Rerun a command whenever a file changes

With entr (a small package, apt install entr):

ls *.py | entr -c python3 main.py

It reruns the command every time one of the listed files changes; -c clears the screen first. Watch a whole tree with find:

find src -name '*.ts' | entr -c npm test

Without entr, inotifywait from inotify-tools in a loop:

while inotifywait -qr -e modify,create,delete src; do npm test; done

And for the "just rerun it every two seconds" case, there's watch -n 2 ls -l.