[thirdparty/systemd.git] / man / systemd.special.xml

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<refentry id="systemd.special">

  <refentryinfo>
    <title>systemd.special</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.special</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.special</refname>
    <refpurpose>Special systemd units</refpurpose>
  </refnamediv>

  <refsynopsisdiv><para>
    <!-- sort alphabetically, targets first --><filename>basic.target</filename>,
    <filename>bluetooth.target</filename>,
    <filename>cryptsetup-pre.target</filename>,
    <filename>cryptsetup.target</filename>,
    <filename>ctrl-alt-del.target</filename>,
    <filename>default.target</filename>,
    <filename>emergency.target</filename>,
    <filename>exit.target</filename>,
    <filename>final.target</filename>,
    <filename>getty.target</filename>,
    <filename>getty-pre.target</filename>,
    <filename>graphical.target</filename>,
    <filename>halt.target</filename>,
    <filename>hibernate.target</filename>,
    <filename>hybrid-sleep.target</filename>,
    <filename>suspend-then-hibernate.target</filename>,
    <filename>initrd-fs.target</filename>,
    <filename>initrd-root-device.target</filename>,
    <filename>initrd-root-fs.target</filename>,
    <filename>kbrequest.target</filename>,
    <filename>kexec.target</filename>,
    <filename>local-fs-pre.target</filename>,
    <filename>local-fs.target</filename>,
    <filename>machines.target</filename>
    <filename>multi-user.target</filename>,
    <filename>network-online.target</filename>,
    <filename>network-pre.target</filename>,
    <filename>network.target</filename>,
    <filename>nss-lookup.target</filename>,
    <filename>nss-user-lookup.target</filename>,
    <filename>paths.target</filename>,
    <filename>poweroff.target</filename>,
    <filename>printer.target</filename>,
    <filename>reboot.target</filename>,
    <filename>remote-cryptsetup.target</filename>,
    <filename>remote-fs-pre.target</filename>,
    <filename>remote-fs.target</filename>,
    <filename>rescue.target</filename>,
    <filename>rpcbind.target</filename>,
    <filename>runlevel2.target</filename>,
    <filename>runlevel3.target</filename>,
    <filename>runlevel4.target</filename>,
    <filename>runlevel5.target</filename>,
    <filename>shutdown.target</filename>,
    <filename>sigpwr.target</filename>,
    <filename>sleep.target</filename>,
    <filename>slices.target</filename>,
    <filename>smartcard.target</filename>,
    <filename>sockets.target</filename>,
    <filename>sound.target</filename>,
    <filename>suspend.target</filename>,
    <filename>swap.target</filename>,
    <filename>sysinit.target</filename>,
    <filename>system-update.target</filename>,
    <filename>time-sync.target</filename>,
    <filename>timers.target</filename>,
    <filename>umount.target</filename>,
    <!-- slices --><filename>-.slice</filename>,
    <filename>system.slice</filename>,
    <filename>user.slice</filename>,
    <filename>machine.slice</filename>,
    <!-- the rest --><filename>-.mount</filename>,
    <filename>dbus.service</filename>,
    <filename>dbus.socket</filename>,
    <filename>display-manager.service</filename>,
    <filename>init.scope</filename>,
    <filename>syslog.socket</filename>,
    <filename>system-update-cleanup.service</filename>
  </para></refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>A few units are treated specially by systemd. Many of them have
    special internal semantics and cannot be renamed, while others simply
    have a standard meaning and should be present on all systems.</para>
  </refsect1>

  <refsect1>
    <title>Special System Units</title>

    <variablelist>
      <varlistentry>
        <term><filename>-.mount</filename></term>
        <listitem>
          <para>The root mount point, i.e. the mount unit for the <filename>/</filename> path. This unit is
          unconditionally active, during the entire time the system is up, as this mount point is where the basic
          userspace is running from.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>basic.target</filename></term>
        <listitem>
          <para>A special target unit covering basic boot-up.</para>

