A locking wrapper scripthttp://code.google.com/p/withlock/
withlock is a locking wrapper script to make sure that some program isn't run more than once. It is ideal to prevent periodic jobs spawned by cron from stacking up.
The locks created are valid only while the wrapper is running, and thus will never require a cleanup, as after a reboot. Thus, the wrapper is safe and easy to use, and much better than implementing half-hearted locking within scripts.