man/systemd.device.xml

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<refentry id="systemd.device">
  <refentryinfo>
    <title>systemd.device</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </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.</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
    <literal>[Unit]</literal> and <literal>[Install]</literal>
    sections. A separate <literal>[Device]</literal> 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 "systemd" udev tag (by default
    all block and network devices, and a few others). This may be used
    to define dependencies between devices and other units. 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 are named after the <filename>/sys</filename>
    and <filename>/dev</filename> paths they control. Example: the
    device <filename noindex='true'>/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>
  </refsect1>

  <refsect1>
    <title>Automatic 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>
  </refsect1>

  <refsect1>
    <title>The udev Database</title>

    <para>The settings of device units may either be configured via
    unit files, or directly from the udev database (which is
    recommended). The following udev device properties are understood
    by systemd:</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 all listed
        units. The first form is used by the system systemd instance,
        the second by user systemd instances. Those settings may be
        used to activate arbitrary units when a specific device
        becomes available.</para>

        <para>Note that this and the other tags are not taken into
        account unless the device is tagged with the
        <literal>systemd</literal> string 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 influence on
        which udev event to trigger the dependencies.
        </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. This property has
        no influence on the behavior when a device disappears from the
        udev tree.</para>

        <para>This option is useful to support 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>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>