          <para>systemd automatically adds dependency of the type
          <varname>After=</varname> for this target unit to all
          services (except for those with
          <varname>DefaultDependencies=no</varname>).</para>

          <para>Usually, this should pull-in all local mount points plus
          <filename>/var</filename>, <filename>/tmp</filename> and
          <filename>/var/tmp</filename>, swap devices, sockets, timers,
          path units and other basic initialization necessary for general
          purpose daemons. The mentioned mount points are special cased
          to allow them to be remote.
          </para>

          <para>This target usually does not pull in any non-target units
          directly, but rather does so indirectly via other early boot targets.
          It is instead meant as a synchronization point for late boot
          services. Refer to
          <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
          for details on the targets involved.
          </para>

        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>ctrl-alt-del.target</filename></term>
        <listitem>
          <para>systemd starts this target whenever Control+Alt+Del is
          pressed on the console. Usually, this should be aliased
          (symlinked) to <filename>reboot.target</filename>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>cryptsetup.target</filename></term>
        <listitem>
          <para>A target that pulls in setup services for all
          encrypted block devices.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>dbus.service</filename></term>
        <listitem>
          <para>A special unit for the D-Bus bus daemon. As soon as
          this service is fully started up systemd will connect to it
          and register its service.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>dbus.socket</filename></term>
        <listitem>
          <para>A special unit for the D-Bus system bus socket. All
          units with <varname>Type=dbus</varname> automatically gain a
          dependency on this unit.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>default.target</filename></term>
        <listitem>
          <para>The default unit systemd starts at bootup. Usually,
          this should be aliased (symlinked) to
          <filename>multi-user.target</filename> or
          <filename>graphical.target</filename>.</para>

          <para>The default unit systemd starts at bootup can be
          overridden with the <varname>systemd.unit=</varname> kernel
          command line option.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>display-manager.service</filename></term>
        <listitem>
          <para>The display manager service. Usually, this should be
          aliased (symlinked) to <filename>gdm.service</filename> or a
          similar display manager service.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>emergency.target</filename></term>
        <listitem>
          <para>A special target unit that starts an emergency shell on the main console. This target does not pull in
          any services or mounts. It is the most minimal version of starting the system in order to acquire an
          interactive shell; the only processes running are usually just the system manager (PID 1) and the shell
          process. This unit is supposed to be used with the kernel command line option
          <varname>systemd.unit=</varname>; it is also used when a file system check on a required file system fails,
          and boot-up cannot continue. Compare with <filename>rescue.target</filename>, which serves a similar purpose,
          but also starts the most basic services and mounts all file systems.</para>

          <para>Use the <literal>systemd.unit=emergency.target</literal> kernel command line option to boot into this
          mode. A short alias for this kernel command line option is <literal>emergency</literal>, for compatibility
          with SysV.</para>

          <para>In many ways booting into <filename>emergency.target</filename> is similar to the effect of booting
          with <literal>init=/bin/sh</literal> on the kernel command line, except that emergency mode provides you with
          the full system and service manager, and allows starting individual units in order to continue the boot
          process in steps.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>exit.target</filename></term>
        <listitem>
          <para>A special service unit for shutting down the system or
          user service manager. It is equivalent to
          <filename>poweroff.target</filename> on non-container
          systems, and also works in containers.</para>

          <para>systemd will start this unit when it receives the
          <constant>SIGTERM</constant> or <constant>SIGINT</constant>
          signal when running as user service daemon.</para>

          <para>Normally, this (indirectly) pulls in
          <filename>shutdown.target</filename>, which in turn should be
          conflicted by all units that want to be scheduled for
          shutdown when the service manager starts to exit.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>final.target</filename></term>
        <listitem>
          <para>A special target unit that is used during the shutdown
          logic and may be used to pull in late services after all
          normal services are already terminated and all mounts
          unmounted.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>getty.target</filename></term>
        <listitem>
          <para>A special target unit that pulls in statically
          configured local TTY <filename>getty</filename> instances.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>graphical.target</filename></term>
        <listitem>
          <para>A special target unit for setting up a graphical login
          screen. This pulls in
          <filename>multi-user.target</filename>.</para>

