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 │ typically uses remote clock │
│sources, boot process not │ sources, inserts dependencies on │
│affected by availability of │ remote resources into boot │
│external resources │ process │
├────────────────────────────┼──────────────────────────────────┤
│reliable, because local │ unreliable, because typically │
│ │ network involved │
├────────────────────────────┼──────────────────────────────────┤
│typically guarantees an │ typically guarantees an accurate │
│approximate and roughly │ clock │
│monotonic clock only │ │
├────────────────────────────┼──────────────────────────────────┤
│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