systemd.special(7)
NAME
systemd.special - Special systemd units
SYNOPSIS
basic.target, bluetooth.target, ctrl-alt-del.target, cryptsetup.target, cryptsetup-pre.target, dbus.service, dbus.socket, default.target, display-manager.service, emergency.target, exit.target, final.target, getty.target, graphical.target, halt.target, hibernate.target, hybrid-sleep.target, initrd-fs.target, kbrequest.target, kexec.target, local-fs.target, local-fs-pre.target, multi-user.target, network.target, network-online.target, network-pre.target, nss-lookup.target, nss-user-lookup.target, paths.target, poweroff.target, printer.target, reboot.target, remote-fs.target, remote-fs-pre.target, rescue.target, initrd-root-device.target, initrd-root-fs.target, rpcbind.target, runlevel2.target, runlevel3.target, runlevel4.target, runlevel5.target, shutdown.target, sigpwr.target, sleep.target, slices.target, smartcard.target, sockets.target, sound.target, suspend.target, swap.target, sysinit.target, syslog.socket, system-update.target, time-sync.target, timers.target, umount.target, -.slice, system.slice, user.slice, machine.slice
DESCRIPTION
A few units are treated specially by systemd. They have special internal semantics and cannot be renamed.
SPECIAL SYSTEM UNITS
basic.target
A special target unit covering basic boot-up.
systemd automatically adds dependency of the type After= for this
target unit to all services (except for those with
DefaultDependencies=no).
Usually, this should pull-in all local mount points plus /var, /tmp
and /var/tmp, 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.
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 bootup(7) for details on the targets involved.
ctrl-alt-del.target
systemd starts this target whenever Control+Alt+Del is pressed on
the console. Usually, this should be aliased (symlinked) to
reboot.target.
cryptsetup.target
A target that pulls in setup services for all encrypted block
devices.
dbus.service
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.
dbus.socket
A special unit for the D-Bus system bus socket. All units with
Type=dbus automatically gain a dependency on this unit.
default.target
The default unit systemd starts at bootup. Usually, this should be
aliased (symlinked) to multi-user.target or graphical.target.
The default unit systemd starts at bootup can be overridden with
the systemd.unit= kernel command line option.
display-manager.service
The display manager service. Usually, this should be aliased
(symlinked) to gdm.service or a similar display manager service.
emergency.target
A special target unit that starts an emergency shell on the main
console. This target does not pull in any services or mounts. It is
the most minimal version of starting the system in order to acquire
an interactive shell; the only processes running are usually just
the system manager (PID 1) and the shell process. This unit is
supposed to be used with the kernel command line option
systemd.unit=; it is also used when a file system check on a
required file system fails, and boot-up cannot continue. Compare
with rescue.target, which serves a similar purpose, but also starts
the most basic services and mounts all file systems.
Use the "systemd.unit=emergency.target" kernel command line option
to boot into this mode. A short alias for this kernel command line
option is "emergency", for compatibility with SysV.
In many ways booting into emergency.target is similar to the effect
of booting with "init=/bin/sh" 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.
exit.target
A special service unit for shutting down the system or user service
manager. It is equivalent to poweroff.target on non-container
systems, and also works in containers.
systemd will start this unit when it receives a request to shut
down over D-Bus or a SIGTERM or SIGINT signal when running as user
service daemon.
Normally, this (indirectly) pulls in shutdown.target, which in turn
should be conflicted by all units that want to be scheduled for
shutdown when the service manager starts to exit.
final.target
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.
getty.target
A special target unit that pulls in statically configured local TTY
getty instances.
graphical.target
A special target unit for setting up a graphical login screen. This
pulls in multi-user.target.
Units that are needed for graphical logins shall add Wants=
dependencies for their unit to this unit (or multi-user.target)
during installation. This is best configured via
WantedBy=graphical.target in the unit's "[Install]" section.
hibernate.target
A special target unit for hibernating the system. This pulls in
sleep.target.
hybrid-sleep.target
A special target unit for hibernating and suspending the system at
the same time. This pulls in sleep.target.
halt.target
A special target unit for shutting down and halting the system.
Note that this target is distinct from poweroff.target in that it
generally really just halts the system rather than powering it
down.
Applications wanting to halt the system should start this unit.
initrd-fs.target
systemd-fstab-generator(3) automatically adds dependencies of type
Before= to sysroot-usr.mount and all mount points found in
/etc/fstab that have x-initrd.mount and not have noauto mount
options set.
kbrequest.target
systemd starts this target whenever Alt+ArrowUp is pressed on the
console. This is a good candidate to be aliased (symlinked) to
rescue.target.
kexec.target
A special target unit for shutting down and rebooting the system
via kexec.