          <para>Units that are needed for graphical logins shall add
          <varname>Wants=</varname> dependencies for their unit to
          this unit (or <filename>multi-user.target</filename>) during
          installation. This is best configured via
          <varname>WantedBy=graphical.target</varname> in the unit's
          <literal>[Install]</literal> section.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>hibernate.target</filename></term>
        <listitem>
          <para>A special target unit for hibernating the system. This
          pulls in <filename>sleep.target</filename>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>hybrid-sleep.target</filename></term>
        <listitem>
          <para>A special target unit for hibernating and suspending
          the system at the same time. This pulls in
          <filename>sleep.target</filename>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>suspend-then-hibernate.target</filename></term>
        <listitem>
          <para>A special target unit for suspending the system for a period
          of time, waking it and putting it into hibernate. This pulls in
          <filename>sleep.target</filename>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>halt.target</filename></term>
        <listitem>
          <para>A special target unit for shutting down and halting
          the system. Note that this target is distinct from
          <filename>poweroff.target</filename> in that it generally
          really just halts the system rather than powering it
          down.</para>

          <para>Applications wanting to halt the system should not start this unit
          directly, but should instead execute <command>systemctl halt</command>
          (possibly with the <option>--no-block</option> option) or call
          <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
          <command>org.freedesktop.systemd1.Manager.Halt</command> D-Bus method
          directly.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>init.scope</filename></term>
        <listitem>
          <para>This scope unit is where the system and service manager (PID 1) itself resides. It is active as long as
          the system is running.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>initrd-fs.target</filename></term>
        <listitem>
          <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          automatically adds dependencies of type
          <varname>Before=</varname> to
          <filename>sysroot-usr.mount</filename> and all mount points
          found in <filename>/etc/fstab</filename> that have
          <option>x-initrd.mount</option> and not have
          <option>noauto</option> mount options set.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>initrd-root-device.target</filename></term>
        <listitem>
          <para>A special initrd target unit that is reached when the root filesystem device is available, but before
          it has been mounted.
          <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          and
          <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          automatically setup the appropriate dependencies to make this happen.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>initrd-root-fs.target</filename></term>
        <listitem>
          <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          automatically adds dependencies of type
          <varname>Before=</varname> to the
          <filename>sysroot.mount</filename> unit, which is generated
          from the kernel command line.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>kbrequest.target</filename></term>
        <listitem>
          <para>systemd starts this target whenever Alt+ArrowUp is
          pressed on the console. Note that any user with physical access
          to the machine will be able to do this, without authentication,
          so this should be used carefully.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>kexec.target</filename></term>
        <listitem>
          <para>A special target unit for shutting down and rebooting
          the system via kexec.</para>

          <para>Applications wanting to reboot the system should not start this unit
          directly, but should instead execute <command>systemctl kexec</command>
          (possibly with the <option>--no-block</option> option) or call
          <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
          <command>org.freedesktop.systemd1.Manager.KExec</command> D-Bus method
          directly.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>local-fs.target</filename></term>
        <listitem>
          <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          automatically adds dependencies of type
          <varname>Before=</varname> to all mount units that refer to
          local mount points for this target unit. In addition, it
          adds dependencies of type <varname>Wants=</varname> to this
          target unit for those mounts listed in
          <filename>/etc/fstab</filename> that have the
          <option>auto</option> mount option set.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>machines.target</filename></term>
        <listitem>
          <para>A standard target unit for starting all the containers
          and other virtual machines. See <filename>systemd-nspawn@.service</filename>
          for an example.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>multi-user.target</filename></term>
        <listitem>
          <para>A special target unit for setting up a multi-user
          system (non-graphical). This is pulled in by
          <filename>graphical.target</filename>.</para>

