man/systemd.target.xml

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  23
  24 <refentry id="systemd.target">
  25   <refentryinfo>
  26     <title>systemd.target</title>
  27     <productname>systemd</productname>
  28
  29     <authorgroup>
  30       <author>
  31         <contrib>Developer</contrib>
  32         <firstname>Lennart</firstname>
  33         <surname>Poettering</surname>
  34         <email>lennart@poettering.net</email>
  35       </author>
  36     </authorgroup>
  37   </refentryinfo>
  38
  39   <refmeta>
  40     <refentrytitle>systemd.target</refentrytitle>
  41     <manvolnum>5</manvolnum>
  42   </refmeta>
  43
  44   <refnamediv>
  45     <refname>systemd.target</refname>
  46     <refpurpose>Target unit configuration</refpurpose>
  47   </refnamediv>
  48
  49   <refsynopsisdiv>
  50     <para><filename><replaceable>target</replaceable>.target</filename></para>
  51   </refsynopsisdiv>
  52
  53   <refsect1>
  54     <title>Description</title>
  55
  56     <para>A unit configuration file whose name ends in
  57     <literal>.target</literal> encodes information about a target unit
  58     of systemd, which is used for grouping units and as well-known
  59     synchronization points during start-up.</para>
  60
  61     <para>This unit type has no specific options. See
  62     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  63     for the common options of all unit configuration files. The common
  64     configuration items are configured in the generic [Unit] and
  65     [Install] sections. A separate [Target] section does not exist,
  66     since no target-specific options may be configured.</para>
  67
  68     <para>Target units do not offer any additional functionality on
  69     top of the generic functionality provided by units. They exist
  70     merely to group units via dependencies (useful as boot targets),
  71     and to establish standardized names for synchronization points
  72     used in dependencies between units. Among other things, target
  73     units are a more flexible replacement for SysV runlevels in the
  74     classic SysV init system. (And for compatibility reasons special
  75     target units such as <filename>runlevel3.target</filename> exist
  76     which are used by the SysV runlevel compatibility code in systemd.
  77     See
  78     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  79     for details).</para>
  80   </refsect1>
  81
  82   <refsect1>
  83     <title>Implicit Dependencies</title>
  84
  85     <para>There are no implicit dependencies for target units.</para>
  86   </refsect1>
  87
  88   <refsect1>
  89     <title>Default Dependencies</title>
  90
  91     <para>The following dependencies are added unless
  92     <varname>DefaultDependencies=no</varname> is set:</para>
  93
  94     <itemizedlist>
  95       <listitem><para>Target units will automatically complement all
  96       configured dependencies of type <varname>Wants=</varname> or
  97       <varname>Requires=</varname> with dependencies of type
  98       <varname>After=</varname>. Note that <varname>Wants=</varname> or
  99       <varname>Requires=</varname> must be defined in the target unit itself — if
 100       you for example define <varname>Wants=</varname>some.target in
 101       some.service, the automatic ordering will not be added.</para></listitem>
 102
 103       <listitem><para>Target units automatically gain <varname>Conflicts=</varname>
 104       dependency against <filename>shutdown.target</filename>.</para></listitem>
 105     </itemizedlist>
 106   </refsect1>
 107
 108   <refsect1>
 109     <title>Example</title>
 110
 111     <example>
 112       <title>Simple standalone target</title>
 113
 114       <programlisting># emergency-net.target
 115
 116 [Unit]
 117 Description=Emergency Mode with Networking
 118 Requires=emergency.target systemd-networkd.service
 119 After=emergency.target systemd-networkd.service
 120 AllowIsolate=yes</programlisting>
 121
 122       <para>When adding dependencies to other units, it's important to check if they set
 123       <varname>DefaultDependencies=</varname>. Service units, unless they set
 124       <varname>DefaultDependencies=no</varname>, automatically get a dependency on
 125       <filename>sysinit.target</filename>. In this case, both
 126       <filename>emergency.target</filename> and <filename>systemd-networkd.service</filename>
 127       have <varname>DefaultDependencies=no</varname>, so they are suitable for use
 128       in this target, and do not pull in <filename>sysinit.target</filename>.</para>
 129
 130       <para>You can now switch into this emergency mode by running <varname>systemctl
 131       isolate emergency-net.target</varname> or by passing the option
 132       <varname>systemd.unit=emergency-net.target</varname> on the kernel command
 133       line.</para>
 134
 135       <para>Other units can have <varname>WantedBy=emergency-net.target</varname> in the
 136       <varname>[Install]</varname> section. After they are enabled using
 137       <command>systemctl enable</command>, they will be started before
 138       <varname>emergency-net.target</varname> is started. It is also possible to add
 139       arbitrary units as dependencies of <filename>emergency.target</filename> without
 140       modifying them by using <command>systemctl add-wants</command>.
 141       </para>
 142     </example>
 143   </refsect1>
 144
 145   <refsect1>
 146       <title>See Also</title>
 147       <para>
 148         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 149         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 150         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 151         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 152         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 153       </para>
 154   </refsect1>
 155
 156 </refentry>