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

Start and stop services

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

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.

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.

pdo debian eng sw systemd-truelite

2017-09-23 00:00:00+02:00