          <para>Units that are needed for a multi-user system shall
          add <varname>Wants=</varname> dependencies for their unit to
          this unit during installation. This is best configured via
          <varname>WantedBy=multi-user.target</varname> in the unit's
          <literal>[Install]</literal> section.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>network-online.target</filename></term>
        <listitem>
          <para>Units that strictly require a configured network
          connection should pull in
          <filename>network-online.target</filename> (via a
          <varname>Wants=</varname> type dependency) and order
          themselves after it. This target unit is intended to pull in
          a service that delays further execution until the network is
          sufficiently set up. What precisely this requires is left to
          the implementation of the network managing service.</para>

          <para>Note the distinction between this unit and
          <filename>network.target</filename>. This unit is an active
          unit (i.e. pulled in by the consumer rather than the
          provider of this functionality) and pulls in a service which
          possibly adds substantial delays to further execution. In
          contrast, <filename>network.target</filename> is a passive
          unit (i.e. pulled in by the provider of the functionality,
          rather than the consumer) that usually does not delay
          execution much. Usually, <filename>network.target</filename>
          is part of the boot of most systems, while
          <filename>network-online.target</filename> is not, except
          when at least one unit requires it. Also see <ulink
          url="https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running
          Services After the Network is up</ulink> for more
          information.</para>

          <para>All mount units for remote network file systems
          automatically pull in this unit, and order themselves after
          it. Note that networking daemons that simply provide
          functionality to other hosts generally do not need to pull
          this in.</para>

          <para>Note that this unit is only useful during the original system start-up logic. After the system has
          completed booting up, it will not track the online state of the system anymore. Due to this it cannot be used
          as a network connection monitor concept, it is purely a one-time system start-up concept.</para>
          </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>paths.target</filename></term>
        <listitem>
          <para>A special target unit that sets up all path units (see
          <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details) that shall be active after boot.</para>

          <para>It is recommended that path units installed by
          applications get pulled in via <varname>Wants=</varname>
          dependencies from this unit. This is best configured via a
          <varname>WantedBy=paths.target</varname> in the path unit's
          <literal>[Install]</literal> section.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>poweroff.target</filename></term>
        <listitem>
          <para>A special target unit for shutting down and powering
          off the system.</para>

          <para>Applications wanting to power off the system should not start this unit
          directly, but should instead execute <command>systemctl poweroff</command>
          (possibly with the <option>--no-block</option> option) or call
          <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
          <command>org.freedesktop.login1.Manager.PowerOff</command> D-Bus method
          directly.</para>

          <para><filename>runlevel0.target</filename> is an alias for
          this target unit, for compatibility with SysV.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>reboot.target</filename></term>
        <listitem>
          <para>A special target unit for shutting down and rebooting
          the system.</para>

          <para>Applications wanting to reboot the system should not start this unit
          directly, but should instead execute <command>systemctl reboot</command>
          (possibly with the <option>--no-block</option> option) or call
          <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
          <command>org.freedesktop.login1.Manager.Reboot</command> D-Bus method
          directly.</para>

          <para><filename>runlevel6.target</filename> is an alias for
          this target unit, for compatibility with SysV.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>remote-cryptsetup.target</filename></term>
        <listitem>
          <para>Similar to <filename>cryptsetup.target</filename>, but for encrypted
          devices which are accessed over the network. It is used for
          <citerefentry><refentrytitle>crypttab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
          entries marked with <option>_netdev</option>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>remote-fs.target</filename></term>
        <listitem>
          <para>Similar to <filename>local-fs.target</filename>, but
          for remote mount points.</para>

