I have that aliased to 'cack'. Then ruby is 'rack', python is 'pack', go is 'gack', etc.
(I've never needed to use the rackup "rack" command directly, fortunately, if you do you ought to use a different alias)
(I've never needed to use the rackup "rack" command directly, fortunately, if you do you ought to use a different alias)
Or escape alias \rack
(Presumably because in zsh, `which which` says it's a shell built-in, whereas in bash it finds `/usr/bin/which`, so bash doesn't seem to be caring about your aliases.)