To create a background service, a systemd service file is needed. It can be create in ~/.config/systemd/user/ with the extension .service for user-level services, or in /etc/systemd/system/ with the extension .service for system-level ones.

The content of the service file should include the [Unit], [Service], and [Install] sections. For example:

[Unit]
Description=My Background Service

[Service]
ExecStart=/path/to/your/executable
Restart=always/on-failure/...

[Install]
WantedBy=default.target/multi-user.target/...

The key fields to note are ExecStart, Restart, and WantedBy.

• ExecStart specifies the command to run the service.
• Restart specifies when you want the service to restart: always (after quit), when quited unexpectedly, etc.
• WantedBy specifies the target to depend on. The perhaps most common one, default.target, means the service will start when user logs in.

Here is my Syncthing service file:

[Unit]
Description=Syncthing
After=network.target

[Service]
ExecStart=/opt/syncthing/syncthing serve --no-browser
Restart=on-failure
RestartSec=5

[Install]
WantedBy=default.target

Interpretation: The file defines a service that is called Syncthing. It auto starts when user logs in by calling syncthing serve --no-browser (the command to start the Syncthing server), after the network is up. It restarts everytime it crashes after waiting for 5 seconds.