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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd.device">
+ <refentryinfo>
+ <title>systemd.device</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>systemd.device</refentrytitle>
+ <manvolnum>5</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>systemd.device</refname>
+ <refpurpose>Device unit configuration</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <para><filename><replaceable>device</replaceable>.device</filename></para>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <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
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ 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-udevd.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
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
+ <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>
+
+ <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>
+ </refsect1>
+
+ <refsect1>
+ <title>Automatic Dependencies</title>
+
+ <refsect2>
+ <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>
+ </refsect2>
+
+ <refsect2>
+ <title>Default Dependencies</title>
+
+ <para>There are no default dependencies for device units.</para>
+ </refsect2>
+ </refsect1>
+
+ <refsect1>
+ <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'>
+ <varlistentry>
+ <term><varname>SYSTEMD_WANTS=</varname></term>
+ <term><varname>SYSTEMD_USER_WANTS=</varname></term>
+ <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>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>SYSTEMD_ALIAS=</varname></term>
+ <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>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>SYSTEMD_READY=</varname></term>
+ <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
+ tree.</para>
+
+ <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>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
+ <term><varname>ID_MODEL=</varname></term>
+
+ <listitem><para>If set, this property is used as description
+ string for the device unit.</para></listitem>
+ </varlistentry>
+
+ </variablelist>
+ </refsect1>
+
+ <refsect1>
+ <title>Options</title>
+
+ <para>Device unit files may include [Unit] and [Install] sections, which are described in
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. No
+ options specific to this file type are supported.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>