How do I check if a variable is set in zsh?

Question

What is the canonical / idiomatic way to test if a variable has been set in zsh?

if ....something.... ; then
    print "Sweet, that variable is defined"
else
    print "That variable is not defined"
fi

Here's the SO answer for bash.

Answer 1

The typical way to do this in Zsh is:

if (( ${+SOME_VARIABLE} )); then

For example:

if (( ${+commands[brew]} )); then
    brew install mypackage
else
    print "Please install Homebrew first."
fi

In Zsh 5.3 and later, you can use the -v operator in a conditional expression:

if [[ -v SOME_VARIABLE ]]; then

The POSIX-compliant test would be

if [ -n "${SOME_VARIABLE+1}" ]; then

Answer 2

Use the -v (introduced in zsh 5.3) operator in a conditional expression.

% unset foo
% [[ -v foo ]] && echo "foo is set"
% foo=
% [[ -v foo ]] && echo "foo is set"
foo is set