Baffling Zsh behavior on = (equal sign) precedes a Zsh $variable

Question

When a = sign precedes a Zsh $variable expansion, I get the following behavior:

% m=$PATH;  echo =$m=
zsh: /usr/local/bin:/usr/sbin:/usr/bin= not found

Quite perplexing. What is happening, and why?

Answer 1

You've discovered = filename expansion.

Per 14.7.3 ‘=’ expansion from the zsh docs:

If a word begins with an unquoted ‘=’ and the EQUALS option is set, the remainder of the word is taken as the name of a command. If a command exists by that name, the word is replaced by the full pathname of the command.

To disable this behavior, unset the EQUALS option:

unsetopt EQUALS

Here's an example of both behaviors:

$ m=date

$ setopt EQUALS
$ echo =$m
/usr/bin/date

$ unsetopt EQUALS
$ echo =$m
=date