Test if a variable is set in bash when using “set -o nounset”

Question

The following code exits with a unbound variable error. How to fix this, while still using the set -o nounset option?

#!/bin/bash

set -o nounse         


        

Answer 1

Assumptions:

$ echo $SHELL
/bin/bash
$ /bin/bash --version | head -1
GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
$ set -o nounset

If you want a non-interactive script to print an error and exit if a variable is null or not set:

$ [[ "${HOME:?}" ]]

$ [[ "${IAMUNBOUND:?}" ]]
bash: IAMUNBOUND: parameter null or not set

$ IAMNULL=""
$ [[ "${IAMNULL:?}" ]]
bash: IAMNULL: parameter null or not set

If you don't want the script to exit:

$ [[ "${HOME:-}" ]] || echo "Parameter null or not set."

$ [[ "${IAMUNBOUND:-}" ]] || echo "Parameter null or not set."
Parameter null or not set.

$ IAMNULL=""
$ [[ "${IAMUNNULL:-}" ]] || echo "Parameter null or not set."
Parameter null or not set.

You can even use [ and ] instead of [[ and ]] above, but the latter is preferable in Bash.

Note what the colon does above. From the docs:

Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

There is apparently no need for -n or -z.

In summary, I may typically just use [[ "${VAR:?}" ]]. Per the examples, this prints an error and exits if a variable is null or not set.