Ubuntu Manpage: systemd.special - Special systemd units

Provided by: systemd_249.11-0ubuntu3.12_amd64

NAME

       systemd.special - Special systemd units

SYNOPSIS

       basic.target, bluetooth.target, cryptsetup-pre.target, cryptsetup.target, veritysetup-pre.target,
       veritysetup.target, ctrl-alt-del.target, blockdev@.target, boot-complete.target, default.target,
       emergency.target, exit.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,
       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.service

DESCRIPTION

       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
-.mount
The 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.

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 initramfs, 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
systemd-nspawn@.service 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.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.

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.

blockdev@.target
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 blockdev@.target, 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 blockdev@.target 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. blockdev@dev-mapper-foobar.target 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:

• 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.

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
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 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, │ typically uses remote clock sources, │
│boot process not affected by │ inserts dependencies on remote │
│availability of external resources │ resources into boot process │
├────────────────────────────────────┼───────────────────────────────────────┤
│reliable, because local │ unreliable, because typically network │
│ │ involved │
├────────────────────────────────────┼───────────────────────────────────────┤
│typically guarantees an approximate │ typically guarantees an accurate │
│and roughly monotonic clock only │ clock │
├────────────────────────────────────┼───────────────────────────────────────┤
│implemented by │ implemented by │
│systemd-timesyncd.service │ systemd-time-wait-sync.service │
└────────────────────────────────────┴───────────────────────────────────────┘

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. See systemd.slice(7) for details about slice units.

-.slice
The root slice is the root of the slice hierarchy. It usually does not contain units directly, but
may be used to set defaults for the whole tree.

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.target
This is the main target of the user session, started by default. Various services that compose the
normal user session should be pulled into this target. In this regard, 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.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.

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.

-.slice
The root slice is the root of the user's slice hierarchy. It usually does not contain units directly,
but may be used to set defaults for the whole tree.

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.

NOTES