bash-variables set, unset, empty, +/-defaults (Stackoverflow answer)

bash-variable-test-conditions.md

SO Answer

How to check if a variable is set in Bash?
Q: from 2011 A: from 2021

https://stackoverflow.com/a/65482850/202553

if test "${name+x}"; then
    echo 'name is set'
else
    echo 'name is not set'
fi

This question already has a lot of answers, but none of them offered bona fide Boolean expressions to clearly differentiate between variables values.

Here are some unambiguous expressions that I worked out:

Expression in script	name='fish'	name=''	unset name
test "$name"	TRUE	f	f
test -n "$name"	TRUE	f	f
test ! -z "$name"	TRUE	f	f
test ! "${name-x}"	f	TRUE	f
test ! "${name+x}"	f	f	TRUE

By the way, these expressions are equivalent: test <=> [ ]

Other ambiguous expressions to be used with caution:

Expression in script	name='fish'	name=''	unset name
test "${name+x}"	TRUE	TRUE	f
test "${name-x}"	TRUE	f	TRUE
test -z "$name"	f	TRUE	TRUE
test ! "$name"	f	TRUE	TRUE
test ! -n "$name"	f	TRUE	TRUE
test "$name" = ''	f	TRUE	TRUE