Show HN: Teetail – like tee and like tail

I want to pass tons of stuff through a pipeline, and also store it in a file so I can debug what happened if things unexpectedly die. There's way too much data to put it all in a file, so I just want the last bit to end up in the file.

Wrote a quick bit of C to solve that, posting it here to save the next person to have this particular need a bit of time.