Systemd

A system and service manager for Linux. Runs as a daemon with PID 1.

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Units

Units are systemd tasks. The most common units are services (.service files). See man systemd.unit for documentation.

Locations:

~/.config/systemd/user: normal units
/etc/systemd/system: root units

Commands:

systemctl daemon-reload: load new unit files
systemctl start/stop/restart <service>: control a service
systemctl enable/disable <service>: run a service on boot
systemctl status <service>: get the status of a service

Service file example:

[Unit]
Description=Run foo
## wait for network
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
Restart=always
Environment=FOO=BAR
EnvironmentFile=.env
ExecStart=/usr/local/bin/foo

[Install]
WantedBy=multi-user.target # run on boot

Targets (.target files) are groups of units. For example, graphical.target contains all units that are necessary to run GUI. At boot time, systemd activates default.target.

Timers

https://wiki.archlinux.org/title/systemd/Timers

An alternative to cron jobs. By convention, a timer unit starts a service unit with the same name, e.g. foo.timer starts foo.service.

Timer file example:

[Unit]
Description=Run foo weekly

[Timer]
OnCalendar=Weekly
Persistent=true

[Install]
WantedBy=timers.target

Logs

Use journalctl to view systemd logs:

journalctl --unit foo.service