Guard is a command line tool to easily handle events on file system modifications. http://guardgem.org
Github: https://github.com/guard/guard
Guard can be used for many different things related to file system modifications. Our most common use cases will be live-reload and stylesheet compilation.
Just installing guard alone won't give you all the features. You need to install guard as well as the plugins you want to use with it. Jeffrey Way has a great video on installing/using guard. guard-is-your-best-friend