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

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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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>veritysetup-pre.target</filename>,
    <filename>veritysetup.target</filename>,
    <filename>ctrl-alt-del.target</filename>,
    <filename>blockdev@.target</filename>,
    <filename>boot-complete.target</filename>,
    <filename>default.target</filename>,
    <filename>emergency.target</filename>,
    <filename>exit.target</filename>,
    <filename>factory-reset.target</filename>,
    <filename>final.target</filename>,
    <filename>first-boot-complete.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.target</filename>,
    <filename>initrd-fs.target</filename>,
    <filename>initrd-root-device.target</filename>,
    <filename>initrd-root-fs.target</filename>,
    <filename>initrd-usr-fs.target</filename>,
    <filename>integritysetup-pre.target</filename>,
    <filename>integritysetup.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-veritysetup.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-set.target</filename>,
    <filename>time-sync.target</filename>,
    <filename>timers.target</filename>,
    <filename>umount.target</filename>,
    <filename>usb-gadget.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 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>boot-complete.target</filename></term>
          <listitem>
            <para>This target is intended as generic synchronization point for services that shall determine or act on
            whether the boot process completed successfully. Order units that are required to succeed for a boot process
            to be considered successful before this unit, and add a <varname>Requires=</varname> dependency from the
            target unit to them. Order units that shall only run when the boot process is considered successful after the
            target unit and pull in the target from it, also with <varname>Requires=</varname>. Note that by default this
            target unit is not part of the initial boot transaction, but is supposed to be pulled in only if required by
            units that want to run only on successful boots.</para>

            <para>See
            <citerefentry><refentrytitle>systemd-boot-check-no-failures.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
            for a service that implements a generic system health check and orders itself before
            <filename>boot-complete.target</filename>.</para>

            <para>See
            <citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
            for a service that propagates boot success information to the boot loader, and orders itself after
            <filename>boot-complete.target</filename>.</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>veritysetup.target</filename></term>
          <listitem>
            <para>A target that pulls in setup services for all
            verity integrity protected 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>. See
            <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
            more discussion.</para>

            <para>The default unit systemd starts at bootup can be overridden with the
            <varname>systemd.unit=</varname> kernel command line option, or more conveniently, with the short
            names like <varname>single</varname>, <varname>rescue</varname>, <varname>1</varname>,
            <varname>3</varname>, <varname>5</varname>, …; see
            <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</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 other 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 may be used
            by specifying <varname>emergency</varname> on the kernel command line; 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>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>

            <para>Note that depending on how <filename>emergency.target</filename> is reached, the root file
            system might be mounted read-only or read-write (no remounting is done specially for this
            target). For example, the system may boot with root mounted read-only when <varname>ro</varname>
            is used on the kernel command line and remain this way for <filename>emergency.target</filename>,
            or the system may transition to <filename>emergency.target</filename> after the system has been
            partially booted and disks have already been remounted read-write.</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>factory-reset.target</filename></term>
          <listitem>
            <para>A special target to trigger a factory reset.</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
            [Install] 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.target</filename></term>
          <listitem>
            <para>This is the default target in the initrd, similar to <filename>default.target</filename> in
            the main system. It is used to mount the real root and transition to it. See
            <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
            more discussion.</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 the <option>x-initrd.mount</option> mount option set
            and do not have the <option>noauto</option> mount option set. It is also indirectly ordered after
            <filename>sysroot.mount</filename>. Thus, once this target is reached the
            <filename>/sysroot/</filename> hierarchy is fully set up, in preparation for the transition to
            the host OS.</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's
            <varname>root=</varname> setting (or equivalent).</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><filename>initrd-usr-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>sysusr-usr.mount</filename> unit, which is generated from the kernel command line's
            <varname>usr=</varname> switch. Services may order themselves after this target unit in order to
            run once the <filename>/sysusr/</filename> hierarchy becomes available, on systems that come up
            initially without a root file system, but with an initialized <filename>/usr/</filename> and need
            to access that before setting up the root file system to ultimately switch to. On systems where
            <varname>usr=</varname> is not used this target is ordered after
            <filename>sysroot.mount</filename> and thus mostly equivalent to
            <filename>initrd-root-fs.target</filename>. In effect on any system once this target is reached
            the file system backing <filename>/usr/</filename> is mounted, though possibly at two different
            locations, either below the <filename>/sysusr/</filename> or the <filename>/sysroot/</filename>
            hierarchies.</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
            [Install] 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://systemd.io/NETWORK_ONLINE">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 <emphasis>provide</emphasis>
            functionality to other hosts (as opposed to <emphasis>consume</emphasis> functionality of 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
            [Install] 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-veritysetup.target</filename></term>
          <listitem>
            <para>Similar to <filename>veritysetup.target</filename>, but for verity
            integrity protected devices which are accessed over the network. It is used for
            <citerefentry><refentrytitle>veritytab</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 always be active after boot. By default the generic
            <filename>system.slice</filename> slice unit as well as the root slice unit
            <filename>-.slice</filename> are pulled in and ordered before this unit (see
            below).</para>