          <para>systemd automatically adds dependencies of type
          <varname>After=</varname> for this target unit to all SysV
          init script service units with an LSB header referring to
          the <literal>$remote_fs</literal> facility.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>rescue.target</filename></term>
        <listitem>
          <para>A special target unit that pulls in the base system (including system mounts) and spawns a rescue
          shell. Isolate to this target in order to administer the system in single-user mode with all file systems
          mounted but with no services running, except for the most basic. Compare with
          <filename>emergency.target</filename>, which is much more reduced and does not provide the file systems or
          most basic services. Compare with <filename>multi-user.target</filename>, this target could be seen as
          <filename>single-user.target</filename>.</para>

          <para><filename>runlevel1.target</filename> is an alias for this target unit, for compatibility with
          SysV.</para>

          <para>Use the <literal>systemd.unit=rescue.target</literal> kernel command line option to boot into this
          mode. A short alias for this kernel command line option is <literal>1</literal>, for compatibility with
          SysV.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>runlevel2.target</filename></term>
        <term><filename>runlevel3.target</filename></term>
        <term><filename>runlevel4.target</filename></term>
        <term><filename>runlevel5.target</filename></term>
        <listitem>
          <para>These are targets that are called whenever the SysV
          compatibility code asks for runlevel 2, 3, 4, 5,
          respectively. It is a good idea to make this an alias for
          (i.e. symlink to) <filename>graphical.target</filename>
          (for runlevel 5) or <filename>multi-user.target</filename>
          (the others).</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>shutdown.target</filename></term>
        <listitem>
          <para>A special target unit that terminates the services on
          system shutdown.</para>

          <para>Services that shall be terminated on system shutdown
          shall add <varname>Conflicts=</varname> and
          <varname>Before=</varname> dependencies to this unit for
          their service unit, which is implicitly done when
          <varname>DefaultDependencies=yes</varname> is set (the
          default).</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>sigpwr.target</filename></term>
        <listitem>
          <para>A special target that is started when systemd receives
          the SIGPWR process signal, which is normally sent by the
          kernel or UPS daemons when power fails.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>sleep.target</filename></term>
        <listitem>
          <para>A special target unit that is pulled in by
          <filename>suspend.target</filename>,
          <filename>hibernate.target</filename> and
          <filename>hybrid-sleep.target</filename> and may be used to
          hook units into the sleep state logic.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>slices.target</filename></term>
        <listitem>
          <para>A special target unit that sets up all slice units (see
          <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
          details) that shall be active after boot. By default the generic <filename>system.slice</filename>
          slice unit, as well as the root slice unit <filename>-.slice</filename>, is pulled in and ordered before
          this unit (see below).</para>

          <para>It's a good idea to add <varname>WantedBy=slices.target</varname> lines to the <literal>[Install]</literal>
          section of all slices units that may be installed dynamically.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>sockets.target</filename></term>
        <listitem>
          <para>A special target unit that sets up all socket
          units (see
          <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details) that shall be active after boot.</para>

          <para>Services that can be socket-activated shall add
          <varname>Wants=</varname> dependencies to this unit for
          their socket unit during installation. This is best
          configured via a <varname>WantedBy=sockets.target</varname>
          in the socket unit's <literal>[Install]</literal>
          section.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>suspend.target</filename></term>
        <listitem>
          <para>A special target unit for suspending the system. This
          pulls in <filename>sleep.target</filename>.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>swap.target</filename></term>
        <listitem>
          <para>Similar to <filename>local-fs.target</filename>, but
          for swap partitions and swap files.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>sysinit.target</filename></term>
        <listitem>
          <para>systemd automatically adds dependencies of the types
          <varname>Requires=</varname> and <varname>After=</varname>
          for this target unit to all services (except for those with
          <varname>DefaultDependencies=no</varname>).</para>

