man/systemd.device.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7 -->
   8
   9 <refentry id="systemd.device">
  10   <refentryinfo>
  11     <title>systemd.device</title>
  12     <productname>systemd</productname>
  13
  14     <authorgroup>
  15       <author>
  16         <contrib>Developer</contrib>
  17         <firstname>Lennart</firstname>
  18         <surname>Poettering</surname>
  19         <email>lennart@poettering.net</email>
  20       </author>
  21     </authorgroup>
  22   </refentryinfo>
  23
  24   <refmeta>
  25     <refentrytitle>systemd.device</refentrytitle>
  26     <manvolnum>5</manvolnum>
  27   </refmeta>
  28
  29   <refnamediv>
  30     <refname>systemd.device</refname>
  31     <refpurpose>Device unit configuration</refpurpose>
  32   </refnamediv>
  33
  34   <refsynopsisdiv>
  35     <para><filename><replaceable>device</replaceable>.device</filename></para>
  36   </refsynopsisdiv>
  37
  38   <refsect1>
  39     <title>Description</title>
  40
  41     <para>A unit configuration file whose name ends in
  42     <literal>.device</literal> encodes information about a device unit
  43     as exposed in the
  44     sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  45     device tree.</para>
  46
  47     <para>This unit type has no specific options. See
  48     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  49     for the common options of all unit configuration files. The common
  50     configuration items are configured in the generic
  51     <literal>[Unit]</literal> and <literal>[Install]</literal>
  52     sections. A separate <literal>[Device]</literal> section does not
  53     exist, since no device-specific options may be configured.</para>
  54
  55     <para>systemd will dynamically create device units for all kernel
  56     devices that are marked with the "systemd" udev tag (by default
  57     all block and network devices, and a few others). This may be used
  58     to define dependencies between devices and other units. To tag a
  59     udev device, use <literal>TAG+="systemd"</literal> in the udev
  60     rules file, see
  61     <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  62     for details.</para>
  63
  64     <para>Device units are named after the <filename>/sys</filename>
  65     and <filename>/dev</filename> paths they control. Example: the
  66     device <filename noindex='true'>/dev/sda5</filename> is exposed in
  67     systemd as <filename>dev-sda5.device</filename>. For details about
  68     the escaping logic used to convert a file system path to a unit
  69     name see
  70     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
  71
  72     <para>Device units will be reloaded by systemd whenever the
  73     corresponding device generates a <literal>changed</literal> event.
  74     Other units can use <varname>ReloadPropagatedFrom=</varname> to react
  75     to that event</para>
  76   </refsect1>
  77
  78   <refsect1>
  79     <title>Automatic Dependencies</title>
  80
  81     <refsect2>
  82       <title>Implicit Dependencies</title>
  83
  84       <para>Many unit types automatically acquire dependencies on device
  85       units of devices they require. For example,
  86       <filename>.socket</filename> unit acquire dependencies on the
  87       device units of the network interface specified in
  88       <varname>BindToDevice=</varname>. Similar, swap and mount units
  89       acquire dependencies on the units encapsulating their backing
  90       block devices.</para>
  91     </refsect2>
  92
  93     <refsect2>
  94       <title>Default Dependencies</title>
  95
  96       <para>There are no default dependencies for device units.</para>
  97     </refsect2>
  98   </refsect1>
  99
 100   <refsect1>
 101     <title>The udev Database</title>
 102
 103     <para>Unit settings of device units may either be configured via unit files, or directly from the udev
 104     database. The following udev device properties are understood by the service manager:</para>
 105
 106     <variablelist class='udev-directives'>
 107       <varlistentry>
 108         <term><varname>SYSTEMD_WANTS=</varname></term>
 109         <term><varname>SYSTEMD_USER_WANTS=</varname></term>
 110         <listitem><para>Adds dependencies of type <varname>Wants=</varname> from the device unit to the specified
 111         units. <varname>SYSTEMD_WANTS=</varname> is read by the system service manager,
 112         <varname>SYSTEMD_USER_WANTS=</varname> by user service manager instances. These properties may be used to
 113         activate arbitrary units when a specific device becomes available.</para>
 114
 115         <para>Note that this and the other udev device properties are not taken into account unless the device is
 116         tagged with the <literal>systemd</literal> tag in the udev database, because otherwise the device is not
 117         exposed as a systemd unit (see above).</para>
 118
 119         <para>Note that systemd will only act on <varname>Wants=</varname> dependencies when a device first becomes
 120         active. It will not act on them if they are added to devices that are already active. Use
 121         <varname>SYSTEMD_READY=</varname> (see below) to configure when a udev device shall be considered active, and
 122         thus when to trigger the dependencies.</para>
 123
 124         <!-- Note that we don't document here that we actually apply unit_name_mangle() to all specified names, since
 125              that's kinda ugly, and people should instead specify correctly escaped names -->
 126
 127         <para>The specified property value should be a space-separated list of valid unit names. If a unit template
 128         name is specified (that is, a unit name containing an <literal>@</literal> character indicating a unit name to
 129         use for multiple instantiation, but with an empty instance name following the <literal>@</literal>), it will be
 130         automatically instantiated by the device's <literal>sysfs</literal> path (that is: the path is escaped and
 131         inserted as instance name into the template unit name). This is useful in order to instantiate a specific
 132         template unit once for each device that appears and matches specific properties.</para></listitem>
 133       </varlistentry>
 134
 135       <varlistentry>
 136         <term><varname>SYSTEMD_ALIAS=</varname></term>
 137         <listitem><para>Adds an additional alias name to the device
 138         unit. This must be an absolute path that is automatically
 139         transformed into a unit name. (See above.)</para></listitem>
 140       </varlistentry>
 141
 142       <varlistentry>
 143         <term><varname>SYSTEMD_READY=</varname></term>
 144         <listitem><para>If set to 0, systemd will consider this device unplugged even if it shows up in the udev
 145         tree. If this property is unset or set to 1, the device will be considered plugged if it is visible in the udev
 146         tree.</para>
 147
 148         <para>This option is useful for devices that initially show up in an uninitialized state in the tree, and for
 149         which a <literal>changed</literal> event is generated the moment they are fully set up. Note that
 150         <varname>SYSTEMD_WANTS=</varname> (see above) is not acted on as long as <varname>SYSTEMD_READY=0</varname> is
 151         set for a device.</para></listitem>
 152       </varlistentry>
 153
 154       <varlistentry>
 155         <term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
 156         <term><varname>ID_MODEL=</varname></term>
 157
 158         <listitem><para>If set, this property is used as description
 159         string for the device unit.</para></listitem>
 160       </varlistentry>
 161
 162     </variablelist>
 163
 164   </refsect1>
 165
 166   <refsect1>
 167       <title>See Also</title>
 168       <para>
 169         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 170         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 171         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 172         <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 173         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 174       </para>
 175   </refsect1>
 176
 177 </refentry>