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

There is quite a lot of material, so I split them into a series of posts, running once a day for the next 9 days.

Units

Everything managed by systemd is called a unit (see man systemd.unit), and each unit is described by a configuration in ini-style format.

For example, this unit continuously plays an alarm sound when the system is in emergency or rescue mode:

[Unit]
Description=Beeps when in emergency or rescue mode
DefaultDependencies=false
StopWhenUnneeded=true

[Install]
WantedBy=emergency.target rescue.target

[Service]
Type=simple
ExecStart=/bin/sh -ec 'while true; do /usr/bin/aplay -q /tmp/beep.wav; sleep 2; done'

Units can be described by configuration files, which have different extensions based on what kind of thing they describe:

System unit files can be installed in:

Unit files in /etc/ override unit files in /lib/. Note that while Debian uses /lib/, other distributions may use /usr/lib/ instead.

If there is a directory with the same name as the unit file plus a .d suffix, any file *.conf it contains is parsed after the unit, and can be used to add or override configuration options.

For example:

Similarly, a unitname.wants/ or unitname.requires/ directory can be used to extend Wants= and Requires= dependencies on other units, by placing symlinks to other units in them.

See also:

pdo debian eng sw systemd-truelite

2017-09-22 00:00:00+02:00