man: split systemd.special(7) into separate system/user sections

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

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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  SPDX-License-Identifier: LGPL-2.1+
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<refentry id="systemd.special">

  <refentryinfo>
    <title>systemd.special</title>
    <productname>systemd</productname>
  </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>system-update-pre.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>Units managed by the system's service manager</title>

    <refsect2>
      <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>systemd automatically adds dependencies of type <varname>Wants=</varname> and
            <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>

            <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-pre.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. The
            main units executing the update should order themselves after
            <filename>system-update-pre.target</filename> but not pull it in. Services which want to
            run during system updates only, but before the actual system update is executed should
            order themselves before this unit and pull it in. 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>
    </refsect2>

    <refsect2>
      <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>
    </refsect2>

    <refsect2>
      <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>
          </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 UNIX 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 regular UNIX
            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. All services which provide parts of the
            user/group database should be ordered before this target, and pull it in. Note that this
            unit is only relevant for regular users and groups — system users and groups are
            required to be resolvable during earliest boot already, and hence do not need 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>
    </refsect2>

    <refsect2>
      <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>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Units managed by the user's service manager</title>

    <refsect2>
      <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>
    </refsect2>

    <refsect2>
      <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>
    </refsect2>
  </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>