          <para>This target pulls in the services required for system
          initialization. System services pulled in by this target should
          declare <varname>DefaultDependencies=no</varname> and specify
          all their dependencies manually, including access to anything
          more than a read only root filesystem. For details on the
          dependencies of this target, refer to
          <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>syslog.socket</filename></term>
        <listitem>
          <para>The socket unit syslog implementations should listen
          on. All userspace log messages will be made available on
          this socket. For more information about syslog integration,
          please consult the <ulink
          url="https://www.freedesktop.org/wiki/Software/systemd/syslog">Syslog
          Interface</ulink> document.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>system-update.target</filename></term>
        <term><filename>system-update-cleanup.service</filename></term>
        <listitem>
          <para>A special target unit that is used for offline system updates.
          <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
          will redirect the boot process to this target if <filename>/system-update</filename>
          exists. For more information see
          <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
          </para>

          <para>Updates should happen before the <filename>system-update.target</filename> is
          reached, and the services which implement them should cause the machine to reboot. As
          a safety measure, if this does not happen, and <filename>/system-update</filename>
          still exists after <filename>system-update.target</filename> is reached,
          <filename>system-update-cleanup.service</filename> will remove this symlink and
          reboot the machine.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>timers.target</filename></term>
        <listitem>
          <para>A special target unit that sets up all timer units
          (see
          <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
          for details) that shall be active after boot.</para>

          <para>It is recommended that timer units installed by
          applications get pulled in via <varname>Wants=</varname>
          dependencies from this unit. This is best configured via
          <varname>WantedBy=timers.target</varname> in the timer
          unit's <literal>[Install]</literal> section.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>umount.target</filename></term>
        <listitem>
          <para>A special target unit that unmounts all mount and
          automount points on system shutdown.</para>

          <para>Mounts that shall be unmounted on system shutdown
          shall add Conflicts dependencies to this unit for their
          mount unit, which is implicitly done when
          <varname>DefaultDependencies=yes</varname> is set (the
          default).</para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>Special System Units for Devices</title>

    <para>Some target units are automatically pulled in as devices of
    certain kinds show up in the system. These may be used to
    automatically activate various services based on the specific type
    of the available hardware.</para>

    <variablelist>
      <varlistentry>
        <term><filename>bluetooth.target</filename></term>
        <listitem>
          <para>This target is started automatically as soon as a
          Bluetooth controller is plugged in or becomes available at
          boot.</para>

          <para>This may be used to pull in Bluetooth management
          daemons dynamically when Bluetooth hardware is found.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>printer.target</filename></term>
        <listitem>
          <para>This target is started automatically as soon as a
          printer is plugged in or becomes available at boot.</para>

          <para>This may be used to pull in printer management daemons
          dynamically when printer hardware is found.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>smartcard.target</filename></term>
        <listitem>
          <para>This target is started automatically as soon as a
          smartcard controller is plugged in or becomes available at
          boot.</para>

          <para>This may be used to pull in smartcard management
          daemons dynamically when smartcard hardware is found.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>sound.target</filename></term>
        <listitem>
          <para>This target is started automatically as soon as a
          sound card is plugged in or becomes available at
          boot.</para>

          <para>This may be used to pull in audio management daemons
          dynamically when audio hardware is found.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Special Passive System Units </title>

    <para>A number of special system targets are defined that can be
    used to properly order boot-up of optional services. These targets
    are generally not part of the initial boot transaction, unless
    they are explicitly pulled in by one of the implementing services.
    Note specifically that these <emphasis>passive</emphasis> target
    units are generally not pulled in by the consumer of a service,
    but by the provider of the service. This means: a consuming
    service should order itself after these targets (as appropriate),
    but not pull it in. A providing service should order itself before
    these targets (as appropriate) and pull it in (via a
    <varname>Wants=</varname> type dependency).</para>

    <para>Note that these passive units cannot be started manually,
    i.e. <literal>systemctl start time-sync.target</literal> will fail
    with an error. They can only be pulled in by dependency. This is
    enforced since they exist for ordering purposes only and thus are
    not useful as only unit within a transaction.</para>

