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Prevent Unset Variables in Your Shell / Bash Scripts with set nounset


This is handy to prevent typos because this option will halt your script as soon as an unset variable is found.

Quick Jump: Demo Video

Imagine this scenario:

#!/usr/bin/env sh


echo "${REFERER}"

That’s going to echo an empty string because the variable is defined as REFERRER but it’s referenced as REFERER (it’s missing the double “R”s).

If you’re a fan of using ShellCheck, it will warn you of this but you can go the extra step and add set -o nounset (or set -u) near the top of your script and now if you run it you’ll get ./demo: 7: REFERER: parameter not set. The extra protection is nice and it’s POSIX compliant.

If you want an optional variable that might be undefined, you can do that too:

#!/usr/bin/env sh

set -o nounset


[ -n "${DEBUG}" ] && echo "A helpful debug message!"

Now you can run ./demo or DEBUG=1 ./demo without errors.

This is something I’ll be adding to all of my scripts.

Demo Video


  • 0:32 – ShellCheck can help us here but we can do better
  • 0:43 – Protecting ourselves from variable typos with the nounset option
  • 1:52 – Handling optional variables by setting an “empty” default
  • 3:43 – Defining all of your variables is a good practice to adhere to

Will you be setting this in all of your scripts? Let us know!

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