“Exit Traps” Can Make Your Bash Scripts Way More Robust and Reliable

I used an exit trap to kill an SSH agent that I am running, and I noticed that dash did not kill if the script was interrupted, but only if it ran to successful completion. I asked on the mailing list if this was expected behavior, and it turns out that POSIX only requires EXIT to run on a clean shutdown; to catch interruptions, add more signals. trap 'eval $(ssh-agent -k)' EXIT INT ABRT KILL TERM

Traps are neat but beware, they aren't completely reliable. You can't trap SIGKILL or SIGSTOP. In the article's examples, a SIGKILL would (1) leave temp directories around, (2) fail to restart a service, and (3) leave an expensive AWS resource running. Remember that SIGKILL isn't always the result of a human typing `kill -9` either: the Linux OOM killer sends it; all unixes potentially send it during shutdown and runlevel switching; programs like timeout(1) send it as a last resort…