Linux Shell Script Decision making Statements

 These are the statements which redirect the logic of a script on the basis of some condition.

If

If the condition is satisfied then if block gets executed otherwise if statements will not execute

Example

Find the total free space on the root directory and if it is less than 20% (free/total *100) then send a notification message then "Alert! disk space is too low"

usedspace=`df / | awk '{print $5}' | sed '1d' | cut -c1`

threshold=2

if [ $usedspace -gt $threshold ]

then

        echo "Alert ! used space $usedspace which is more than $threshold"

fi

Example with command-line value for the threshold

usedspace=`df / | awk '{print $5}' | sed '1d' | cut -c1`

threshold=$1

if [ $usedspace -gt $threshold ]

then

        echo "Alert ! used space $usedspace which is more than $threshold"

fi

Execute

./alert.sh 80

If - else

usedspace=`df / | awk '{print $5}' | sed '1d' | cut -c1`

threshold=$1

if [ $usedspace -gt $threshold ]

then

        echo "Alert ! used space $usedspace which is more than $threshold"

else

        echo "You have sufficient free space :)"

fi

Execute

./alert.sh 80

If -elif 

For multiple if conditions

Example 

Perform certain operation on apache service

status=`systemctl status apache2 | grep "active (running)" | wc -l`

if [ $# -lt 1 ]

then

        var1="restart"

        echo "No commandline argument is passed so default functionality is restarted"

else

       var1="$1"

fi

echo "-----------Checking Apache Status------------"

if [ $status -ge 1 ] && [  $var1 == 'stop' ]

then

        echo "apache is running now it is getting stopped"

        systemctl stop apache2

        echo "apache is in stopped mode"

elif [ $status -ge 1 ] && [  $var1 == 'restart' ]

then

       echo "apache is running but the request is to restart it so restarting the apache"

       systemctl restart apache2

       echo "apache is in started mode"

elif [ $status -lt 1 ] && [  $var1 == 'restart' ]

then

       echo "current status of apache is in stopped mode so as per request restarting the apache"

       systemctl restart apache2

             echo "apache is in started mode"

fi

echo "-------------Apache Operations are done---------"

Execute

./apachestatus.sh

output

No commandline argument is passed so default functionality is restarted

-----------Checking Apache Status------------

apache is running but the request is to restart it so restarting the apache

apache is in started mode

-------------Apache Operations are done---------

Execute

 ./apachestatus.sh stop

output

-----------Checking Apache Status------------

apache is running now it is getting stopped

apache is in stopped mode

-------------Apache Operations are done---------

case..esac