This a template for simple sysmtemd service to change the ownership of a file, since in my case the file is in /sys and it is generated on boot, so using acl didn’t help me. I had to use this hack to change the ownership of a file on each boot.
What I need is a write permissions to a file in order to change the brightness on my laptop with i3wm.

The file is /etc/systemd/system/brightness.service
but there is symlink from the /etc/systemd/system/multi-user.target.wants directory.

This are the contents of the service file:

[Unit]
Description=Alter permissionsfor brightness

[Service]
ExecStart=/bin/chmod go+rw /sys/class/backlight/intel_backlight/brightness
ExecStop=/bin/chmod go+rw /sys/class/backlight/intel_backlight/brightness

[Install]
WantedBy=multi-user.target

You will also want to enable and start the service with:

systemctl enable brightness.service
systemctl start brightness.service

All of this action is happening on Ubuntu 18.04