Is there feature like "if file x is changed, then run script y"?

[k4leg]

For example, I have an emacs daemon that needs to be reloaded to read changed config, and to do this I need to do something like:

if [ "$(systemctl --user is-active emacs.service)" = "active" ]; then
        systemctl --user restart emacs.service
fi

[twpayne]

No, there's no such feature built in, but you can achieve this with a run_after_ script that checks the checksum of the config file and restarts the service if the checksum has changed.