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systemd-stable/man/systemd.special.xml
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<refentry id="systemd.special">
<refentryinfo>
<title>systemd.special</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>systemd.special</refentrytitle>
<manvolnum>7</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.special</refname>
<refpurpose>Special systemd units</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>basic.target</filename>,
<filename>bluetooth.target</filename>,
<filename>ctrl-alt-del.target</filename>,
<filename>cryptsetup.target</filename>,
<filename>cryptsetup-pre.target</filename>,
<filename>dbus.service</filename>,
<filename>dbus.socket</filename>,
<filename>default.target</filename>,
<filename>display-manager.service</filename>,
<filename>emergency.target</filename>,
<filename>exit.target</filename>,
<filename>final.target</filename>,
<filename>getty.target</filename>,
<filename>graphical.target</filename>,
<filename>halt.target</filename>,
<filename>hibernate.target</filename>,
<filename>hybrid-sleep.target</filename>,
<filename>initrd-fs.target</filename>,
<filename>kbrequest.target</filename>,
<filename>kexec.target</filename>,
<filename>local-fs.target</filename>,
<filename>local-fs-pre.target</filename>,
<filename>multi-user.target</filename>,
<filename>network.target</filename>,
<filename>network-online.target</filename>,
<filename>network-pre.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-fs.target</filename>,
<filename>remote-fs-pre.target</filename>,
<filename>rescue.target</filename>,
<filename>initrd-root-fs.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>smartcard.target</filename>,
<filename>sockets.target</filename>,
<filename>sound.target</filename>,
<filename>suspend.target</filename>,
<filename>swap.target</filename>,
<filename>sysinit.target</filename>,
<filename>syslog.socket</filename>,
<filename>system-update.target</filename>,
<filename>time-sync.target</filename>,
<filename>timers.target</filename>,
<filename>umount.target</filename>,
<filename>-.slice</filename>,
<filename>system.slice</filename>,
<filename>user.slice</filename>,
<filename>machine.slice</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>A few units are treated specially by systemd. They have
special internal semantics and cannot be renamed.</para>
</refsect1>
<refsect1>
<title>Special System Units</title>
<variablelist>
<varlistentry>
<term><filename>basic.target</filename></term>
<listitem>
<para>A special target unit covering basic boot-up.</para>
<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>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 unit is supposed to be used with
the kernel command line option
<varname>systemd.unit=</varname> and has otherwise little
use.
</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 a
request to shut down over D-Bus or a
<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>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 start
this unit.</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>kbrequest.target</filename></term>
<listitem>
<para>systemd starts this target whenever Alt+ArrowUp is
pressed on the console. This is a good candidate to be
aliased (symlinked) to
<filename>rescue.target</filename>.</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 with kexec
should start this unit.</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>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="http://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>
</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
start this unit.</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 start
this unit.</para>
<para><filename>runlevel6.target</filename> is an alias for
this target unit, for compatibility with SysV.</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 for setting up the base system
and a rescue shell.</para>
<para><filename>runlevel1.target</filename> is an alias for
this target unit, for compatibility with SysV.</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>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>multi-user.target</filename>
(for runlevel 2) or <filename>graphical.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> 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>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>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="http://www.freedesktop.org/wiki/Software/systemd/syslog">Syslog
Interface</ulink> document.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>system-update.target</filename></term>
<listitem>
<para>A special target unit that is used for off-line 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 the <ulink
url="http://freedesktop.org/wiki/Software/systemd/SystemUpdates">System
Updates Specification</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>timers.target</filename></term>
<listitem>
<para>A special target unit that sets up all timer units
(see
<citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details) that shall be active after boot.</para>
<para>It is recommended that timer units installed by
applications get pulled in via <varname>Wants=</varname>
dependencies from this unit. This is best configured via
<varname>WantedBy=timers.target</varname> in the timer
unit's <literal>[Install]</literal> section.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>umount.target</filename></term>
<listitem>
<para>A special target unit that unmounts all mount and
automount points on system shutdown.</para>
<para>Mounts that shall be unmounted on system shutdown
shall add Conflicts dependencies to this unit for their
mount unit, which is implicitly done when
<varname>DefaultDependencies=yes</varname> is set (the
default).</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Special System Units for Devices</title>
<para>Some target units are automatically pulled in as devices of
certain kinds show up in the system. These may be used to
automatically activate various services based on the specific type
of the available hardware.</para>
<variablelist>
<varlistentry>
<term><filename>bluetooth.target</filename></term>
<listitem>
<para>This target is started automatically as soon as a
Bluetooth controller is plugged in or becomes available at
boot.</para>
<para>This may be used to pull in Bluetooth management
daemons dynamically when Bluetooth hardware is found.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>printer.target</filename></term>
<listitem>
<para>This target is started automatically as soon as a
printer is plugged in or becomes available at boot.</para>
<para>This may be used to pull in printer management daemons
dynamically when printer hardware is found.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>smartcard.target</filename></term>
<listitem>
<para>This target is started automatically as soon as a
smartcard controller is plugged in or becomes available at
boot.</para>
<para>This may be used to pull in smartcard management
daemons dynamically when smartcard hardware is found.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>sound.target</filename></term>
<listitem>
<para>This target is started automatically as soon as a
sound card is plugged in or becomes available at
boot.</para>
<para>This may be used to pull in audio management daemons
dynamically when audio hardware is found.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Special Passive System Units </title>
<para>A number of special system targets are defined that can be
used to properly order boot-up of optional services. These targets
are generally not part of the initial boot transaction, unless
they are explicitly pulled in by one of the implementing services.
