Systemd service units

These are the notes of a training course on systemd I gave as part of my work with Truelite.

.service units

Describe how to start and stop system services, like daemons.

Services are described in a [Service] section. The Type= configuration describes how the service wants to be brought up:

• Type=simple: a program is started and runs forever, providing the service. systemd takes care of daemonizing it properly in the background, creating a pidfile, stopping it and so on. The service is considered active as soon as it has started.
• Type=forking: a traditional daemons that forks itself, creates a pidfile and so on. The server is considered active as soon as the parent process ends.
• Type=oneshot: a program is run once, and the service is considered started after the program ends. This can be used, for example, to implement a service to do one-off configuration, like checking a file system.
• Type=dbus: like simple but for D-Bus services: the service is considered active as soon as it appears on the D-Bus bus.
• Type=notify: like simple, but the service tells sytemd when it has finished initialization and is ready. Notification can happen via the sd_notify C function, or the systemd-notify command.
• Type=idle: like simple, but it is run after all other services has been started on a transaction. You can use this, for example, to start a shell on a terminal after the boot, so that the prompt doesn't get flooded with boot messages, or to play a happy trumped sound after the system has finished booting.

There are a lot more configuration options to fine-tune how the program should be managed, to limit its resource access or capabilities to harden the system security, to run setup/cleanup scripts before or after it started, and after it gets stopped, to control what signals to send to ask for reload or quit, and quite a lot more.

See: man systemd.service, man systemd.exec, man systemd.resource-control, and man systemd.kill.

See systemctl --all -t service for examples.