man/systemd.device.xml

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  24
  25 <refentry id="systemd.device">
  26   <refentryinfo>
  27     <title>systemd.device</title>
  28     <productname>systemd</productname>
  29
  30     <authorgroup>
  31       <author>
  32         <contrib>Developer</contrib>
  33         <firstname>Lennart</firstname>
  34         <surname>Poettering</surname>
  35         <email>lennart@poettering.net</email>
  36       </author>
  37     </authorgroup>
  38   </refentryinfo>
  39
  40   <refmeta>
  41     <refentrytitle>systemd.device</refentrytitle>
  42     <manvolnum>5</manvolnum>
  43   </refmeta>
  44
  45   <refnamediv>
  46     <refname>systemd.device</refname>
  47     <refpurpose>Device unit configuration</refpurpose>
  48   </refnamediv>
  49
  50   <refsynopsisdiv>
  51     <para><filename><replaceable>device</replaceable>.device</filename></para>
  52   </refsynopsisdiv>
  53
  54   <refsect1>
  55     <title>Description</title>
  56
  57     <para>A unit configuration file whose name ends in
  58     <literal>.device</literal> encodes information about a device unit
  59     as exposed in the
  60     sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  61     device tree.</para>
  62
  63     <para>This unit type has no specific options. See
  64     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  65     for the common options of all unit configuration files. The common
  66     configuration items are configured in the generic
  67     <literal>[Unit]</literal> and <literal>[Install]</literal>
  68     sections. A separate <literal>[Device]</literal> section does not
  69     exist, since no device-specific options may be configured.</para>
  70
  71     <para>systemd will dynamically create device units for all kernel
  72     devices that are marked with the "systemd" udev tag (by default
  73     all block and network devices, and a few others). This may be used
  74     to define dependencies between devices and other units. To tag a
  75     udev device, use <literal>TAG+="systemd"</literal> in the udev
  76     rules file, see
  77     <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  78     for details.</para>
  79
  80     <para>Device units are named after the <filename>/sys</filename>
  81     and <filename>/dev</filename> paths they control. Example: the
  82     device <filename noindex='true'>/dev/sda5</filename> is exposed in
  83     systemd as <filename>dev-sda5.device</filename>. For details about
  84     the escaping logic used to convert a file system path to a unit
  85     name see
  86     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
  87
  88   </refsect1>
  89
  90   <refsect1>
  91     <title>The udev Database</title>
  92
  93     <para>The settings of device units may either be configured via
  94     unit files, or directly from the udev database (which is
  95     recommended). The following udev device properties are understood
  96     by systemd:</para>
  97
  98     <variablelist class='udev-directives'>
  99       <varlistentry>
 100         <term><varname>SYSTEMD_WANTS=</varname></term>
 101         <term><varname>SYSTEMD_USER_WANTS=</varname></term>
 102         <listitem><para>Adds dependencies of type
 103         <varname>Wants</varname> from the device unit to all listed
 104         units. The first form is used by the system systemd instance,
 105         the second by user systemd instances. Those settings may be
 106         used to activate arbitrary units when a specific device
 107         becomes available.</para>
 108
 109         <para>Note that this and the other tags are not taken into
 110         account unless the device is tagged with the
 111         <literal>systemd</literal> string in the udev database,
 112         because otherwise the device is not exposed as a systemd unit
 113         (see above).</para>
 114
 115         <para>Note that systemd will only act on
 116         <varname>Wants</varname> dependencies when a device first
 117         becomes active. It will not act on them if they are added to
 118         devices that are already active. Use
 119         <varname>SYSTEMD_READY=</varname> (see below) to influence on
 120         which udev event to trigger the dependencies.
 121         </para></listitem>
 122       </varlistentry>
 123
 124       <varlistentry>
 125         <term><varname>SYSTEMD_ALIAS=</varname></term>
 126         <listitem><para>Adds an additional alias name to the device
 127         unit. This must be an absolute path that is automatically
 128         transformed into a unit name. (See above.)</para></listitem>
 129       </varlistentry>
 130
 131       <varlistentry>
 132         <term><varname>SYSTEMD_READY=</varname></term>
 133         <listitem><para>If set to 0, systemd will consider this device
 134         unplugged even if it shows up in the udev tree. If this
 135         property is unset or set to 1, the device will be considered
 136         plugged if it is visible in the udev tree. This property has
 137         no influence on the behavior when a device disappears from the
 138         udev tree.</para>
 139
 140         <para>This option is useful to support devices that initially
 141         show up in an uninitialized state in the tree, and for which a
 142         <literal>changed</literal> event is generated the moment they
 143         are fully set up. Note that <varname>SYSTEMD_WANTS=</varname>
 144         (see above) is not acted on as long as
 145         <varname>SYSTEMD_READY=0</varname> is set for a
 146         device.</para></listitem>
 147       </varlistentry>
 148
 149       <varlistentry>
 150         <term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
 151         <term><varname>ID_MODEL=</varname></term>
 152
 153         <listitem><para>If set, this property is used as description
 154         string for the device unit.</para></listitem>
 155       </varlistentry>
 156
 157     </variablelist>
 158
 159   </refsect1>
 160
 161   <refsect1>
 162       <title>See Also</title>
 163       <para>
 164         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 165         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 166         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 167         <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 168         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 169       </para>
 170   </refsect1>
 171
 172 </refentry>