How to test that an environment variable is set in bash?

The goal here is to test that a variable both exists and is not empty. You can use the following to achieve that:

${THEVAR:+x}

Which will expand to “x” if THEVAR is set and not empty.

Alternatively, this test can be used:

if [ ! -v THEVAR ]; then
    echo "THEVAR is not set"
fi

But that test will also pass if THEVAR is set but empty.