Shell Scripting: If Variable Is Not Defined, Set Default Variable

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To check if a variable is defined in a shell script, you can use the if [ -z "$VAR" ] statement. Here, -z tests if the length of the variable $VAR is zero or not. If the variable is not defined or has a length of zero, the statement will be true.

To set a default variable value in case the variable is not defined, you can use the ${VAR:-default} syntax. Here, ${VAR} returns the value of the variable $VAR, and :-default specifies the default value to use if $VAR is not defined.

Here’s an example script that sets a default value for a variable:

#!/bin/bash

if [ -z "$VAR" ]; then
VAR="default_value"
fi

echo "The value of VAR is: $VAR"

Alternatively, you can use a shorter version of the above script using the ${VAR:=default} syntax, which sets $VAR to default if it is not defined:

#!/bin/bash

echo "The value of VAR is: ${VAR:=default_value}"

In this version, the variable is set to default_value if it is not defined and then printed.

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