    <variablelist>
      <varlistentry>
        <term><filename>cryptsetup-pre.target</filename></term>
        <listitem>
          <para>This passive target unit may be pulled in by services
          that want to run before any encrypted block device is set
          up. All encrypted block devices are set up after this target
          has been reached. Since the shutdown order is implicitly the
          reverse start-up order between units, this target is
          particularly useful to ensure that a service is shut down
          only after all encrypted block devices are fully
          stopped.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>getty-pre.target</filename></term>
        <listitem>
          <para>A special passive target unit. Users of this target
          are expected to pull it in the boot transaction via
          a dependency (e.g. <varname>Wants=</varname>). Order your
          unit before this unit if you want to make use of the console
          just before <filename>getty</filename> is started.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>local-fs-pre.target</filename></term>
        <listitem>
          <para>This target unit is
          automatically ordered before
          all local mount points marked
          with <option>auto</option>
          (see above). It can be used to
          execute certain units before
          all local mounts.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>network.target</filename></term>
        <listitem>
          <para>This unit is supposed to indicate when network
          functionality is available, but it is only very weakly
          defined what that is supposed to mean, with one exception:
          at shutdown, a unit that is ordered after
          <filename>network.target</filename> will be stopped before
          the network — to whatever level it might be set up then —
          is shut down. It is hence useful when writing service files
          that require network access on shutdown, which should order
          themselves after this target, but not pull it in. Also see
          <ulink url="https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running
          Services After the Network is up</ulink> for more
          information. Also see
          <filename>network-online.target</filename> described
          above.</para>

          <para>systemd automatically adds dependencies of type
          <varname>After=</varname> for this target unit to all SysV
          init script service units with an LSB header referring to
          the <literal>$network</literal> facility.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>network-pre.target</filename></term>
        <listitem>
          <para>This passive target unit may be pulled in by services
          that want to run before any network is set up, for example
          for the purpose of setting up a firewall. All network
          management software orders itself after this target, but
          does not pull it in.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>nss-lookup.target</filename></term>
        <listitem>
          <para>A target that should be used as synchronization point
          for all host/network name service lookups. Note that this is
          independent of user/group name lookups for which
          <filename>nss-user-lookup.target</filename> should be used.
          All services for which the availability of full host/network
          name resolution is essential should be ordered after this
          target, but not pull it in. systemd automatically adds
          dependencies of type <varname>After=</varname> for this
          target unit to all SysV init script service units with an
          LSB header referring to the <literal>$named</literal>
          facility.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>nss-user-lookup.target</filename></term>
        <listitem>
          <para>A target that should be used as synchronization point
          for all user/group name service lookups. Note that this is
          independent of host/network name lookups for which
          <filename>nss-lookup.target</filename> should be used. All
          services for which the availability of the full user/group
          database is essential should be ordered after this target,
          but not pull it in. Note that system users are always
          resolvable, and hence do not require any special ordering
          against this target.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>remote-fs-pre.target</filename></term>
        <listitem>
          <para>This target unit is automatically ordered before all
          mount point units (see above) and cryptsetup devices
          marked with the <option>_netdev</option>. It can be used to run
          certain units before remote encrypted devices and mounts are established.
          Note that this unit is generally not part of the initial
          transaction, unless the unit that wants to be ordered before
          all remote mounts pulls it in via a
          <varname>Wants=</varname> type dependency. If the unit wants
          to be pulled in by the first remote mount showing up, it
          should use <filename>network-online.target</filename> (see
          above).</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>rpcbind.target</filename></term>
        <listitem>
          <para>The portmapper/rpcbind pulls in this target and orders
          itself before it, to indicate its availability. systemd
          automatically adds dependencies of type
          <varname>After=</varname> for this target unit to all SysV
          init script service units with an LSB header referring to
          the <literal>$portmap</literal> facility.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>time-sync.target</filename></term>
        <listitem>
          <para>Services responsible for synchronizing the system
          clock from a remote source (such as NTP client
          implementations) should pull in this target and order
          themselves before it. All services where correct time is
          essential should be ordered after this unit, but not pull it
          in. systemd automatically adds dependencies of type
          <varname>After=</varname> for this target unit to all SysV
          init script service units with an LSB header referring to
          the <literal>$time</literal> facility. </para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Special User Units</title>

