Systemd device units

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

.device units

Several devices are automatically represented inside systemd by .device units, which can be used to activate services when a given device exists in the file system.

See systemctl --all --full -t device to see a list of all decives for which systemd has a unit in your system.

For example, this .service unit plays a sound as long as a specific USB key is plugged in my system:

[Unit]
Description=Beeps while a USB key is plugged
DefaultDependencies=false
StopWhenUnneeded=true

[Install]
WantedBy=dev-disk-by\x2dlabel-ERLUG.device

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

If you need to work with a device not seen by default by systemd, you can add a udev rule that makes it available, by adding the systemd tag to the device with TAG+="systemd".

It is also possible to give the device an extra alias using ENV{SYSTEMD_ALIAS}="/dev/my-alias-name".

To figure out all you can use for matching a device:

  1. Run udevadm monitor --environment and plug the device
  2. Look at the DEVNAME= values and pick one that addresses your device the way you prefer
  3. udevadm info --attribute-walk --name=*the value of devname* will give you all you can use for matching in the udev rule.

See: