Bash – Need for the `builtin` builtin

zsh is one of the few shells (the other ones being tcsh (which originated as a csh script for csh users, which also had its limitation, tcsh made it a builtin as an improvement)) where which does something sensible since it's a shell builtin, but somehow you or your OS (via some rc file) broke it by replacing it with a call to the system which command which can't do anything sensible reliably since it doesn't have access to the interns of the shell so can't know how that shell interprets a command name.

In zsh, all of which, type, whence and where are builtin commands that are all used to find out about what commands are, but with different outputs. They're all there for historical reason, you can get all of their behaviours with different flags to the whence command.

You can get the details of what each does by running:

info -f zsh --index-search=which

Or type info zsh, then bring up the index with i, and enter the builtin name (completion is available).

And avoid using /usr/bin/which. There's no shell nowadays where that which is needed. As Timothy says, use the builtin that your shell provides for that. Most POSIX shells will have the type command, and you can use command -v to only get the path of a command (though both type and command -v are optional in POSIX (but not Unix, and not any longer in LSB), they are available in most if not all the Bourne-like shells you're likely to ever come across).

(BTW, it looks like /usr/bin appears twice in your $PATH, you could add a typeset -U path to your ~/.zshrc)

I can't imagine any reason to write this code, and I'm not quite sure what the person who wrote this code was trying to achieve. wait here does nothing -- from its perspective, there are no child processes, so it will just exit immediately and basically act as a noop (wait itself executes in a child process due to command substitution, but that's unrelated).

As an aside, parsing the output of ls is quite fragile. Instead, consider just doing this:

for file in *.txt; do
    ...
done