The Listen gem listens for file modifications and notifies about changes.

- OS-optimized adapters on MRI for Mac OS X 10.6+, Linux, *BSD and Windows
- Detects file modification, addition and removal
- You can watch multiple directories
- Regexp-patterns for ignoring paths for more accuracy and speed
- Increased change detection accuracy on OS X HFS and VFAT volumes

WWW: https://github.com/sass/listen