Applications wanting to reboot the system with kexec should start
this unit.
local-fs.target
systemd-fstab-generator(3) automatically adds dependencies of type
Before= to all mount units that refer to local mount points for
this target unit. In addition, it adds dependencies of type Wants=
to this target unit for those mounts listed in /etc/fstab that have
the auto mount option set.
multi-user.target
A special target unit for setting up a multi-user system
(non-graphical). This is pulled in by graphical.target.
Units that are needed for a multi-user system shall add Wants=
dependencies for their unit to this unit during installation. This
is best configured via WantedBy=multi-user.target in the unit's
"[Install]" section.
network-online.target
Units that strictly require a configured network connection should
pull in network-online.target (via a Wants= 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.
Note the distinction between this unit and network.target. 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,
network.target 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, network.target is part of the boot
of most systems, while network-online.target is not, except when at
least one unit requires it. Also see Running Services After the
Network is up[1] for more information.
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.
paths.target
A special target unit that sets up all path units (see
systemd.path(5) for details) that shall be active after boot.
It is recommended that path units installed by applications get
pulled in via Wants= dependencies from this unit. This is best
configured via a WantedBy=paths.target in the path unit's
"[Install]" section.
poweroff.target
A special target unit for shutting down and powering off the
system.
Applications wanting to power off the system should start this
unit.
runlevel0.target is an alias for this target unit, for
compatibility with SysV.
reboot.target
A special target unit for shutting down and rebooting the system.
Applications wanting to reboot the system should start this unit.
runlevel6.target is an alias for this target unit, for
compatibility with SysV.
remote-fs.target
Similar to local-fs.target, but for remote mount points.
systemd automatically adds dependencies of type After= for this
target unit to all SysV init script service units with an LSB
header referring to the "$remote_fs" facility.
rescue.target
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 emergency.target, which is much more reduced
and does not provide the file systems or most basic services.
runlevel1.target is an alias for this target unit, for
compatibility with SysV.
Use the "systemd.unit=rescue.target" kernel command line option to
boot into this mode. A short alias for this kernel command line
option is "1", for compatibility with SysV.
initrd-root-device.target
A special initrd target unit that is reached when the root
filesystem device is available, but before it has been mounted.
systemd-fstab-generator(3) and systemd-gpt-auto-generator(3)
automatically setup the appropriate dependencies to make this
happen.
initrd-root-fs.target
systemd-fstab-generator(3) automatically adds dependencies of type
Before= to the sysroot.mount unit, which is generated from the
kernel command line.
runlevel2.target, runlevel3.target, runlevel4.target, runlevel5.target
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) graphical.target (for
runlevel 5) or multi-user.target (the others).
shutdown.target
A special target unit that terminates the services on system
shutdown.
Services that shall be terminated on system shutdown shall add
Conflicts= and Before= dependencies to this unit for their service
unit, which is implicitly done when DefaultDependencies=yes is set
(the default).
sigpwr.target
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.
sleep.target
A special target unit that is pulled in by suspend.target,
hibernate.target and hybrid-sleep.target and may be used to hook
units into the sleep state logic.
slices.target
A special target unit that sets up all slice units (see
systemd.slice(5) for details) that shall be active after boot. By
default the generic user.slice, system.slice, machines.slice slice
units, as well as the root slice unit -.slice are pulled in and
ordered before this unit (see below).
It's a good idea to add WantedBy=slices.target lines to the
"[Install]" section of all slices units that may be installed
dynamically.
sockets.target
A special target unit that sets up all socket units (see
systemd.socket(5) for details) that shall be active after boot.
Services that can be socket-activated shall add Wants= dependencies
to this unit for their socket unit during installation. This is
best configured via a WantedBy=sockets.target in the socket unit's
"[Install]" section.
suspend.target
A special target unit for suspending the system. This pulls in
sleep.target.
swap.target
Similar to local-fs.target, but for swap partitions and swap files.
sysinit.target
systemd automatically adds dependencies of the types Requires= and
After= for this target unit to all services (except for those with
DefaultDependencies=no).
This target pulls in the services required for system
initialization. System services pulled in by this target should
declare DefaultDependencies=no 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 bootup(7).
syslog.socket
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
Syslog Interface[2] document.
system-update.target
A special target unit that is used for off-line system updates.
systemd-system-update-generator(8) will redirect the boot process
to this target if /system-update exists. For more information see
the System Updates Specification[3].
timers.target
A special target unit that sets up all timer units (see
systemd.timer(5) for details) that shall be active after boot.
It is recommended that timer units installed by applications get
pulled in via Wants= dependencies from this unit. This is best
configured via WantedBy=timers.target in the timer unit's
"[Install]" section.
umount.target
A special target unit that unmounts all mount and automount points
on system shutdown.
Mounts that shall be unmounted on system shutdown shall add
Conflicts dependencies to this unit for their mount unit, which is
implicitly done when DefaultDependencies=yes is set (the default).
