man/systemd.target.xml

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   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd.target">
   7   <refentryinfo>
   8     <title>systemd.target</title>
   9     <productname>systemd</productname>
  10   </refentryinfo>
  11
  12   <refmeta>
  13     <refentrytitle>systemd.target</refentrytitle>
  14     <manvolnum>5</manvolnum>
  15   </refmeta>
  16
  17   <refnamediv>
  18     <refname>systemd.target</refname>
  19     <refpurpose>Target unit configuration</refpurpose>
  20   </refnamediv>
  21
  22   <refsynopsisdiv>
  23     <para><filename><replaceable>target</replaceable>.target</filename></para>
  24   </refsynopsisdiv>
  25
  26   <refsect1>
  27     <title>Description</title>
  28
  29     <para>A unit configuration file whose name ends in <literal>.target</literal> encodes information about a
  30     target unit of systemd. Target units are used to group units and to set synchronization points for
  31     ordering dependencies with other unit files.</para>
  32
  33     <para>This unit type has no specific options. See
  34     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for the
  35     common options of all unit configuration files. The common configuration items are configured in the
  36     generic [Unit] and [Install] sections. A separate [Target] section does not exist, since no
  37     target-specific options may be configured.</para>
  38
  39     <para>Target units do not offer any additional functionality on top of the generic functionality provided
  40     by units. They merely group units, allowing a single target name to be used in <varname>Wants=</varname>
  41     and <varname>Requires=</varname> settings to establish a dependency on a set of units defined by the
  42     target, and in <varname>Before=</varname> and <varname>After=</varname> settings to establish ordering.
  43     Targets establish standardized names for synchronization points during boot and shutdown. Importantly,
  44     see <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  45     for examples and descriptions of standard systemd targets.</para>
  46
  47     <para>Target units provide a more flexible replacement for SysV runlevels in the classic SysV init
  48     system. For compatibility reasons special target units such as <filename>runlevel3.target</filename>
  49     exist which are used by the SysV runlevel compatibility code in systemd, see
  50     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
  51     details.</para>
  52
  53     <para>Note that a target unit file must not be empty, lest it be considered a masked unit. It is
  54     recommended to provide a [Unit] section which includes informative <varname>Description=</varname> and
  55     <varname>Documentation=</varname> options.</para>
  56   </refsect1>
  57
  58   <refsect1>
  59     <title>Automatic Dependencies</title>
  60
  61     <refsect2>
  62       <title>Implicit Dependencies</title>
  63
  64       <para>There are no implicit dependencies for target units.</para>
  65     </refsect2>
  66
  67     <refsect2>
  68       <title>Default Dependencies</title>
  69
  70       <para>The following dependencies are added unless
  71       <varname>DefaultDependencies=no</varname> is set:</para>
  72
  73       <itemizedlist>
  74         <listitem><para>Target units will automatically complement all configured dependencies of type
  75         <varname>Wants=</varname> or <varname>Requires=</varname> with dependencies of type
  76         <varname>After=</varname> unless <varname>DefaultDependencies=no</varname> is set in the specified
  77         units.</para>
  78
  79         <para>Note that the reverse is not true. For example, defining <option>Wants=that.target</option> in
  80         <filename index='false'>some.service</filename> will not automatically add the
  81         <option>After=that.target</option> ordering dependency for <filename>some.service</filename>.
  82         Instead, <filename>some.service</filename> should use the primary synchronization function of target
  83         type units, by setting a specific <option>After=that.target</option> or
  84         <option>Before=that.target</option> ordering dependency in its .service unit file.
  85         </para></listitem>
  86
  87         <listitem><para>Target units automatically gain <varname>Conflicts=</varname>
  88         and <varname>Before=</varname> dependencies against
  89         <filename>shutdown.target</filename>.</para></listitem>
  90       </itemizedlist>
  91     </refsect2>
  92   </refsect1>
  93
  94   <refsect1>
  95     <title>Options</title>
  96
  97     <para>Target unit files may include [Unit] and [Install] sections, which are described in
  98     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
  99     No options specific to this file type are supported.</para>
 100   </refsect1>
 101
 102   <refsect1>
 103     <title>Example</title>
 104
 105     <example>
 106       <title>Simple standalone target</title>
 107
 108       <programlisting># emergency-net.target
 109
 110 [Unit]
 111 Description=Emergency Mode with Networking
 112 Requires=emergency.target systemd-networkd.service
 113 After=emergency.target systemd-networkd.service
 114 AllowIsolate=yes</programlisting>
 115
 116       <para>When adding dependencies to other units, it's important to check if they set
 117       <varname>DefaultDependencies=</varname>. Service units, unless they set
 118       <varname>DefaultDependencies=no</varname>, automatically get a dependency on
 119       <filename>sysinit.target</filename>. In this case, both
 120       <filename>emergency.target</filename> and <filename>systemd-networkd.service</filename>
 121       have <varname>DefaultDependencies=no</varname>, so they are suitable for use
 122       in this target, and do not pull in <filename>sysinit.target</filename>.</para>
 123
 124       <para>You can now switch into this emergency mode by running <varname>systemctl
 125       isolate emergency-net.target</varname> or by passing the option
 126       <varname>systemd.unit=emergency-net.target</varname> on the kernel command
 127       line.</para>
 128
 129       <para>Other units can have <varname>WantedBy=emergency-net.target</varname> in the
 130       <varname>[Install]</varname> section. After they are enabled using
 131       <command>systemctl enable</command>, they will be started before
 132       <varname>emergency-net.target</varname> is started. It is also possible to add
 133       arbitrary units as dependencies of <filename>emergency.target</filename> without
 134       modifying them by using <command>systemctl add-wants</command>.
 135       </para>
 136     </example>
 137   </refsect1>
 138
 139   <refsect1>
 140       <title>See Also</title>
 141       <para>
 142         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 143         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 144         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 145         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 146         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 147       </para>
 148   </refsect1>
 149
 150 </refentry>