I ended up writing very similar code to do the same thing: https://gist.github.com/jhlb/83999422a8d7e8a6e28e https://gist.github.com/jhlb/83999422a8d7e8a6e28e

I ended up writing very similar code to do the same thing: https://gist.github.com/jhlb/83999422a8d7e8a6e28e https://gist.github.com/jhlb/83999422a8d7e8a6e28e EDIT: as mentioned elsewhere in the thread, the pv program has a way to watch a PID as well that makes this program unnecessary.