robbyrussell/oh-my-zsh — pull request #3123 by quodlibetor (2014-09-11 14:46:48+00:00)
Title: `test -d` to make sure awscli is actually installed

brew --prefix shows the location that a package is or would be
installed:

https://github.com/Homebrew/homebrew/blob/a8582c58/share/man/man1/brew.1#L448-L449