linux - How can I make bash treat undefined variables as errors?

Question

Please note: there are many questions about how to test a single shell variable on this site. This question is about testing a script for any undefined variable.

You can use an undefined variable in bash without seeing any error at execution:

#!/bin/bash

echo ${UNDEF_FILE}
ls -l ${UNDEF_FILE}

exit 0

I've found this very error prone. If I want to change the name of a variable in a large script, or remove that variable, all the previous stale references will cause errors in the script. Sometimes this is not obvious to debug or you find out when it's too late.

Why is this allowed? Is there any way to flag undefined variables?

Answer 1

You can use:

set -u

at the start of your script to throw an error when using undefined variables.

-u

Treat unset variables and parameters other than the special parameters "@" and "*" as an error when performing parameter expansion. If expansion is attempted on an unset variable or parameter, the shell prints an error message, and, if not interactive, exits with a non-zero status.