NAME
systemd.special - Special systemd unitsSYNOPSIS
basic.target, bluetooth.target, cryptsetup-pre.target, cryptsetup.target, veritysetup-pre.target, veritysetup.target, ctrl-alt-del.target, [email protected], boot-complete.target, default.target, emergency.target, exit.target, factory-reset.target, final.target, first-boot-complete.target, getty.target, getty-pre.target, graphical.target, halt.target, hibernate.target, hybrid-sleep.target, suspend-then-hibernate.target, initrd.target, initrd-fs.target, initrd-root-device.target, initrd-root-fs.target, initrd-usr-fs.target, integritysetup-pre.target, integritysetup.target, kbrequest.target, kexec.target, local-fs-pre.target, local-fs.target, machines.target multi-user.target, network-online.target, network-pre.target, network.target, nss-lookup.target, nss-user-lookup.target, paths.target, poweroff.target, printer.target, reboot.target, remote-cryptsetup.target, remote-veritysetup.target, remote-fs-pre.target, remote-fs.target, rescue.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, system-update.target, system-update-pre.target, time-set.target, time-sync.target, timers.target, umount.target, usb-gadget.target, -.slice, system.slice, user.slice, machine.slice, -.mount, dbus.service, dbus.socket, display-manager.service, init.scope, syslog.socket, system-update-cleanup.serviceDESCRIPTION
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.UNITS MANAGED BY THE SYSTEM SERVICE MANAGER
Special System Units
-.mountThe root mount point, i.e. the mount unit for
the / 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.
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.
boot-complete.target
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
Requires= 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 Requires=. 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.
See systemd-boot-check-no-failures.service(8) for a service that
implements a generic system health check and orders itself before
boot-complete.target.
See systemd-bless-boot.service(8) for a service that propagates boot
success information to the boot loader, and orders itself after
boot-complete.target.
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.
veritysetup.target
A target that pulls in setup services for all
verity integrity protected 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. See bootup(7) for more discussion.
The default unit systemd starts at bootup can be overridden with the
systemd.unit= kernel command line option, or more conveniently, with
the short names like single, rescue, 1, 3,
5, ...; see systemd(1).
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 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 emergency 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 rescue.target, which serves a similar purpose, but also starts
the most basic services and mounts all file systems.
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.
Note that depending on how emergency.target 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 ro is used on the kernel command line and remain this way for
emergency.target, or the system may transition to emergency.target after the
system has been partially booted and disks have already been remounted
read-write.
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 the 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.
factory-reset.target
A special target to trigger a factory
reset.
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.
suspend-then-hibernate.target
A special target unit for suspending the
system for a period of time, waking it and putting it into hibernate. 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 not start this unit directly, but
should instead execute systemctl halt (possibly with the
--no-block option) or call systemd(1)'s
org.freedesktop.systemd1.Manager.Halt D-Bus method directly.
init.scope
This scope unit is where the system and
service manager (PID 1) itself resides. It is active as long as the system is
running.
initrd.target
This is the default target in the initrd,
similar to default.target in the main system. It is used to mount the real
root and transition to it. See bootup(7) for more discussion.
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 the x-initrd.mount
mount option set and do not have the noauto mount option set. It is
also indirectly ordered after sysroot.mount. Thus, once this target is reached
the /sysroot/ hierarchy is fully set up, in preparation for the transition to
the host OS.
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's root= setting
(or equivalent).
initrd-usr-fs.target
systemd-fstab-generator(3)
automatically adds dependencies of type Before= to the sysusr-usr.mount
unit, which is generated from the kernel command line's usr= switch.
Services may order themselves after this target unit in order to run once the
/sysusr/ hierarchy becomes available, on systems that come up initially
without a root file system, but with an initialized /usr/ and need to access
that before setting up the root file system to ultimately switch to. On
systems where usr= is not used this target is ordered after
sysroot.mount and thus mostly equivalent to initrd-root-fs.target. In effect
on any system once this target is reached the file system backing /usr/ is
mounted, though possibly at two different locations, either below the /sysusr/
or the /sysroot/ hierarchies.
kbrequest.target
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.
kexec.target
A special target unit for shutting down and
rebooting the system via kexec.
