Systemd on the command line

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

Exploring the state of a system

• systemctl status [unitname [unitname..]] show status of one or more units, or of the whole system. Glob patterns also work: systemctl status "systemd-fsck@*"
• systemctl list-units or just systemctl show a table with all units, their status and their description
• systemctl list-sockets lists listening sockets managed by systemd and what they activate
• systemctl list-timers lists timer-activated units, with information about when they last ran and when they will run again
• systemctl is-active [pattern] checks if one or more units are in active state
• systemctl is-enabled [pattern] checks if one or more units are enabled
• systemctl is-failed [pattern] checks if one or more units are in failed state
• systemctl list-dependencies [unitname] lists the dependencies of a unit, or a system-wide dependency tree
• systemctl is-system-running check if the system is running correctly, or if some unit is in a failed state
• systemd-cgtop like top but processes are aggregated by unit
• systemd-analyze produces reports on boot time, per-unit boot time charts, dependency graphs, and more

Start and stop services

Similar to the System V service command, systemctl provides commands to start/stop/restart/reload units or services:

• start: starts a unit if it is not already started
• stop: stops a unit
• restart: starts or restarts a unit
• reload: tell a unit to reload its configuration (if it supports it)
• try-restart: restarts a unit only if it is already active, otherwise do nothing, to prevent accidentally starting a service
• reload-or-restart: tell a unit to reload its configuration if supported, otherwise restart it
• try-reload-or-restart: tell a unit to reload its configuration if supported, otherwise restart it. If the unit is not already active, do nothing to prevent accidentally starting a service.

Changing global system state

systemctl has halt, poweroff, reboot, suspend, hibernate, and hybrid-sleep commands to tell systemd to reboot, power off, suspend and so on. kexec and switch-root also work.

The rescue and emergency commands switch the system to rescue and emergency mode (see man systemd.special. systemctl default switches to the default mode, which also happens when exiting the rescue or emergency shell.

Run services at boot

systemd does not implement runlevels, and services start at boot based on their dependencies.

To start a service at boot, you add to its .service file a WantedBy= dependency on a well-known .target unit.

At boot, systemd brings up the whole chain of dependency started from a default unit, and that will eventually activate also your service.

See systemctl get-default for what unit is currently the default in your system. You can change it via the systemd.unit= kernel command line, so you can configure multiple entries in the boot loader that boot the system running different services. For example systemd.unit=rescue.target for a rescue mode, systemd.unit=multi-user.target for a non-graphical mode, or add your own .target file to implement new system modes.

See systemctl list-units -t target --all for a list of all currently available targets in your system.

• systemctl enable unitname enables the unit to start at boot, by creating symlinks to it in the .wants directory of the units listed in its WantedBy= configuration
• systemctl disable unitname removes the symlinks created by enable
• systemctl reenable unitname removes and readds the symlinks for when you changed WantedBy=

Notes: systemctl start activates a unit right now, but does not automatically enable it at boot systemctl enable enables a unit at boot, but does not automatically start it right now * a disabled unit can still be activated if another unit depends on it

To disable a unit so that it will never get started even if another unit depends on it, use systemctl mask unitname. Use systemctl unmask unitname to undo the masking.

Reloading / restarting systemd

systemctl daemon-reload tells systemd to reload its configuration.

systemctl daemon-reexec tells systemd to restart iself.