With the listen gem you can write crossplatform scripts (http://rubygems.org/gems/listen).

For those of Perl persuasion, there are:

    * File::ChangeNotify (https://www.metacpan.org/module/File::ChangeNotify)
    * Filesys::Notify::Simple (https://www.metacpan.org/module/Filesys::Notify::Simple)

Hadn't come across listen, that's perfect for what I need. I've used notify on Linux but I wanted something a bit more portable for OS X.