Executing jobs on file-changes with Systemd

For the last couple of months I’ve been using Facebook’s watchman for executing a deployment script when my CD pipeline pushed a file onto the target system. Watchman is a bit weird when it comes to running it as a service, though, so I started looking around thinking that this should be something that Systemd should be able to handle all by itself. And to actually nobody’s surprise I found the feature I was looking for: systemd.path.

Similar to a timer you can tell Systemd to activate a service if something happens to a file-system path. A sample setup would look like this:

$cat /etc/systemd/system/app-update.service
Description=app-update executor

$ cat /etc/systemd/system/app-update.path


Here /srv/app/scripts/update.sh is triggered when /srv/app/etc/main.yml is modified. If you don’t want to trigger the service whenever the file is modified but only when the file descriptor is closed again after a modification, use PathChanged instead.

Just make sure to enable and start the path file when you’re done. Otherwise nothing will happen when the file changes πŸ˜…

$ systemctl enable app-update.path
$ systemctl start app-update.path