Nosier monitors paths for changes and then runs a specified command.
Intended for automatically running tests upon code changes, it can however be useful for running any command upon changes to files.
Alternatives are Jeff Winkler's original nosy script and Gary Bernhardt's improved nosy script. It appears that both of these are not packaged for easy installation.
Nosier can be installed simply with pip:
pip install nosier
Or with easy_install:
Nosier uses the Linux inotify facility for monitoring paths which has a couple of implications.
First, it doesn't put as much load on the cpu and disk as some alternatives which will regularly scan and calculate check sums.
Second, inotify is Linux 2.6 specific and hence limited to that platform.