            <para>Adding slice units to <filename>slices.target</filename> is generally not
            necessary. Instead, when some unit that uses <varname>Slice=</varname> is started, the
            specified slice will be started automatically. Adding
            <varname>WantedBy=slices.target</varname> lines to the [Install]
            section should only be done for units that need to be always active. In that case care
            needs to be taken to avoid creating a loop through the automatic dependencies on
            "parent" slices.</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 [Install]
            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> or
            <filename>/etc/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> or
            <filename>/etc/system-update</filename> still exists after
            <filename>system-update.target</filename> is reached,
            <filename>system-update-cleanup.service</filename> will remove the symlinks 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 [Install] 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>
        <varlistentry>
          <term><filename>usb-gadget.target</filename></term>
          <listitem>
            <para>This target is started automatically as soon as a
            USB Device Controller becomes available at boot.</para>

            <para>This may be used to pull in usb gadget
            dynamically when UDC 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>blockdev@.target</filename></term>
          <listitem><para>This template unit is used to order mount units and other consumers of block
          devices after services that synthesize these block devices. In particular, this is intended to be
          used with storage services (such as
          <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>/
          <citerefentry><refentrytitle>systemd-veritysetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
          that allocate and manage a virtual block device. Storage services are ordered before an instance of
          <filename>blockdev@.target</filename>, and the consumer units after it. The ordering is
          particularly relevant during shutdown, as it ensures that the mount is deactivated first and the
          service backing the mount later. The <filename>blockdev@.target</filename> instance should be
          pulled in via a <option>Wants=</option> dependency of the storage daemon and thus generally not be
          part of any transaction unless a storage daemon is used. The instance name for instances of this
          template unit must be a properly escaped block device node path, e.g.
          <filename index="false">blockdev@dev-mapper-foobar.target</filename> for the storage device
          <filename index="false">/dev/mapper/foobar</filename>.</para></listitem>
        </varlistentry>
        <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>veritysetup-pre.target</filename></term>
          <listitem>
            <para>This passive target unit may be pulled in by services
            that want to run before any verity integrity protected block
            device is set up. All verity integrity protected 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 verity integrity
            protected block devices are fully stopped.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><filename>first-boot-complete.target</filename></term>
          <listitem>
            <para>This passive target is intended as a synchronization point for units that need to run once
            during the first boot.  Only after all units ordered before this target have finished, will the
            <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
            be committed to disk, marking the first boot as completed.  If the boot is aborted at any time
            before that, the next boot will re-run any units with <varname>ConditionFirstBoot=yes</varname>.
            </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. However, the following should apply at
            minimum:</para>

            <itemizedlist>
              <listitem><para>At start-up, any configured synthetic network devices (i.e. not physical ones
              that require hardware to show up and be probed, but virtual ones like bridge devices and
              similar which are created programmatically) that do not depend on any underlying hardware
              should be allocated by the time this target is reached. It is not necessary for these
              interfaces to also have completed IP level configuration by the time
              <filename>network.target</filename> is reached.</para></listitem>

              <listitem><para>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 by 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://systemd.io/NETWORK_ONLINE">Running Services After the Network Is Up</ulink> for
              more information.</para></listitem>
            </itemizedlist>

            <para>It must emphasized that at start-up there's no guarantee that hardware-based devices have
            shown up by the time this target is reached, or even acquired complete IP configuration. For that
            purpose use <filename>network-online.target</filename> as 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. Also see <ulink
            url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is Up</ulink> for more
            information.</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/veritysetup 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-set.target</filename></term>
          <listitem>
            <para>Services responsible for setting the system clock (<constant>CLOCK_REALTIME</constant>)
            from a local source (such as a maintained timestamp file or imprecise real-time clock) should
            pull in this target and order themselves before it. Services where approximate, roughly monotonic
            time is desired should be ordered after this unit, but not pull it in.</para>

            <para>This target does not provide the accuracy guarantees of
            <filename>time-sync.target</filename> (see below), however does not depend on remote clock
            sources to be reachable, i.e. the target is typically not delayed by network problems and
            similar. Use of this target is recommended for services where approximate clock accuracy and
            rough monotonicity is desired but activation shall not be delayed for possibly unreliable network
            communication.</para>

            <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
            this target unit to all timer units with at least one <varname>OnCalendar=</varname>
            directive.</para>

            <para>The
            <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
            service is a simple daemon that pulls in this target and orders itself before it. Besides
            implementing the SNTP network protocol it maintains a timestamp file on disk whose modification
            time is regularlary updated. At service start-up the local system clock is set from that modification time,
            ensuring it increases roughly monotonically.</para>

