`md5sum` is not available on OS X by default. I have an alias for it: https://github.com/mathiasbynens/dotfiles/blob/4ce66fd1281037a4e3381c79818510e2d7f750b6/.aliases#L76-L77 But of course, the alias won’t be available the first time `bootstrap.sh` is run on a clean, dotfile-free OS X install.