Frequently Asked Question

What is a drop-in override and how is it different from editing a unit file?

A drop-in is a small partial unit file that supplements an existing one. Instead of editing (and therefore replacing) the vendor-shipped unit at /usr/lib/systemd/system/ nginx.service, you create a directory /etc/systemd/system/nginx.service.d/ and put .conf files inside it. systemd merges them on top of the original unit. The shortcut is sudo systemctl edit nginx, which opens an editor on an override file in the right place and runs daemon-reload for you when you save.

Drop-ins are the right way to customise a packaged service: they survive package upgrades (the vendor unit can be updated without losing your changes), they make your modifications easy to find and audit, and they keep the original unit intact for reference. systemctl cat nginx prints the effective merged unit so you can see what systemd is actually using. To replace a single directive entirely, set it to empty first (e.g. ExecStart= then ExecStart=/new/command) because some directives otherwise accumulate.