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<refentry id="systemd.device">
<refpurpose>Device unit configuration</refpurpose>
<para>A unit configuration file whose name ends in <literal>.device</literal> encodes information about a
device unit as exposed in the
sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> device
tree. This may be used to define dependencies between devices and other units.</para>
<para>This unit type has no specific options. See
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.</para>
<para>systemd will dynamically create device units for all kernel devices that are marked with the
<literal>systemd</literal> udev tag (by default all block and network devices, and a few others). Note
that <emphasis>if <filename>systemd-udev.service</filename> is not running, no device units will be
available (for example in a typical container)</emphasis>.</para>
<para>Device units are named after the <filename>/sys/</filename>
and <filename>/dev/</filename> paths they control. Example: the
device <filename index="false">/dev/sda5</filename> is exposed in
systemd as <filename>dev-sda5.device</filename>. For details about
the escaping logic used to convert a file system path to a unit
name see
<para>To tag a udev device, use <literal>TAG+="systemd"</literal> in the udev rules file, see
<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details.
<para>Device units will be reloaded by systemd whenever the
corresponding device generates a <literal>changed</literal> event.
Other units can use <varname>ReloadPropagatedFrom=</varname> to react
to that event.</para>
<title>Automatic Dependencies</title>
<title>Implicit Dependencies</title>
<para>Many unit types automatically acquire dependencies on device
units of devices they require. For example,
<filename>.socket</filename> unit acquire dependencies on the
device units of the network interface specified in
<varname>BindToDevice=</varname>. Similar, swap and mount units
acquire dependencies on the units encapsulating their backing
block devices.</para>
<title>Default Dependencies</title>
<para>There are no default dependencies for device units.</para>
<title>The udev Database</title>
<para>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:</para>
<variablelist class='udev-directives'>
<listitem><para>Adds dependencies of type <varname>Wants=</varname> from the device unit to the specified
units. <varname>SYSTEMD_WANTS=</varname> is read by the system service manager,
<varname>SYSTEMD_USER_WANTS=</varname> by user service manager instances. These properties may be used to
activate arbitrary units when a specific device becomes available.</para>
<para>Note that this and the other udev device properties are not taken into account unless the device is
tagged with the <literal>systemd</literal> tag in the udev database, because otherwise the device is not
exposed as a systemd unit (see above).</para>
<para>Note that systemd will only act on <varname>Wants=</varname> dependencies when a device first becomes
active. It will not act on them if they are added to devices that are already active. Use
<varname>SYSTEMD_READY=</varname> (see below) to configure when a udev device shall be considered active, and
thus when to trigger the dependencies.</para>
<!-- Note that we don't document here that we actually apply unit_name_mangle() to all specified names, since
that's kinda ugly, and people should instead specify correctly escaped names -->
<para>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 <literal>@</literal> character indicating a unit name to
use for multiple instantiation, but with an empty instance name following the <literal>@</literal>), it will be
automatically instantiated by the device's <literal>sysfs</literal> 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.</para></listitem>
<listitem><para>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.)</para></listitem>
<listitem><para>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
<para>This option is useful for devices that initially show up in an uninitialized state in the tree, and for
which a <literal>changed</literal> event is generated the moment they are fully set up. Note that
<varname>SYSTEMD_WANTS=</varname> (see above) is not acted on as long as <varname>SYSTEMD_READY=0</varname> is
set for a device.</para></listitem>
<listitem><para>If set, this property is used as description
string for the device unit.</para></listitem>
<title>See Also</title>