SYSTEMD.SPECIAL(7) systemd.special SYSTEMD.SPECIAL(7)
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, 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.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.
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 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. 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 │ typically uses remote │
│sources, boot process not │ clock sources, inserts │
│affected by availability │ dependencies on remote │
│of external resources │ resources into boot │
│ │ process │
├───────────────────────────┼────────────────────────────────┤
│reliable, because local │ unreliable, because │
│ │ typically network involved │
├───────────────────────────┼────────────────────────────────┤
│typically guarantees an │ typically guarantees an │
│approximate and roughly │ accurate 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.
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
https://systemd.io/NETWORK_ONLINE
2. Syslog Interface
https://www.freedesktop.org/wiki/Software/systemd/syslog
3. Desktop Environments
https://systemd.io/DESKTOP_ENVIRONMENTS
systemd 252 SYSTEMD.SPECIAL(7)
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