Big Sur - detect when Finder window has closed
I figured out how to solve this.
I repeatedly run the following command in my shell script in a loop, with a short sleep between loop iterations:
/usr/bin/osascript -e 'tell application "Finder" to get the name of every window' | grep '/path/to/directory'
As soon as it stops finding a match, I break out of the loop, since it means that the Finder window for /path/to/directory has closed.