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151 lines
7.0 KiB
XML
151 lines
7.0 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/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd.target">
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<refentryinfo>
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<title>systemd.target</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.target</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.target</refname>
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<refpurpose>Target unit configuration</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename><replaceable>target</replaceable>.target</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>.target</literal> encodes information about a
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target unit of systemd. Target units are used to group units and to set synchronization points for
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ordering dependencies with other unit files.</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> for the
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common options of all unit configuration files. The common configuration items are configured in the
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generic [Unit] and [Install] sections. A separate [Target] section does not exist, since no
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target-specific options may be configured.</para>
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<para>Target units do not offer any additional functionality on top of the generic functionality provided
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by units. They merely group units, allowing a single target name to be used in <varname>Wants=</varname>
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and <varname>Requires=</varname> settings to establish a dependency on a set of units defined by the
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target, and in <varname>Before=</varname> and <varname>After=</varname> settings to establish ordering.
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Targets establish standardized names for synchronization points during boot and shutdown. Importantly,
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see <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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for examples and descriptions of standard systemd targets.</para>
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<para>Target units provide a more flexible replacement for SysV runlevels in the classic SysV init
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system. For compatibility reasons special target units such as <filename>runlevel3.target</filename>
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exist which are used by the SysV runlevel compatibility code in systemd, see
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
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details.</para>
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<para>Note that a target unit file must not be empty, lest it be considered a masked unit. It is
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recommended to provide a [Unit] section which includes informative <varname>Description=</varname> and
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<varname>Documentation=</varname> options.</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>There are no implicit dependencies for target units.</para>
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</refsect2>
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<refsect2>
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<title>Default Dependencies</title>
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<para>The following dependencies are added unless
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<varname>DefaultDependencies=no</varname> is set:</para>
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<itemizedlist>
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<listitem><para>Target units will automatically complement all configured dependencies of type
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<varname>Wants=</varname> or <varname>Requires=</varname> with dependencies of type
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<varname>After=</varname> unless <varname>DefaultDependencies=no</varname> is set in the specified
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units.</para>
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<para>Note that the reverse is not true. For example, defining <option>Wants=that.target</option> in
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<filename index='false'>some.service</filename> will not automatically add the
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<option>After=that.target</option> ordering dependency for <filename>some.service</filename>.
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Instead, <filename>some.service</filename> should use the primary synchronization function of target
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type units, by setting a specific <option>After=that.target</option> or
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<option>Before=that.target</option> ordering dependency in its .service unit file.
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</para></listitem>
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<listitem><para>Target units automatically gain <varname>Conflicts=</varname>
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and <varname>Before=</varname> dependencies against
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<filename>shutdown.target</filename>.</para></listitem>
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</itemizedlist>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<para>Target 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>.
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No options specific to this file type are supported.</para>
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</refsect1>
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<refsect1>
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<title>Example</title>
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<example>
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<title>Simple standalone target</title>
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<programlisting># emergency-net.target
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[Unit]
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Description=Emergency Mode with Networking
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Requires=emergency.target systemd-networkd.service
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After=emergency.target systemd-networkd.service
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AllowIsolate=yes</programlisting>
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<para>When adding dependencies to other units, it's important to check if they set
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<varname>DefaultDependencies=</varname>. Service units, unless they set
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<varname>DefaultDependencies=no</varname>, automatically get a dependency on
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<filename>sysinit.target</filename>. In this case, both
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<filename>emergency.target</filename> and <filename>systemd-networkd.service</filename>
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have <varname>DefaultDependencies=no</varname>, so they are suitable for use
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in this target, and do not pull in <filename>sysinit.target</filename>.</para>
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<para>You can now switch into this emergency mode by running <varname>systemctl
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isolate emergency-net.target</varname> or by passing the option
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<varname>systemd.unit=emergency-net.target</varname> on the kernel command
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line.</para>
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<para>Other units can have <varname>WantedBy=emergency-net.target</varname> in the
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<varname>[Install]</varname> section. After they are enabled using
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<command>systemctl enable</command>, they will be started before
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<varname>emergency-net.target</varname> is started. It is also possible to add
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arbitrary units as dependencies of <filename>emergency.target</filename> without
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modifying them by using <command>systemctl add-wants</command>.
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</para>
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</example>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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