Checking for empty string in Bash

In Bash you quite often need to check to see if a variable has been set or has a value other than an empty string. This can be done using the -n or -z string comparison operators.

The -n operator checks whether the string is not null. Effectively, this will return true for every case except where the string contains no characters. ie:

VAR="hello"
if [ -n "$VAR" ]; then
    echo "VAR is not empty"
fi

Similarly, the -z operator checks whether the string is null. ie:

VAR=""
if [ -z "$VAR" ]; then
    echo "VAR is empty"
fi

Note the spaces around the square brackets. Bash will complain if the spaces are not there.

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