systemd.device - Device unit configuration
A unit configuration file whose name ends in ".device" encodes information about a device unit as exposed in the sysfs/udev(7) device tree. This may be used to define dependencies between devices and other units.
This unit type has no specific options. See systemd.unit(5) for the common options of all unit configuration files. The common configuration items are configured in the generic [Unit] and [Install] sections. A separate [Device] section does not exist, since no device-specific options may be configured.
systemd will dynamically create device units for all kernel devices that are marked with the "systemd" udev tag (by default all block and network devices, and a few others). Note that if systemd-udev.service is not running, no device units will be available (for example in a typical container).
Device units are named after the /sys/ and /dev/ paths they control. Example: the device /dev/sda5 is exposed in systemd as dev-sda5.device. For details about the escaping logic used to convert a file system path to a unit name see systemd.unit(5).
To tag a udev device, use "TAG+="systemd"" in the udev rules file, see udev(7) for details.
Device units will be reloaded by systemd whenever the corresponding device generates a "changed" event. Other units can use ReloadPropagatedFrom= to react to that event.
Many unit types automatically acquire dependencies on device units of devices they require. For example, .socket unit acquire dependencies on the device units of the network interface specified in BindToDevice=. Similar, swap and mount units acquire dependencies on the units encapsulating their backing block devices.
There are no default dependencies for device units.
Unit settings of device units may either be configured via unit files, or directly from the udev database. The following udev device properties are understood by the service manager:
Note that this and the other udev device properties are not taken into account unless the device is tagged with the "systemd" tag in the udev database, because otherwise the device is not exposed as a systemd unit (see above).
Note that systemd will only act on Wants= dependencies when a device first becomes active. It will not act on them if they are added to devices that are already active. Use SYSTEMD_READY= (see below) to configure when a udev device shall be considered active, and thus when to trigger the dependencies.
The specified property value should be a space-separated list of valid unit names. If a unit template name is specified (that is, a unit name containing an "@" character indicating a unit name to use for multiple instantiation, but with an empty instance name following the "@"), it will be automatically instantiated by the device's "sysfs" path (that is: the path is escaped and inserted as instance name into the template unit name). This is useful in order to instantiate a specific template unit once for each device that appears and matches specific properties.
This option is useful for devices that initially show up in an uninitialized state in the tree, and for which a "changed" event is generated the moment they are fully set up. Note that SYSTEMD_WANTS= (see above) is not acted on as long as SYSTEMD_READY=0 is set for a device.