Systemd Truelite course

pdo debian eng sw systemd

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:

.service: daemons
.socket: communication sockets
.device: hardware devices
.mount: mount points
.automount: automounting
.swap: swap files or partitions
.target: only dependencies, like Debian metapackages
.path: inotify monitoring of paths
.timer: cron-like activation
.slice: group processes for common resource management
.scope: group processes for common resource management

System unit files can be installed in:

/lib/systemd/system/: for units provided by packaged software
/run/systemd/system/: runtime-generated units
/etc/systemd/system/: for units provided by systemd administrators

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:

/lib/systemd/system/beep.service.d/foo.conf can be used to tweak the contents of /lib/systemd/system/beep.service, so it is possible for a package to distribute a tweak to the configuration of another package.
/etc/systemd/system/beep.service.d/foo.conf can be used to tweak the contents of /lib/systemd/system/beep.service, so it is possible a system administrator to extend a packaged unit without needing to replace it entirely.

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.