Applications wanting to reboot the system should not start this unit directly,
but should instead execute systemctl kexec (possibly with the
--no-block option) or call systemd(1)'s
org.freedesktop.systemd1.Manager.KExec D-Bus method directly.
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.
machines.target
A standard target unit for starting all the
containers and other virtual machines. See [email protected] for an
example.
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 (as opposed to consume
functionality of other hosts) generally do not need to pull this in.
systemd automatically adds dependencies of type Wants= and After=
for this target unit to all SysV init script service units with an LSB header
referring to the "$network" facility.
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.
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 not start this unit
directly, but should instead execute systemctl poweroff (possibly with
the --no-block option) or call systemd-logind(8)'s
org.freedesktop.login1.Manager.PowerOff D-Bus method directly.
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 not start this unit directly,
but should instead execute systemctl reboot (possibly with the
--no-block option) or call systemd-logind(8)'s
org.freedesktop.login1.Manager.Reboot D-Bus method directly.
runlevel6.target is an alias for this target unit, for compatibility with
SysV.
remote-cryptsetup.target
Similar to cryptsetup.target, but for
encrypted devices which are accessed over the network. It is used for
crypttab(8) entries marked with _netdev.
remote-veritysetup.target
Similar to veritysetup.target, but for verity
integrity protected devices which are accessed over the network. It is used
for veritytab(8) entries marked with _netdev.
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. Compare with multi-user.target, this
target could be seen as single-user.target.
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.
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 always be active
after boot. By default the generic system.slice slice unit as well as the root
slice unit -.slice are pulled in and ordered before this unit (see below).
Adding slice units to slices.target is generally not necessary. Instead, when
some unit that uses Slice= is started, the specified slice will be
started automatically. Adding WantedBy=slices.target 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.
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, system-update-pre.target, system-update-cleanup.service
A special target unit that is used for offline
system updates. systemd-system-update-generator(8) will redirect the
boot process to this target if /system-update exists. For more information see
systemd.offline-updates(7).
Updates should happen before the system-update.target is reached, and the
services which implement them should cause the machine to reboot. The main
units executing the update should order themselves after
system-update-pre.target 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 /system-update still exists after system-update.target is
reached, system-update-cleanup.service will remove this symlink and reboot the
machine.
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.
usb-gadget.target
This target is started automatically as soon
as a USB Device Controller becomes available at boot.
This may be used to pull in usb gadget dynamically when UDC 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. [email protected]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 systemd-cryptsetup@.service(5)/
systemd-veritysetup@.service(5)) that allocate and manage a virtual
block device. Storage services are ordered before an instance of
[email protected], 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
[email protected] instance should be pulled in via a Wants= 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.
[email protected] for the storage device
/dev/mapper/foobar.
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.
veritysetup-pre.target
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.
first-boot-complete.target
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
machine-id(5) 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 ConditionFirstBoot=yes.
getty-pre.target
A special passive target unit. Users of this
target are expected to pull it in the boot transaction via a dependency (e.g.
Wants=). Order your unit before this unit if you want to make use of
the console just before getty is started.
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. However, the following should apply at minimum:
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 network-online.target as
described above.
network-pre.target
•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
network.target is reached.
•At shutdown, a unit that is ordered
after network.target 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
Running Services After the Network Is Up[1] for more information.
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 Running
Services After the Network Is Up[1] for more information.
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 UNIX 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 "$named" facility.
nss-user-lookup.target
A target that should be used as
synchronization point for all regular UNIX user/group name service lookups.
