In Bash, how do I test if a variable is defined in “-u” mode

Question

I just discovered set -u in bash and it helped me find several previously unseen bugs. But I also have a scenario where I need to test if a variable is defined

Answer 1

In bash 4.2 and newer there is an explicit way to check whether a variable is set, which is to use -v. The example from the question could then be implemented like this:

if [[ ! -v variable ]]; then
   variable="$(...)"
fi

See http://www.gnu.org/software/bash/manual/bashref.html#Bash-Conditional-Expressions

If you only want to set the variable, if it is not already set you are probably better of doing something along these lines:

variable="${variable-$(...)}"

Note that this does not deal with a defined but empty variable.