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Conditional statements

Conditional statements help you to execute parts of a shell script only if certain conditions hold. Below, we give the syntax and examples for three common variants of if statements as well as the case statement which acts like switch statements in other languages.

if ... fi

General syntax:

if [ expression ]
then
   Statement(s) to be executed if expression is true
fi

Here, expression often takes the form of comparing strings using operators == or != or comparing numbers using operators -eq (equal), -ne (not equal), -lt (less than), -gt (greater than), or -ge (greater than or equal to).

However, there are other conditional expressions, many related to files (for example -e test.txt returns true if the file test.txt exists). For a full listing, see this manual page.

Example:

test.sh
#!/bin/bash

if [ $1 == $2 ]
then
   echo "$1 is equal to $2"
fi

This is the result of a sample run.

input
bash ./test.sh
output
 is equal to
input
bash ./test.sh 1 2
bash ./test.sh 12 12
output
12 is equal to 12

if ... else ... fi

General syntax: 

if [ expression ]
then
   Statement(s) to be executed if expression is true
else
   Statement(s) to be executed if expression is not true
fi

Example:

test.sh
#!/bin/bash

if [ $1 == $2 ]
then
   echo "$1 is equal to $2"
else
   echo "$1 is not equal to $2"
fi

This is the result of a sample run.

input
bash ./test.sh
output
 is equal to
input
bash ./test.sh 1 2
output
1 is not equal to 2
input
bash ./test.sh 12 12
output
12 is equal to 12

if ... elif ... else ... fi

General syntax: 

if [ expression 1 ]
then
   Statement(s) to be executed if expression 1 is true
elif [ expression 2 ]
then
   Statement(s) to be executed if expression 2 is true
elif [ expression 3 ]
then
   Statement(s) to be executed if expression 3 is true
else
   Statement(s) to be executed if no expression is true
fi

Example:

test.sh
#!/bin/bash

if [ $1 -eq $2 ]
then
   echo "$1 is equal to $2"
elif [ $1 -gt $2 ]
then
   echo "$1 is greater than $2"
elif [ $1 -lt $2 ]
then
   echo "$1 is less than $2"
else
   echo "None of the condition met"
fi

This is the result of a sample run.

input
bash ./test.sh
output
 is equal to
input
bash ./test.sh 1 2
output
1 is less than 2
input
bash ./test.sh 12 2
output
12 is greater than 2
input
bash ./test.sh 12 12
output
12 is equal to 12

case ... esac

General syntax:

case word in
    patterns ) commands ;;
esac

Example:

test.sh
#!/bin/bash

read -p "Enter a number between 1 and 3 inclusive > " character
case $character in
    1 ) echo "You entered one."
        ;;
    2 ) echo "You entered two."
        ;;
    3 ) echo "You entered three."
        ;;
    * ) echo "You did not enter a number between 1 and 3."
esac

This is the result of a sample run.

input
bash test.sh
output
Enter a number between 1 and 3 inclusive > 1
You entered one.
input
bash test.sh
output
Enter a number between 1 and 3 inclusive > 2
You entered two.
input
bash test.sh
output
Enter a number between 1 and 3 inclusive > 3
You entered three.
input
bash test.sh
output
Enter a number between 1 and 3 inclusive > 4
You did not enter a number between 1 and 3.