    <para>When systemd runs as a user instance, the following special
    units are available, which have similar definitions as their
    system counterparts:
    <filename>exit.target</filename>,
    <filename>default.target</filename>,
    <filename>shutdown.target</filename>,
    <filename>sockets.target</filename>,
    <filename>timers.target</filename>,
    <filename>paths.target</filename>,
    <filename>bluetooth.target</filename>,
    <filename>printer.target</filename>,
    <filename>smartcard.target</filename>,
    <filename>sound.target</filename>.</para>
  </refsect1>

  <refsect1>
    <title>Special Passive User Units</title>

    <variablelist>
      <varlistentry>
        <term><filename>graphical-session.target</filename></term>
        <listitem>
          <para>This target is active whenever any graphical session is running. It is used to stop user services which
          only apply to a graphical (X, Wayland, etc.) session when the session is terminated. Such services should
          have <literal>PartOf=graphical-session.target</literal> in their <literal>[Unit]</literal> section. A target
          for a particular session (e. g.  <filename>gnome-session.target</filename>) starts and stops
          <literal>graphical-session.target</literal> with <literal>BindsTo=graphical-session.target</literal>.</para>

          <para>Which services are started by a session target is determined by the <literal>Wants=</literal> and
          <literal>Requires=</literal> dependencies.  For services that can be enabled independently, symlinks in
          <literal>.wants/</literal> and <literal>.requires/</literal> should be used, see
          <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.  Those
          symlinks should either be shipped in packages, or should be added dynamically after installation, for example
          using <literal>systemctl add-wants</literal>, see
          <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
          </para>

          <example>
            <title>Nautilus as part of a GNOME session</title>

            <para><literal>gnome-session.target</literal> pulls in Nautilus as top-level service:</para>

            <programlisting>[Unit]
Description=User systemd services for GNOME graphical session
Wants=nautilus.service
BindsTo=graphical-session.target</programlisting>

            <para><literal>nautilus.service</literal> gets stopped when the session stops:</para>

            <programlisting>[Unit]
Description=Render the desktop icons with Nautilus
PartOf=graphical-session.target

[Service]
…</programlisting>
          </example>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>graphical-session-pre.target</filename></term>
        <listitem>
          <para>This target contains services which set up the environment or global configuration of a graphical
          session, such as SSH/GPG agents (which need to export an environment variable into all desktop processes) or
          migration of obsolete d-conf keys after an OS upgrade (which needs to happen before starting any process that
          might use them). This target must be started before starting a graphical session like
          <filename>gnome-session.target</filename>.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1>
    <title>Special Slice Units</title>

    <para>There are four <literal>.slice</literal> units which form the basis of the hierarchy for assignment of
    resources for services, users, and virtual machines or containers. See
    <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details about slice
    units.</para>

    <variablelist>
      <varlistentry>
        <term><filename>-.slice</filename></term>
        <listitem>
          <para>The root slice is the root of the slice hierarchy. It usually does not contain units directly, but may
          be used to set defaults for the whole tree.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>system.slice</filename></term>
        <listitem>
          <para>By default, all system services started by
          <command>systemd</command> are found in this slice.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>user.slice</filename></term>
        <listitem>
          <para>By default, all user processes and services started on
          behalf of the user, including the per-user systemd instance
          are found in this slice.  This is pulled in by
          <filename>systemd-logind.service</filename></para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>machine.slice</filename></term>
        <listitem>
          <para>By default, all virtual machines and containers
          registered with <command>systemd-machined</command> are
          found in this slice.  This is pulled in by
          <filename>systemd-machined.service</filename></para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para>
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
        <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>