Editing Unit Files
While the specific format for unit files is outside of the scope of this tutorial, systemctl provides built-in mechanisms for editing and modifying unit files if you need to make adjustments. This functionality was added in systemd version 218.
The edit command, by default, will open a unit file snippet for the unit in question:
sudo systemctl edit nginx.service
This will be a blank file that can be used to override or add directives to the unit definition. A directory will be created within the /etc/systemd/system directory which contains the name of the unit with .d appended. For instance, for the nginx.service, a directory called nginx.service.d will be created.
Within this directory, a snippet will be created called override.conf. When the unit is loaded, systemd will, in memory, merge the override snippet with the full unit file. The snippet’s directives will take precedence over those found in the original unit file.
If you wish to edit the full unit file instead of creating a snippet, you can pass the –full flag:
sudo systemctl edit --full nginx.service
This will load the current unit file into the editor, where it can be modified. When the editor exits, the changed file will be written to /etc/systemd/system, which will take precedence over the system’s unit definition (usually found somewhere in /lib/systemd/system).
To remove any additions you have made, either delete the unit’s .d configuration directory or the modified service file from /etc/systemd/system. For instance, to remove a snippet, we could type:
sudo rm -r /etc/systemd/system/nginx.service.d
To remove a full modified unit file, we would type:
sudo rm /etc/systemd/system/nginx.service
After deleting the file or directory, you should reload the systemd process so that it no longer attempts to reference these files and reverts back to using the system copies. You can do this by typing:
sudo systemctl daemon-reload
systemd – ArchWiki (archlinux.org)
How To Use Systemctl to Manage Systemd Services and Units | DigitalOcean