THE UDEV DATABASE
− Device unit configuration
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.
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.
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
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.
THE UDEV DATABASE
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
Adds dependencies of type
Wants from the device unit to all listed units. The
first form is used by the system systemd instance, the
second by user systemd instances. Those settings may be used
to activate arbitrary units when a specific device becomes
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).
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.
Adds an additional alias name
to the device unit. This must be an absolute path that is
automatically transformed into a unit name. (See above.)
If set to 0, systemd will
consider this device unplugged even if it shows up in the
udev tree. If this property is unset or set to 1, the device
will be considered plugged if it is visible in the udev
tree. This property has no influence on the behavior when a
device disappears from the udev tree.
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
If set, this property is used
as description string for the device unit.
systemctl(1), systemd.unit(5), udev(7),