            <para>Note that ordering a unit after <filename>time-set.target</filename> only has effect if
            there's actually a service ordered before it that delays it until the clock is adjusted for rough
            monotonicity. Otherwise, this target might get reached before the clock is adjusted to be roughly
            monotonic. Enable
            <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
            or an alternative NTP implementation to delay the target.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><filename>time-sync.target</filename></term>
          <listitem>
            <para>Services indicating completed synchronization of the system clock
            (<constant>CLOCK_REALTIME</constant>) to a remote source should pull in this target and order
            themselves before it. Services where accurate time is essential should be ordered after this
            unit, but not pull it in.</para>

            <para>The service manager 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, as well to all timer units with at least one
            <varname>OnCalendar=</varname> directive.</para>

            <para>This target provides stricter clock accuracy guarantees than
            <filename>time-set.target</filename> (see above), but likely requires
            network communication and thus introduces unpredictable delays.
            Services that require clock accuracy and where network
            communication delays are acceptable should use this target. Services that require a less accurate
            clock, and only approximate and roughly monotonic clock behaviour should use
            <filename>time-set.target</filename> instead.</para>

            <para>Note that ordering a unit after <filename>time-sync.target</filename> only has effect if
            there's actually a service ordered before it that delays it until clock synchronization is
            reached. Otherwise, this target might get reached before the clock is synchronized to any remote
            accurate reference clock. When using
            <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
            enable
            <citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
            to delay the target; or use an equivalent service for other NTP implementations.</para>

            <table>
              <title>Comparison</title>
              <tgroup cols='2' align='left' colsep='1' rowsep='1'>
                <colspec colname="time-set" />
                <colspec colname="time-sync" />
                <thead>
                  <row>
                    <entry><filename>time-set.target</filename></entry>
                    <entry><filename>time-sync.target</filename></entry>
                  </row>
                </thead>
                <tbody>
                  <row>
                    <entry>"quick" to reach</entry>
                    <entry>"slow" to reach</entry>
                  </row>
                  <row>
                    <entry>typically uses local clock sources, boot process not affected by availability of external resources</entry>
                    <entry>typically uses remote clock sources, inserts dependencies on remote resources into boot process</entry>
                  </row>
                  <row>
                    <entry>reliable, because local</entry>
                    <entry>unreliable, because typically network involved</entry>
                  </row>
                  <row>
                    <entry>typically guarantees an approximate and roughly monotonic clock only</entry>
                    <entry>typically guarantees an accurate clock</entry>
                  </row>
                  <row>
                    <entry>implemented by <filename>systemd-timesyncd.service</filename></entry>
                    <entry>implemented by <filename>systemd-time-wait-sync.service</filename></entry>
                  </row>
                </tbody>
              </tgroup>
            </table>

          </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 service manager</title>

    <refsect2>
      <title>Special User Units</title>

      <para>When systemd runs as a user instance, the following special
      units are available:</para>

      <variablelist>
        <varlistentry>
          <term><filename>default.target</filename></term>
          <listitem>
            <para>This is the main target of the user session, started by default. Various services that
            compose the normal user session should be pulled into this target. In this regard,
            <filename>default.target</filename> is similar to <filename>multi-user.target</filename> in the
            system instance, but it is a real unit, not an alias.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>In addition, the following units are available which have definitions similar to their
      system counterparts:
      <filename>exit.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 [Unit]
            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>

        <varlistentry>
          <term><filename>xdg-desktop-autostart.target</filename></term>
          <listitem>
            <para>The XDG specification defines a way to autostart applications using XDG desktop files.
            systemd ships
            <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
            for the XDG desktop files in autostart directories. Desktop Environments can opt-in to use this
            service by adding a <varname>Wants=</varname> dependency on
            <filename>xdg-desktop-autostart.target</filename>.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2>
      <title>Special User Slice Units</title>

      <para>There are four <literal>.slice</literal> units which form the basis of the user hierarchy for
      assignment of resources for user applications and services. See
      <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
      for details about slice units and the documentation about
      <ulink url="https://systemd.io/DESKTOP_ENVIRONMENTS">Desktop Environments</ulink>
      for further information.</para>

      <variablelist>
        <varlistentry>
          <term><filename>-.slice</filename></term>
          <listitem>
            <para>The root slice is the root of the user's 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>app.slice</filename></term>
          <listitem>
            <para>By default, all user services and applications managed by
            <command>systemd</command> are found in this slice.
            All interactively launched applications like web browsers and text editors
            as well as non-critical services should be placed into this slice.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><filename>session.slice</filename></term>
          <listitem>
            <para>All essential services and applications required for the
            session should use this slice.
            These are services that either cannot be restarted easily
            or where latency issues may affect the interactivity of the system and applications.
            This includes the display server, screen readers and other services such as DBus or XDG portals.
            Such services should be configured to be part of this slice by
            adding <varname>Slice=session.slice</varname> to their unit files.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><filename>background.slice</filename></term>
          <listitem>
            <para>All services running low-priority background tasks should use this slice.
            This permits resources to be preferentially assigned to the other slices.
            Examples include non-interactive tasks like file indexing or backup operations
            where latency is not important.</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>,
        <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>