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

Writing .unit files

For reference, the global index with all .unit file directives is at man systemd.directives.

All unit files have a [Unit] section with documentation and dependencies. See man systemd.unit for documentation.

It is worth having a look at existing units to see what they are like. Use systemctl --all -t unittype for a list, and systemctl cat unitname to see its content wherever it is installed.

For example: systemctl cat graphical.target. Note that systemctl cat adds a line of comment at the top so one can see where the unit file is installed.

Most unit files also have an [Install] section (also documented in man systemd.unit) that controls what happens when enabling or disabling the unit.

See also:

.target units

.target units only contain [Unit] and [Install] sections, and can be used to give a name to a given set of dependencies.

For example, one could create a remote-maintenance.target unit, that when brought up activates, via dependencies, a set of services, mounts, network sockets, and so on.

See man systemd.target

See systemctl --all -t target for examples.

special units

man systemd.special has a list of units names that have a standard use associated to them.

For example, ctrl-alt-del.target is a unit that is started whenever Control+Alt+Del is pressed on the console. By default it is symlinked to reboot.target, and you can provide your own version in /etc/systemd/system/ to perform another action when Control+Alt+Del is pressed.

User units

systemd can also be used to manage services on a user session, starting them at login and stopping them at logout.

Add --user to the normal systemd commands to have them work with the current user's session instead of the general system.

See systemd/User in the Arch Wiki for a good description of what it can do.

pdo debian eng sw systemd-truelite

2017-09-24 00:00:00+02:00