This provides a ruby library
WaitPid.wait_pid pid_number # waits until that process exits
and also a binary command
$ wait_pid pid_number # waits until that process exits
Currently it just polls 100 times/s to see if the process is still around. If more granularity is needed let me know [it is possible, at least in windows, or a higher polling capability could be made optional in linux].
Enjoy. Feedback welcome.