What does `This command requires a keg argument` mean?

Question

I want to brew rm mtr and its deps, so I execute

brew rm mtr
brew rm $(join <(brew leaves) <(brew deps mtr))

But it says This command requires a keg argument. What is the keg argument?

Thank you.

Answer 1

I don't think that command is doing what you think it is.

Those < redirects aren't doing what you think and so the brew command getting executed is

brew rm

This requires an argument.

I like your attempt at using join without a temp file. After a glance at the man page, I'm pretty sure you cannot do that.

Try this:

brew leaves > leaves
brew rm $(brew deps mtr | join leaves -)