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fab79a85af
systemd-udevd.service listens to kernel uevents and is needed for device units to be available. systemd-udevd.service is misspelled as systemd-udev.service in a couple places. Fixing typo.
172 lines
8.1 KiB
XML
172 lines
8.1 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd.device">
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<refentryinfo>
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<title>systemd.device</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.device</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.device</refname>
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<refpurpose>Device unit configuration</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename><replaceable>device</replaceable>.device</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>A unit configuration file whose name ends in <literal>.device</literal> encodes information about a
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device unit as exposed in the
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sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> device
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tree. This may be used to define dependencies between devices and other units.</para>
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<para>This unit type has no specific options. See
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for the common options of all unit configuration files. The common
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configuration items are configured in the generic
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[Unit] and [Install]
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sections. A separate [Device] section does not
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exist, since no device-specific options may be configured.</para>
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<para>systemd will dynamically create device units for all kernel devices that are marked with the
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<literal>systemd</literal> udev tag (by default all block and network devices, and a few others). Note
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that <emphasis>if <filename>systemd-udevd.service</filename> is not running, no device units will be
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available (for example in a typical container)</emphasis>.</para>
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<para>Device units are named after the <filename>/sys/</filename>
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and <filename>/dev/</filename> paths they control. Example: the
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device <filename index="false">/dev/sda5</filename> is exposed in
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systemd as <filename>dev-sda5.device</filename>. For details about
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the escaping logic used to convert a file system path to a unit
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name see
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
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<para>To tag a udev device, use <literal>TAG+="systemd"</literal> in the udev rules file, see
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<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details.
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</para>
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<para>Device units will be reloaded by systemd whenever the
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corresponding device generates a <literal>changed</literal> event.
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Other units can use <varname>ReloadPropagatedFrom=</varname> to react
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to that event.</para>
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</refsect1>
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<refsect1>
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<title>Automatic Dependencies</title>
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<refsect2>
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<title>Implicit Dependencies</title>
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<para>Many unit types automatically acquire dependencies on device
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units of devices they require. For example,
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<filename>.socket</filename> unit acquire dependencies on the
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device units of the network interface specified in
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<varname>BindToDevice=</varname>. Similar, swap and mount units
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acquire dependencies on the units encapsulating their backing
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block devices.</para>
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</refsect2>
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<refsect2>
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<title>Default Dependencies</title>
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<para>There are no default dependencies for device units.</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>The udev Database</title>
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<para>Unit settings of device units may either be configured via unit files, or directly from the udev
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database. The following udev device properties are understood by the service manager:</para>
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<variablelist class='udev-directives'>
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<varlistentry>
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<term><varname>SYSTEMD_WANTS=</varname></term>
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<term><varname>SYSTEMD_USER_WANTS=</varname></term>
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<listitem><para>Adds dependencies of type <varname>Wants=</varname> from the device unit to the specified
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units. <varname>SYSTEMD_WANTS=</varname> is read by the system service manager,
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<varname>SYSTEMD_USER_WANTS=</varname> by user service manager instances. These properties may be used to
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activate arbitrary units when a specific device becomes available.</para>
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<para>Note that this and the other udev device properties are not taken into account unless the device is
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tagged with the <literal>systemd</literal> tag in the udev database, because otherwise the device is not
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exposed as a systemd unit (see above).</para>
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<para>Note that systemd will only act on <varname>Wants=</varname> dependencies when a device first becomes
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active. It will not act on them if they are added to devices that are already active. Use
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<varname>SYSTEMD_READY=</varname> (see below) to configure when a udev device shall be considered active, and
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thus when to trigger the dependencies.</para>
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<!-- Note that we don't document here that we actually apply unit_name_mangle() to all specified names, since
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that's kinda ugly, and people should instead specify correctly escaped names -->
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<para>The specified property value should be a space-separated list of valid unit names. If a unit template
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name is specified (that is, a unit name containing an <literal>@</literal> character indicating a unit name to
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use for multiple instantiation, but with an empty instance name following the <literal>@</literal>), it will be
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automatically instantiated by the device's <literal>sysfs</literal> path (that is: the path is escaped and
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inserted as instance name into the template unit name). This is useful in order to instantiate a specific
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template unit once for each device that appears and matches specific properties.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>SYSTEMD_ALIAS=</varname></term>
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<listitem><para>Adds an additional alias name to the device
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unit. This must be an absolute path that is automatically
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transformed into a unit name. (See above.)</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>SYSTEMD_READY=</varname></term>
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<listitem><para>If set to 0, systemd will consider this device unplugged even if it shows up in the udev
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tree. If this property is unset or set to 1, the device will be considered plugged if it is visible in the udev
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tree.</para>
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<para>This option is useful for devices that initially show up in an uninitialized state in the tree, and for
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which a <literal>changed</literal> event is generated the moment they are fully set up. Note that
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<varname>SYSTEMD_WANTS=</varname> (see above) is not acted on as long as <varname>SYSTEMD_READY=0</varname> is
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set for a device.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
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<term><varname>ID_MODEL=</varname></term>
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<listitem><para>If set, this property is used as description
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string for the device unit.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<para>Device unit files may include [Unit] and [Install] sections, which are described in
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. No
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options specific to this file type are supported.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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