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 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). This may be used to define dependencies between devices and other units. To tag a udev device, use "TAG+="systemd"" in the udev rules file, see udev(7) for details.
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).
The settings of device units may either be configured via unit files, or directly from the udev database (which is recommended). The following udev device properties are understood by systemd:
Note that this and the other tags are not taken into account unless the device is tagged with the "systemd" string 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 influence on which udev event to trigger the dependencies.
This option is useful to support 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.