man/systemd.target.xml

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   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
  30     <literal>.target</literal> encodes information about a target unit
  31     of systemd, which is used for grouping units and as well-known
  32     synchronization points during start-up.</para>
  33
  34     <para>This unit type has no specific options. See
  35     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  36     for the common options of all unit configuration files. The common
  37     configuration items are configured in the generic <literal>[Unit]</literal> and
  38     <literal>[Install]</literal> sections. A separate <literal>[Target]</literal> section does not exist,
  39     since no target-specific options may be configured.</para>
  40
  41     <para>Target units do not offer any additional functionality on
  42     top of the generic functionality provided by units. They exist
  43     merely to group units via dependencies (useful as boot targets),
  44     and to establish standardized names for synchronization points
  45     used in dependencies between units. Among other things, target
  46     units are a more flexible replacement for SysV runlevels in the
  47     classic SysV init system. (And for compatibility reasons special
  48     target units such as <filename>runlevel3.target</filename> exist
  49     which are used by the SysV runlevel compatibility code in systemd.
  50     See
  51     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  52     for details).</para>
  53   </refsect1>
  54
  55   <refsect1>
  56     <title>Automatic Dependencies</title>
  57
  58     <refsect2>
  59       <title>Implicit Dependencies</title>
  60
  61       <para>There are no implicit dependencies for target units.</para>
  62     </refsect2>
  63
  64     <refsect2>
  65       <title>Default Dependencies</title>
  66
  67       <para>The following dependencies are added unless
  68       <varname>DefaultDependencies=no</varname> is set:</para>
  69
  70       <itemizedlist>
  71         <listitem><para>Target units will automatically complement all
  72         configured dependencies of type <varname>Wants=</varname> or
  73         <varname>Requires=</varname> with dependencies of type
  74         <varname>After=</varname> unless <varname>DefaultDependencies=no</varname>
  75         is set in the specified units. Note that <varname>Wants=</varname> or
  76         <varname>Requires=</varname> must be defined in the target unit itself — if
  77         you for example define <varname>Wants=</varname>some.target in
  78         some.service, the automatic ordering will not be added.</para></listitem>
  79
  80         <listitem><para>Target units automatically gain <varname>Conflicts=</varname>
  81         and <varname>Before=</varname> dependencies against
  82         <filename>shutdown.target</filename>.</para></listitem>
  83       </itemizedlist>
  84     </refsect2>
  85   </refsect1>
  86
  87   <refsect1>
  88     <title>Example</title>
  89
  90     <example>
  91       <title>Simple standalone target</title>
  92
  93       <programlisting># emergency-net.target
  94
  95 [Unit]
  96 Description=Emergency Mode with Networking
  97 Requires=emergency.target systemd-networkd.service
  98 After=emergency.target systemd-networkd.service
  99 AllowIsolate=yes</programlisting>
 100
 101       <para>When adding dependencies to other units, it's important to check if they set
 102       <varname>DefaultDependencies=</varname>. Service units, unless they set
 103       <varname>DefaultDependencies=no</varname>, automatically get a dependency on
 104       <filename>sysinit.target</filename>. In this case, both
 105       <filename>emergency.target</filename> and <filename>systemd-networkd.service</filename>
 106       have <varname>DefaultDependencies=no</varname>, so they are suitable for use
 107       in this target, and do not pull in <filename>sysinit.target</filename>.</para>
 108
 109       <para>You can now switch into this emergency mode by running <varname>systemctl
 110       isolate emergency-net.target</varname> or by passing the option
 111       <varname>systemd.unit=emergency-net.target</varname> on the kernel command
 112       line.</para>
 113
 114       <para>Other units can have <varname>WantedBy=emergency-net.target</varname> in the
 115       <varname>[Install]</varname> section. After they are enabled using
 116       <command>systemctl enable</command>, they will be started before
 117       <varname>emergency-net.target</varname> is started. It is also possible to add
 118       arbitrary units as dependencies of <filename>emergency.target</filename> without
 119       modifying them by using <command>systemctl add-wants</command>.
 120       </para>
 121     </example>
 122   </refsect1>
 123
 124   <refsect1>
 125       <title>See Also</title>
 126       <para>
 127         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 128         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 129         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 130         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 131         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 132       </para>
 133   </refsect1>
 134
 135 </refentry>