Test for a Bash variable being unset, using a function

Question

A simple Bash variable test goes:

${varName:?    \"${varName} is not defined\"}

I\'d like to re-use this, by putting it in a function. How

Answer 1

You want to use [ -z ${parameter+word} ]

Some part of man bash:

Parameter Expansion
    ...
    In  each  of  the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and
    arithmetic expansion.  When not performing substring expansion, bash tests for a parameter that is unset or null;
    omitting the colon results in a test only for a parameter that is unset.
    ...
    ${parameter:+word}
           Use Alternate Value.  If parameter is null or unset, nothing is substituted, otherwise  the  expansion  of
           word is substituted.
    ...

in other words:

    ${parameter+word}
           Use Alternate Value.  If parameter is unset, nothing is substituted, otherwise  the  expansion  of
           word is substituted.

some examples:

$ set | grep FOOBAR
$ if [ -z "${FOOBAR+something}" ]; then echo "it is unset"; fi
it is unset
$ declare FOOBAR
$ if [ -z "${FOOBAR+something}" ]; then echo "it is unset"; fi
$ FOOBAR=
$ if [ -z "${FOOBAR+something}" ]; then echo "it is unset"; fi
$ FOOBAR=1
$ if [ -z "${FOOBAR+something}" ]; then echo "it is unset"; fi
$ unset FOOBAR
$ if [ -z "${FOOBAR+something}" ]; then echo "it is unset"; fi
it is unset
$