systemd.special - Special systemd units
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
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¶
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.
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.
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).
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.
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.
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.
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.
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.
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.
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 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 "$network" facility.
Note that this unit is only useful during the original system start-up logic. After the system has completed booting up, it will not track the online state of the system anymore. Due to this it cannot be used as a network connection monitor concept, it is purely a one-time system start-up concept.
It is recommended that path units installed by applications get pulled in via Wants= dependencies from this unit. This is best configured via a WantedBy=paths.target in the path unit's [Install] section.
Applications wanting to power off the system should not start this unit directly, but should instead execute systemctl poweroff (possibly with the --no-block option) or call systemd-logind(8)'s org.freedesktop.login1.Manager.PowerOff D-Bus method directly.
runlevel0.target is an alias for this target unit, for compatibility with SysV.
Applications wanting to reboot the system should not start this unit directly, but should instead execute systemctl reboot (possibly with the --no-block option) or call systemd-logind(8)'s org.freedesktop.login1.Manager.Reboot D-Bus method directly.
runlevel6.target is an alias for this target unit, for compatibility with SysV.
systemd automatically adds dependencies of type After= for this target unit to all SysV init script service units with an LSB header referring to the "$remote_fs" facility.
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
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).
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.
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.
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).
system-update.target, system-update-pre.target, system-update-cleanup.service
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.
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.
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.
This may be used to pull in Bluetooth management daemons dynamically when Bluetooth hardware is found.
This may be used to pull in printer management daemons dynamically when printer hardware is found.
This may be used to pull in smartcard management daemons dynamically when smartcard hardware is found.
This may be used to pull in audio management daemons dynamically when audio hardware is found.
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.
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.
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.
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
|"quick" to reach||"slow" to reach|
|typically uses local clock sources, boot process not affected by availability of external resources||typically uses remote clock sources, inserts dependencies on remote resources into boot process|
|reliable, because local||unreliable, because typically network involved|
|typically guarantees an approximate and roughly monotonic clock only||typically guarantees an accurate clock|
|implemented by systemd-timesyncd.service||implemented by systemd-time-wait-sync.service|
Special Slice Units¶
There are 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.
UNITS MANAGED BY THE USER SERVICE MANAGER¶
Special User Units¶
When systemd runs as a user instance, the following special units are available:
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¶
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] ...
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 for further information.
- Running Services After the Network is up
- Syslog Interface
- Desktop Environments