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SYSTEMD.SPECIAL(7) | systemd.special | SYSTEMD.SPECIAL(7) |
NAME¶
systemd.special - Special systemd unitsSYNOPSIS¶
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.sliceDESCRIPTION¶
A few units are treated specially by systemd. They have special internal semantics and cannot be renamed.SPECIAL SYSTEM UNITS¶
basic.targetA special target unit covering basic boot-up.
systemd automatically adds dependencies of the types Requires= and
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.
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
"$local_fs" facility.
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 appropiate 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) multi-user.target (for
runlevel 2) or graphical.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=
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
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.targetThis 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.targetThis 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 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. -.sliceThe 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
- 2.
- Syslog Interface
- 3.
- System Updates Specification
systemd 230 |