systemd/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>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>8</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>