Note specifically that these <emphasis>passive</emphasis> target
units are generally not pulled in by the consumer of a service,
but by the provider of the service. This means: a consuming
service should order itself after these targets (as appropriate),
but not pull it in. A providing service should order itself before
these targets (as appropriate) and pull it in (via a
<varname>Wants=</varname> type dependency).</para>
<para>Note that these passive units cannot be started manually,
i.e. <literal>systemctl start time-sync.target</literal> will fail
with an error. They can only be pulled in by dependency. This is
enforced since they exist for ordering purposes only and thus are
not useful as only unit within a transaction.</para>
<variablelist>
<varlistentry>
<term><filename>cryptsetup-pre.target</filename></term>
<listitem>
<para>This passive target unit may be pulled in by services
that want to run before any encrypted block device is set
up. All encrypted block devices are set up after this target
has been reached. Since the shutdown order is implicitly the
reverse start-up order between units, this target is
particularly useful to ensure that a service is shut down
only after all encrypted block devices are fully
stopped.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>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="http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running
Services After the Network is up</ulink> for more
information. Also see
<filename>network-online.target</filename> described
above.</para>
<para>systemd automatically adds dependencies of type
<varname>After=</varname> for this target unit to all SysV
init script service units with an LSB header referring to
the <literal>$network</literal> facility.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>network-pre.target</filename></term>
<listitem>
<para>This passive target unit may be pulled in by services
that want to run before any network is set up, for example
for the purpose of setting up a firewall. All network
management software orders itself after this target, but
does not pull it in.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>nss-lookup.target</filename></term>
<listitem>
<para>A target that should be used as synchronization point
for all host/network name service lookups. Note that this is
independent of user/group name lookups for which
<filename>nss-user-lookup.target</filename> should be used.
All services for which the availability of full host/network
name resolution is essential should be ordered after this
target, but not pull it in. systemd automatically adds
dependencies of type <varname>After=</varname> for this
target unit to all SysV init script service units with an
LSB header referring to the <literal>$named</literal>
facility.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>nss-user-lookup.target</filename></term>
<listitem>
<para>A target that should be used as synchronization point
for all user/group name service lookups. Note that this is
independent of host/network name lookups for which
<filename>nss-lookup.target</filename> should be used. All
services for which the availability of the full user/group
database is essential should be ordered after this target,
but not pull it in. Note that system users are always
resolvable, and hence do not require any special ordering
against this target.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>remote-fs-pre.target</filename></term>
<listitem>
<para>This target unit is automatically ordered before all
remote mount point units (see above). It can be used to run
certain units before the remote mounts are established. Note
that this unit is generally not part of the initial
transaction, unless the unit that wants to be ordered before
all remote mounts pulls it in via a
<varname>Wants=</varname> type dependency. If the unit wants
to be pulled in by the first remote mount showing up, it
should use <filename>network-online.target</filename> (see
above).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>rpcbind.target</filename></term>
<listitem>
<para>The portmapper/rpcbind pulls in this target and orders
itself before it, to indicate its availability. systemd
automatically adds dependencies of type
<varname>After=</varname> for this target unit to all SysV
init script service units with an LSB header referring to
the <literal>$portmap</literal> facility.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>time-sync.target</filename></term>
<listitem>
<para>Services responsible for synchronizing the system
clock from a remote source (such as NTP client
implementations) should pull in this target and order
themselves before it. All services where correct time is
essential should be ordered after this unit, but not pull it
in. systemd automatically adds dependencies of type
<varname>After=</varname> for this target unit to all SysV
init script service units with an LSB header referring to
the <literal>$time</literal> facility. </para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Special User Units</title>
<para>When systemd runs as a user instance, the following special
units are available, which have similar definitions as their
system counterparts:
<filename>exit.target</filename>,
<filename>default.target</filename>,
<filename>shutdown.target</filename>,
<filename>sockets.target</filename>,
<filename>timers.target</filename>,
<filename>paths.target</filename>,
<filename>bluetooth.target</filename>,
<filename>printer.target</filename>,
<filename>smartcard.target</filename>,
<filename>sound.target</filename>.</para>
</refsect1>
<refsect1>
<title>Special 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.</para>
<variablelist>
<varlistentry>
<term><filename>-.slice</filename></term>
<listitem>
<para>The root slice is the root of the 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.</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.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>