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 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
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
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
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 │ typically uses remote │
│clock sources, boot │ clock sources, inserts │
│process not affected by │ dependencies on remote │
│availability of external │ resources into boot │
│resources │ 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.