SPECIAL SYSTEM UNITS FOR DEVICES
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.
bluetooth.target
This target is started automatically as soon as a Bluetooth
controller is plugged in or becomes available at boot.
This may be used to pull in Bluetooth management daemons
dynamically when Bluetooth hardware is found.
printer.target
This target is started automatically as soon as a printer is
plugged in or becomes available at boot.
This may be used to pull in printer management daemons dynamically
when printer hardware is found.
smartcard.target
This target is started automatically as soon as a smartcard
controller is plugged in or becomes available at boot.
This may be used to pull in smartcard management daemons
dynamically when smartcard hardware is found.
sound.target
This target is started automatically as soon as a sound card is
plugged in or becomes available at boot.
This may be used to pull in audio management daemons dynamically
when audio hardware is found.
SPECIAL PASSIVE SYSTEM UNITS
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 passive 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
Wants= type dependency).
Note that these passive units cannot be started manually, i.e.
"systemctl start time-sync.target" 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.
cryptsetup-pre.target
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.
local-fs-pre.target
This target unit is automatically ordered before all local mount
points marked with auto (see above). It can be used to execute
certain units before all local mounts.
network.target
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 network.target 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 Running Services After the Network is up[1]
for more information. Also see network-online.target described
above.
systemd automatically adds dependencies of type After= for this
target unit to all SysV init script service units with an LSB
header referring to the "$network" facility.
network-pre.target
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.
nss-lookup.target
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 nss-user-lookup.target 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
After= for this target unit to all SysV init script service units
with an LSB header referring to the "$named" facility.
nss-user-lookup.target
A target that should be used as synchronization point for all
user/group name service lookups. Note that this is independent of
host/network name lookups for which nss-lookup.target should be
used. All services for which the availability of the full
user/group database is essential should be ordered after this
target, but not pull it in. Note that system users are always
resolvable, and hence do not require any special ordering against
this target.
remote-fs-pre.target
This target unit is automatically ordered before all remote mount
point units (see above). It can be used to run certain units before
the remote mounts are established. Note that this unit is generally
not part of the initial transaction, unless the unit that wants to
be ordered before all remote mounts pulls it in via a Wants= type
dependency. If the unit wants to be pulled in by the first remote
mount showing up, it should use network-online.target (see above).
rpcbind.target
The portmapper/rpcbind pulls in this target and orders itself
before it, to indicate its availability. systemd automatically adds
dependencies of type After= for this target unit to all SysV init
script service units with an LSB header referring to the "$portmap"
facility.
time-sync.target
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
After= for this target unit to all SysV init script service units
with an LSB header referring to the "$time" facility.
SPECIAL USER UNITS
When systemd runs as a user instance, the following special units are available, which have similar definitions as their system counterparts: exit.target, default.target, shutdown.target, sockets.target, timers.target, paths.target, bluetooth.target, printer.target, smartcard.target, sound.target.
SPECIAL PASSIVE USER UNITS
graphical-session.target 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 "PartOf=graphical-session.target" in their "[Unit]" section. A target for a particular session (e. g. gnome-session.target) starts and stops "graphical-session.target" with "BindsTo=graphical-session.target". Which services are started by a session target is determined by the "Wants=" and "Requires=" dependencies. For services that can be enabled independently, symlinks in ".wants/" and ".requires/" should be used, see systemd.unit(5). Those symlinks should either be shipped in packages, or should be added dynamically after installation, for example using "systemctl add-wants", see systemctl(1). Example 1. Nautilus as part of a GNOME session "gnome-session.target" pulls in Nautilus as top-level service: [Unit] Description=User systemd services for GNOME graphical session Wants=nautilus.service BindsTo=graphical-session.target "nautilus.service" gets stopped when the session stops: [Unit] Description=Render the desktop icons with Nautilus PartOf=graphical-session.target [Service] ... graphical-session-pre.target 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 gnome-session.target.
SPECIAL SLICE UNITS
There are four ".slice" units which form the basis of the hierarchy for
assignment of resources for services, users, and virtual machines or
containers.
-.slice
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.
system.slice
By default, all system services started by systemd are found in
this slice.
user.slice
By default, all user processes and services started on behalf of
the user, including the per-user systemd instance are found in this
slice.
machine.slice
By default, all virtual machines and containers registered with
systemd-machined are found in this slice.
SEE ALSO
systemd(1), systemd.unit(5), systemd.service(5), systemd.socket(5), systemd.target(5), systemd.slice(5), bootup(7), systemd-fstab- generator(8)
NOTES
1. Running Services After the Network is up
http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget
2. Syslog Interface
http://www.freedesktop.org/wiki/Software/systemd/syslog
3. System Updates Specification
http://freedesktop.org/wiki/Software/systemd/SystemUpdates
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