Note that this is independent of host/network name lookups for which
nss-lookup.target should be used. All services for which the availability of
the full user/group database is essential should be ordered after this target,
but not pull it in. All services which provide parts of the user/group
database should be ordered before this target, and pull it in. Note that this
unit is only relevant for regular users and groups — system users and
groups are required to be resolvable during earliest boot already, and hence
do not need any special ordering against this target.
remote-fs-pre.target
This target unit is automatically ordered
before all mount point units (see above) and cryptsetup/veritysetup devices
marked with the _netdev. It can be used to run certain units before
remote encrypted devices and mounts are established. Note that this unit is
generally not part of the initial transaction, unless the unit that wants to
be ordered before all remote mounts pulls it in via a Wants= type
dependency. If the unit wants to be pulled in by the first remote mount
showing up, it should use network-online.target (see above).
rpcbind.target
The portmapper/rpcbind pulls in this target
and orders itself before it, to indicate its availability. systemd
automatically adds dependencies of type After= for this target unit to
all SysV init script service units with an LSB header referring to the
"$portmap" facility.
time-set.target
Services responsible for setting the system
clock ( CLOCK_REALTIME) 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.
This target does not provide the accuracy guarantees of time-sync.target (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.
The service manager automatically adds dependencies of type After= for
this target unit to all timer units with at least one OnCalendar=
directive.
The systemd-timesyncd.service(8) 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.
Note that ordering a unit after time-set.target 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
systemd-timesyncd.service(8), or an alternative NTP implementation to
delay the target.
time-sync.target
Services indicating completed synchronization
of the system clock ( CLOCK_REALTIME) 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.
The service manager 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, as well to all timer units with
at least one OnCalendar= directive.
This target provides stricter clock accuracy guarantees than time-set.target
(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 time-set.target instead.
Note that ordering a unit after time-sync.target 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
systemd-timesyncd.service(8), enable
systemd-time-wait-sync.service(8) to delay the target; or use an
equivalent service for other NTP implementations.
Table 1. Comparison
time-set.target | time-sync.target |
"quick" to reach | "slow" to reach |
typically uses local clock sources, boot process not affected by availability of external resources | typically uses remote clock sources, inserts dependencies on remote resources into boot process |
reliable, because local | unreliable, because typically network involved |
typically guarantees an approximate and roughly monotonic clock only | typically guarantees an accurate clock |
implemented by systemd-timesyncd.service | implemented by systemd-time-wait-sync.service |
Special Slice Units
There are fourThe 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.
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. This is pulled in by systemd-logind.service.
machine.slice
By default, all virtual machines and
containers registered with systemd-machined are found in this slice.
This is pulled in by systemd-machined.service.
UNITS MANAGED BY THE USER SERVICE MANAGER
Special User Units
When systemd runs as a user instance, the following special units are available: default.targetThis 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, default.target is similar
to multi-user.target in the system instance, but it is a real unit, not an
alias.
In addition, the following units are available which have definitions similar to
their system counterparts: exit.target, shutdown.target, sockets.target,
timers.target, paths.target, bluetooth.target, printer.target,
smartcard.target, sound.target.
Special Passive User Units
graphical-session.targetThis 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:
"nautilus.service" gets stopped when the session stops:
graphical-session-pre.target
[Unit] Description=User systemd services for GNOME graphical session Wants=nautilus.service BindsTo=graphical-session.target
[Unit] Description=Render the desktop icons with Nautilus PartOf=graphical-session.target [Service] ...
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.
xdg-desktop-autostart.target
The XDG specification defines a way to
autostart applications using XDG desktop files. systemd ships
systemd-xdg-autostart-generator(8) for the XDG desktop files in
autostart directories. Desktop Environments can opt-in to use this service by
adding a Wants= dependency on xdg-desktop-autostart.target.
Special User Slice Units
There are four ".slice" units which form the basis of the user hierarchy for assignment of resources for user applications and services. See systemd.slice(7) for details about slice units and the documentation about Desktop Environments[3] for further information. -.sliceThe 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.
app.slice
By default, all user services and applications
managed by systemd 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.
session.slice
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
Slice=session.slice to their unit files.
background.slice
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.
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), user@.service(5)NOTES
- 1.
- Running Services After the Network Is Up
- 2.
- Syslog Interface
- 3.
- Desktop Environments
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