man/systemd.device.xml

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  23
  24 <refentry id="systemd.device">
  25   <refentryinfo>
  26     <title>systemd.device</title>
  27     <productname>systemd</productname>
  28
  29     <authorgroup>
  30       <author>
  31         <contrib>Developer</contrib>
  32         <firstname>Lennart</firstname>
  33         <surname>Poettering</surname>
  34         <email>lennart@poettering.net</email>
  35       </author>
  36     </authorgroup>
  37   </refentryinfo>
  38
  39   <refmeta>
  40     <refentrytitle>systemd.device</refentrytitle>
  41     <manvolnum>5</manvolnum>
  42   </refmeta>
  43
  44   <refnamediv>
  45     <refname>systemd.device</refname>
  46     <refpurpose>Device unit configuration</refpurpose>
  47   </refnamediv>
  48
  49   <refsynopsisdiv>
  50     <para><filename><replaceable>device</replaceable>.device</filename></para>
  51   </refsynopsisdiv>
  52
  53   <refsect1>
  54     <title>Description</title>
  55
  56     <para>A unit configuration file whose name ends in
  57     <literal>.device</literal> encodes information about a device unit
  58     as exposed in the
  59     sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  60     device tree.</para>
  61
  62     <para>This unit type has no specific options. See
  63     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  64     for the common options of all unit configuration files. The common
  65     configuration items are configured in the generic
  66     <literal>[Unit]</literal> and <literal>[Install]</literal>
  67     sections. A separate <literal>[Device]</literal> section does not
  68     exist, since no device-specific options may be configured.</para>
  69
  70     <para>systemd will dynamically create device units for all kernel
  71     devices that are marked with the "systemd" udev tag (by default
  72     all block and network devices, and a few others). This may be used
  73     to define dependencies between devices and other units. To tag a
  74     udev device, use <literal>TAG+="systemd"</literal> in the udev
  75     rules file, see
  76     <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  77     for details.</para>
  78
  79     <para>Device units are named after the <filename>/sys</filename>
  80     and <filename>/dev</filename> paths they control. Example: the
  81     device <filename noindex='true'>/dev/sda5</filename> is exposed in
  82     systemd as <filename>dev-sda5.device</filename>. For details about
  83     the escaping logic used to convert a file system path to a unit
  84     name see
  85     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
  86
  87     <para>Device units will be reloaded by systemd whenever the
  88     corresponding device generates a <literal>changed</literal> event.
  89     Other units can use <varname>ReloadPropagatedFrom=</varname> to react
  90     to that event</para>
  91
  92   </refsect1>
  93
  94   <refsect1>
  95     <title>Implicit Dependencies</title>
  96
  97     <para>Many unit types automatically acquire dependencies on device
  98     units of devices they require. For example,
  99     <filename>.socket</filename> unit acquire dependencies on the
 100     device units of the network interface specified in
 101     <varname>BindToDevice=</varname>. Similar, swap and mount units
 102     acquire dependencies on the units encapsulating their backing
 103     block devices.</para>
 104   </refsect1>
 105
 106   <refsect1>
 107     <title>Default Dependencies</title>
 108
 109     <para>There are no default dependencies for device units.</para>
 110   </refsect1>
 111
 112   <refsect1>
 113     <title>The udev Database</title>
 114
 115     <para>Unit settings of device units may either be configured via unit files, or directly from the udev
 116     database. The following udev device properties are understood by the service manager:</para>
 117
 118     <variablelist class='udev-directives'>
 119       <varlistentry>
 120         <term><varname>SYSTEMD_WANTS=</varname></term>
 121         <term><varname>SYSTEMD_USER_WANTS=</varname></term>
 122         <listitem><para>Adds dependencies of type <varname>Wants=</varname> from the device unit to the specified
 123         units. <varname>SYSTEMD_WANTS=</varname> is read by the system service manager,
 124         <varname>SYSTEMD_USER_WANTS=</varname> by user service manager instances. These properties may be used to
 125         activate arbitrary units when a specific device becomes available.</para>
 126
 127         <para>Note that this and the other udev device properties are not taken into account unless the device is
 128         tagged with the <literal>systemd</literal> tag in the udev database, because otherwise the device is not
 129         exposed as a systemd unit (see above).</para>
 130
 131         <para>Note that systemd will only act on <varname>Wants=</varname> dependencies when a device first becomes
 132         active. It will not act on them if they are added to devices that are already active. Use
 133         <varname>SYSTEMD_READY=</varname> (see below) to configure when a udev device shall be considered active, and
 134         thus when to trigger the dependencies.</para>
 135
 136         <!-- Note that we don't document here that we actually apply unit_name_mangle() to all specified names, since
 137              that's kinda ugly, and people should instead specify correctly escaped names -->
 138
 139         <para>The specified property value should be a space-separated list of valid unit names. If a unit template
 140         name is specified (that is, a unit name containing an <literal>@</literal> character indicating a unit name to
 141         use for multiple instantiation, but with an empty instance name following the <literal>@</literal>), it will be
 142         automatically instantiated by the device's <literal>sysfs</literal> path (that is: the path is escaped and
 143         inserted as instance name into the template unit name). This is useful in order to instantiate a specific
 144         template unit once for each device that appears and matches specific properties.</para></listitem>
 145       </varlistentry>
 146
 147       <varlistentry>
 148         <term><varname>SYSTEMD_ALIAS=</varname></term>
 149         <listitem><para>Adds an additional alias name to the device
 150         unit. This must be an absolute path that is automatically
 151         transformed into a unit name. (See above.)</para></listitem>
 152       </varlistentry>
 153
 154       <varlistentry>
 155         <term><varname>SYSTEMD_READY=</varname></term>
 156         <listitem><para>If set to 0, systemd will consider this device unplugged even if it shows up in the udev
 157         tree. If this property is unset or set to 1, the device will be considered plugged if it is visible in the udev
 158         tree.</para>
 159
 160         <para>This option is useful for devices that initially show up in an uninitialized state in the tree, and for
 161         which a <literal>changed</literal> event is generated the moment they are fully set up. Note that
 162         <varname>SYSTEMD_WANTS=</varname> (see above) is not acted on as long as <varname>SYSTEMD_READY=0</varname> is
 163         set for a device.</para></listitem>
 164       </varlistentry>
 165
 166       <varlistentry>
 167         <term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
 168         <term><varname>ID_MODEL=</varname></term>
 169
 170         <listitem><para>If set, this property is used as description
 171         string for the device unit.</para></listitem>
 172       </varlistentry>
 173
 174     </variablelist>
 175
 176   </refsect1>
 177
 178   <refsect1>
 179       <title>See Also</title>
 180       <para>
 181         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 182         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 183         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 184         <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 185         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 186       </para>
 187   </refsect1>
 188
 189 </refentry>