man/systemd.target.xml

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   9 <refentry id="systemd.target">
  10   <refentryinfo>
  11     <title>systemd.target</title>
  12     <productname>systemd</productname>
  13   </refentryinfo>
  14
  15   <refmeta>
  16     <refentrytitle>systemd.target</refentrytitle>
  17     <manvolnum>5</manvolnum>
  18   </refmeta>
  19
  20   <refnamediv>
  21     <refname>systemd.target</refname>
  22     <refpurpose>Target unit configuration</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <para><filename><replaceable>target</replaceable>.target</filename></para>
  27   </refsynopsisdiv>
  28
  29   <refsect1>
  30     <title>Description</title>
  31
  32     <para>A unit configuration file whose name ends in
  33     <literal>.target</literal> encodes information about a target unit
  34     of systemd, which is used for grouping units and as well-known
  35     synchronization points during start-up.</para>
  36
  37     <para>This unit type has no specific options. See
  38     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  39     for the common options of all unit configuration files. The common
  40     configuration items are configured in the generic [Unit] and
  41     [Install] sections. A separate [Target] section does not exist,
  42     since no target-specific options may be configured.</para>
  43
  44     <para>Target units do not offer any additional functionality on
  45     top of the generic functionality provided by units. They exist
  46     merely to group units via dependencies (useful as boot targets),
  47     and to establish standardized names for synchronization points
  48     used in dependencies between units. Among other things, target
  49     units are a more flexible replacement for SysV runlevels in the
  50     classic SysV init system. (And for compatibility reasons special
  51     target units such as <filename>runlevel3.target</filename> exist
  52     which are used by the SysV runlevel compatibility code in systemd.
  53     See
  54     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  55     for details).</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
  75         configured dependencies of type <varname>Wants=</varname> or
  76         <varname>Requires=</varname> with dependencies of type
  77         <varname>After=</varname> unless <varname>DefaultDependencies=no</varname>
  78         is set in the specified units. Note that <varname>Wants=</varname> or
  79         <varname>Requires=</varname> must be defined in the target unit itself — if
  80         you for example define <varname>Wants=</varname>some.target in
  81         some.service, the automatic ordering will not be added.</para></listitem>
  82
  83         <listitem><para>Target units automatically gain <varname>Conflicts=</varname>
  84         and <varname>Before=</varname> dependencies against
  85         <filename>shutdown.target</filename>.</para></listitem>
  86       </itemizedlist>
  87     </refsect2>
  88   </refsect1>
  89
  90   <refsect1>
  91     <title>Example</title>
  92
  93     <example>
  94       <title>Simple standalone target</title>
  95
  96       <programlisting># emergency-net.target
  97
  98 [Unit]
  99 Description=Emergency Mode with Networking
 100 Requires=emergency.target systemd-networkd.service
 101 After=emergency.target systemd-networkd.service
 102 AllowIsolate=yes</programlisting>
 103
 104       <para>When adding dependencies to other units, it's important to check if they set
 105       <varname>DefaultDependencies=</varname>. Service units, unless they set
 106       <varname>DefaultDependencies=no</varname>, automatically get a dependency on
 107       <filename>sysinit.target</filename>. In this case, both
 108       <filename>emergency.target</filename> and <filename>systemd-networkd.service</filename>
 109       have <varname>DefaultDependencies=no</varname>, so they are suitable for use
 110       in this target, and do not pull in <filename>sysinit.target</filename>.</para>
 111
 112       <para>You can now switch into this emergency mode by running <varname>systemctl
 113       isolate emergency-net.target</varname> or by passing the option
 114       <varname>systemd.unit=emergency-net.target</varname> on the kernel command
 115       line.</para>
 116
 117       <para>Other units can have <varname>WantedBy=emergency-net.target</varname> in the
 118       <varname>[Install]</varname> section. After they are enabled using
 119       <command>systemctl enable</command>, they will be started before
 120       <varname>emergency-net.target</varname> is started. It is also possible to add
 121       arbitrary units as dependencies of <filename>emergency.target</filename> without
 122       modifying them by using <command>systemctl add-wants</command>.
 123       </para>
 124     </example>
 125   </refsect1>
 126
 127   <refsect1>
 128       <title>See Also</title>
 129       <para>
 130         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 131         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 132         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 133         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 134         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 135       </para>
 136   </refsect1>
 137
 138 </refentry>