Check Unset Variable

For Shell script, sometimes, we want to find out whether a variable is set or unset. The problem is that there is no easy way to distinguish between:

• a variable is set but the value is null or empty
• an unset variable

To identify them, we use an unique conditions approach using the variable expansion substitution. The idea is that, when a variable is unset, the entire variable expands into nothing; Otherwise, it is substituted with a character. Then, you just need to check against this single character existence to know whether a variable is set or unset.

Check for Set

The easiest way is to use the variable +x expansion. Here is an example:

#!/bin/dash

if [ ! -z ${variable+x} ]; then

        echo "variable is set"

fi

NOTE

• Take important note that the variable is not quoted. We want the variable to expand to nothing, not "", which is a valid empty string.

Check for Unset

Similarly, to check the unset, we use the -z condition. Here is an example:#!/bin/dash

if [ -z ${variable+x} ]; then

        echo "variable is unset"

fi

NOTE

• Take important note that the variable is not quoted. We want the variable to expand to nothing, not "", which is a valid empty string.

That's all for checking a variable is unset or empty.