BASH “switch case” in Linux with practical example

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The switch case in BASH is more relevant and is widely used among the Linux admins/Devops folks to leverage the power of control flow in shell scripts.

As we have seen the if..elif..else..fi Control Structure: Bash If then Else. The switch case has a stronger case where it really simplifies out the control flow by running the specific block of bash code based on the user selection or the input parameters.

Let’s take a look at the simple Switch case as follows:

OPTION=$1
case $OPTION in
choice1)
Choice1 Statements
;;

choice2)
Choice2 Statements
;;

choiceN)
ChoiceN Statements
;;

*)
echo “User Selected Choice not present”
exit 1

esac

The OPTION is generally read from user input and upon this the specific choice case block is invoked.

Explanation:
In the switch command the control flow is forwarded to case keyword and stops here, it checks for the suitable match to pass over the control to relevant OPTION/CHOICE statement block. Upon the execution of the relevant CHOICE statements the case is exited once the control flow encounters esac keyword at the end.

Using the Pattern match
The control flow in bash identifies the case options and proceeds accordingly.
There can be cases where you can match the Here you might have observed that the user input the regular expression and the logical operators using the | for the input case

#! /bin/bash

echo -en "Enter your logins\nUsername: "
read user_name 
echo -en "Password: "
read user_pass 
while [ -n $user_name -a -n $user_pass ]
do

case $user_name in
    ro*|admin)
        if [ "$user_pass" = "Root" ];
        then
            echo -e "Authentication succeeded \ n You Own this Machine"
	    break
        else
            echo -e "Authentication failure"
            exit
        fi
    ;;
    jenk*)
	if [ "$user_pass" = "Jenkins" ];
	then
		echo "Your home directory is /var/lib/jenkins"
	    	break
	else
        	echo -e "Authentication failure"
	fi
        break
    ;;
    *)
        echo -e "An unexpected error has occurred."
        exit
    ;;
esac

done

You should kindly note that the regex used for the cases at ro*|admin and jenk*

We now have demonstrated by entering the username as jenkins and this will get matched with the jenkins case the control flow successfully enters into relevant block of code, checking the password match or not is not relevant for us as we are only concerned till the case choice selection.
We have named the switch case into a script switch-case.sh and run it, Here are the results.

OUTPUT :

[vamshi@node02 switch-case]$ sh switch-case.sh
Enter your logins
Username: jenkins
Password: Jenkins
Your home directory is /var/lib/jenkins

We have entered the correct password and successfully runs the jenkins case block statements

We shall also see the or ro*|admin case, demonstrated as follows.

[vamshi@node02 switch-case]$ sh switch-case.sh 
Enter your logins
Username: root
Password: Root
Authentication succeeded \ n You Own this Machine

We now test the admin username and see the results.

[vamshi@node02 switch-case]$ sh switch-case.sh 
Enter your logins
Username: admin
Password: Root
Authentication succeeded \ n You Own this Machine

Here is a more advanced script used to deploy a python application using the switch case..
Please refer to the Command line arguments section for user input

A complete functional Bash switch case can be seen at https://github.com/rrskris/python-deployment-script/blob/master/deploy-python.sh

Please feel free to share your experiences in comments.

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