Watchman from Facebook is a great little cross-platform file watching service.
When editing documentation in the Sphinx project https://github.com/OSC/ood-documentation I open a separate terminal, and start watchman to watch the source directory. Whenever I save a change watchman rebuilds the application.
watchman-make -p 'source/**/*.rst' --run 'rm -rf build; rake docker:build'
The key is
-p tells what files to watch (rst files recursively) and then
--run is the command to run when a change occurs.