else
 echo “The specified file does not exist”
fi

et us understand how the script executes in both cases. Save the above script in a file ‘script_12.sh’.
Case 1: When test file does not exist

[root@localhost ~]# ll my_file
ls: my_file: No such file or directory
[root@localhost ~]# ./script_12.sh
The specified file does not exist

Case 2: When test file exists

[root@localhost ~]# touch my_file
[root@localhost ~]#
[root@localhost ~]# ll my_file
-rw-r--r-- 1 root root 0 Jun 20 23:56 my_file
[root@localhost ~]# ./script_12.sh
The specified file exists

In some cases, we might be interested in knowing the exit status – this gives a clear indication whether a script (or a command) has executed successfully or not. In most case, for a successful execution, the exit status will be 0. One can get the exit status by using the following command:

$ echo $?

Here is an example to see the exit status after a command execution:

[root@localhost ~]# cal
 June 2010

Su Mo Tu We Th Fr Sa
 1  2  3  4  5
 6  7  8  9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30

[root@localhost ~]# echo $?
0
[root@localhost ~]#

In the above example, we tried to print the exit status after the ‘cal’ command was executed.

For more tutorials from linux User & Developer click here, or go here to see what else featured in issue 90.