Shell provides if and else to run conditional statements depending upon whether the evaluation is true or false. It is useful if we want to perform certain tasks only if a certain condition is true.
The test condition to if can be given using a test condition or [condition]. We have already learned multiple use cases and examples of testing an expression in the previous section, Testing expressions with a test.
The syntax of the if condition is as follows:
if [ conditional_expression ] then statements fi
If conditional_expression is true—that is, the exit status is 0—then the statements inside it get executed. If not, then it will be just be ignored and the next line after fi will be executed.
The syntax of if and else is as follows:
if [ conditional_expression ] then statements else statements fi
Sometimes, when a condition is not true, we might want to execute some statements. In such cases, use if and else. Here, if conditional_statement is true, statements within if get executed. Otherwise, statements within else will be executed.
The following shell script prints the message if a file exists:
#!/bin/bash
# Filename: file_exist.sh
# Description: Print message if file exists
if [ -e /usr/bin/ls ]
then
echo "File /usr/bin/ls exists"
fiThe output after running the script is as follows:
File /usr/bin/ls exists
Another example shows the greater one among two integers as follows:
#!/bin/bash
# Filename: greater_integer.sh
# Description: Determining greater among two integers
echo "Enter two integers a and b"
read a b # Reading input from stdin
echo "a = $a , b = $b"
# Finding greater integer
if test $a -gt $b
then
echo "a is greater than b"
else
echo "b is greater than a"
fiThe following is the output after running the script:
$ sh greater_integer.sh Enter two integers a and b 56 8 a = 56 , b = 8 a is greater than b
In some cases, more than two choices exist, of which only one needs to be executed. The elif allows you to use another if condition instead of using else if a condition is not true. The syntax is as follows:
if [ conditional_expression1 ] then statements elif [ conditional_expression2 ] then statements elif [ conditional_expression3 ] then statements # More elif conditions else statements
The following shell script will make the elif usage more clear. This script asks a user to input a valid file or directory name with the absolute path. On a valid regular file or directory, it displays the following content:
#!/bin/bash # Filename: elif_usage.sh # Description: Display content if user input is a regular file or a directoy echo "Enter a valid file or directory path" read path echo "Entered path is $path" if [ -f $path ] then echo "File is a regular file and its content is:" cat $path elif [ -d $path ] then echo "File is a directory and its content is:" ls $path else echo "Not a valid regular file or directory" fi
The output after running the script is as follows:
Enter a valid file or directory path /home/ Entered path is /home/ File is a directory and its content is: lost+found sinny
In many cases, multiple if conditions are required because the execution of a condition depends upon the result of another condition. The syntax will be as follows:
if [ conditional_expression1 ]
then
if [ conditional_expression2 ]
then
statements
if [conditional_expression3 ]
then
statements
fi
fi
fiThe following script example explains the nested if in more detail. In this script, we will see how to find the greatest one of the three integer values:
#!/bin/bash
# Filename: nested_if.sh
# Description: Finding greatest integer among 3 by making use of nested if
echo "Enter three integer value"
read a b c
echo "a = $a , b = $b, c = $c"
if [ $a -gt $b ]
then
if [ $a -gt $c ]
then
echo "a is the greatest integer"
else
echo "c is the greatest integer"
fi
else
if [ $b -gt $c ]
then
echo "b is the greatest integer"
else
echo "c is the greatest integer"
fi
fiThe output after running the script will be as follows:
Enter three integer value 78 110 7 a = 78 , b = 110, c = 7 b is the greatest integer