How to run a shell script upon user authentication (e.g. screen unlock)?
From https://apple.stackexchange.com/a/159510/76470 :

I found SleepWatcher, a free tool that allows to run a script when the screen goes to sleep or wakes up.

After installing it, you can link a shell script that is run on one of the available events (which include dim screen, wake up screen, computer sleep, wake up computer and some more). From the shell script, I can run a command, or call an AppleScript that I want to run on these events.

Since my screen goes to sleep when I lock my computer, this is exactly what I was looking for.