Apart from simple branches with if, it is also possible to process multiple decision-making operations using the case command. In a case statement, the expression contained in a variable is compared with a number of expressions, and for each expression matched, a command is executed.

It is possible to have multiple branching using the if/elif/else commands. But if more than two or three elif commands are used, then code becomes very complex. When all the different conditions are depending on a single variable, in such cases, the esac statement is used. The interpreter checks the value of the case variable against value1, value2, value3, and so on, till the match is found. If the value is matched then all the statements after that case value are executed till the double semicolon is reached. If nothing is matched then statements after esac are executed. Wildcard characters and pipe (vertical bar for ORing two values) are allowed in the case statement.

A case statement has the following structure:

case variable in
  value1)
    command(s)
    ;;
  value2)
    command(s)
    ;;
  *)
    command(s)
          ;;
esac

For illustrating the switch case scripting example, we will write the case_01.sh script as follows. We will ask the user to enter any number from the range 1–9. We will check the entered number with the case command. If a user enters any other number, then we will display the error by displaying the Invalid key message:

#!/bin/bash

echo "Please enter any number from 1 to 9"
read number

case $number in
  1) echo "ONE"
    ;;
  2) echo "TWO"
    ;;
  3) echo "Three"
    ;;
  4) echo "FOUR"
      ;;
  5) echo "FIVE"
    ;;
  6) echo "SIX"
    ;;
  7) echo "SEVEN"
    ;;
  8) echo "EIGHT"
    ;;
  9) echo "NINE"
    ;;
  *) echo "SOME ANOTHER NUMBER"
    ;;
esac

Let's test the program:

$ chmod +x case_01.sh
$ ./case_01.sh

The following will be the output after executing the preceding commands:

Please enter any number from 1 to 9
5
FIVE

Sometimes, in the Shell script we may need to ask for an e-mail address from the user. In such situations, we need to verify if the address is correct or not. We can use the case command to validate the correct e-mail address as follows:

#!/bin/bash
case $1  in
*@*.com)   echo "valid email address"
    ;;
*)    echo "invalid string"
    ;;
esac

Let's test the program:

$ chmod +x case_02..sh
$ ./ case_02.sh   abc@gmail.com

The following will be the output after executing the preceding commands, if the e-mail address is correct:

valid email address
$ ./ case_02.sh abc.com

The following will be the output after executing the preceding commands, if the e-mail address is not correct:

invalid string

If inside the script, we need to provide file operations such as copy, move, or delete, then we can use the case command for such scripts. The script case_03.sh for file operations is as follows:

#!/bin/bash
echo "Press 1 for copy or 2 for move or 3 for removing the file"
read num
case $num in
1)  echo "We are going to do copy operation"
echo " Enter Source file name"
read source
echo " Enter destination file name"
read destination
cp $source $destination
;;
2)   echo "We are going to do move operation"
echo " Enter Source file name"
read source
echo "Enter destination file name"
read destination
mv $source $destination   ;;
3)   echo "We are going to remove the file"
echo " Enter the name of file to remove"
read source
rm $source   ;;
*) echo "invalid key"
esac

Let's test the program:

$ chmod +x case_03.sh
$ ./case_03.sh

The following will be the output after executing the preceding commands:

Press 1 for copy or 2 for move or 3 for removing the file
1
We are going to do copy operation
 Enter Source file name
File1
 Enter destination file name
File4

In this Shell script case_04.sh, we will ask the user to enter the day of the week. Inside the script, we will detect the text entered and print a detailed description of the day such as First Day is Monday and similar on the screen. Note that we are able to perform pattern matching for the upper case and lower case in the case statement:

#!/bin/bash
echo "Enter Day Of The Week"
read day

case $day in
  [mM][oO][nN][dD][aA][yY])
        echo "First Day is Monday"
        ;;
  [tT][uU][eE][sS][dD][aA][yY])
        echo "Second Day Tuesday"
        ;;
  [wW][eE][dD][nN][eE][sS][dD][aA][yY])
        echo "Third Day Wednesday"
        ;;
  [tT][hH][uU][rR][sS][dD][aA][yY])
        echo " Fourth Day Thursday"
        ;;
  [fF][rR][iI][dD][aA][yY])
        echo "Fifth Day Friday"
        ;;
  [sS][aA][tT][uU][rR][dD][aA][yY])
        echo "Sixth Day Saturday"
        ;;
  [sS][uU][nN][dD][aA][yY])
        echo "Seventh Day Sunday"
        ;;
  *)
    echo "Invalid Day of Week"
    ;;
   esac

Let's test the program:

$ chmod +x case_04.sh
$ ./case_04.sh

The following will be the output after executing the preceding commands:

$ ./ case_04.sh
Enter Day Of The Week
Monday
First Day is Monday

$ ./ case_04.sh
Enter Day Of The Week
Thursday
Fourth Day Thursday

We write the script case_05.sh for printing days in the current month. We will use the date command in the script for finding the current month:

#!/bin/bash
mth=$(date +%m)

case $mth in
02)
  echo "February usually has 28 days."
  echo "If it is a leap year, it has 29 days."
  ;;

04|06|09|11)
  echo "The current month has 30 days."
  ;;

*)
  echo "The current month has 31 days."
  ;;
 esac

Let's test the program:

$ chmod +x case_05.sh
$ ./case_05.sh

The following will be the output after executing the preceding commands:

The current month has 30 days.