Linux "systemd.special" Command Line Options and Examples
Special systemd units
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.SPECIAL SYSTEM UNITS -.
The root mount point, i.e. the mount unit for the / path. This unit is unconditionally active, during theentire time the system is up, as this mount point is where the basic userspace is running from.basic.targetA special target unit covering basic boot-up.systemd automatically adds dependency of the type After= for this target unit to all services (except forthose 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 mentionedmount 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 viaother early boot targets. It is instead meant as a synchronization point for late boot services. Refer tobootup(7) for details on the targets involved.ctrl-alt-del.targetsystemd starts this target whenever Control+Alt+Del is pressed on the console. Usually, this should bealiased (symlinked) to reboot.target.cryptsetup.targetA target that pulls in setup services for all encrypted block devices.dbus.serviceA special unit for the D-Bus bus daemon. As soon as this service is fully started up systemd will connectto it and register its service.dbus.socketA special unit for the D-Bus system bus socket. All units with Type=dbus automatically gain a dependencyon this unit.default.targetThe default unit systemd starts at bootup. Usually, this should be aliased (symlinked) tomulti-user.target or graphical.target.The default unit systemd starts at bootup can be overridden with the systemd.unit= kernel command lineoption.display-manager.serviceThe display manager service. Usually, this should be aliased (symlinked) to gdm.service or a similardisplay manager service.emergency.targetA special target unit that starts an emergency shell on the main console. This target does not pull in anyservices or mounts. It is the most minimal version of starting the system in order to acquire aninteractive shell; the only processes running are usually just the system manager (PID 1) and the shellprocess. This unit is supposed to be used with the kernel command line option systemd.unit=; it is alsoused when a file system check on a required file system fails, and boot-up cannot continue. Compare withrescue.target, which serves a similar purpose, but also starts the most basic services and mounts all filesystems.Use the "systemd.unit=emergency.target" kernel command line option to boot into this mode. A short aliasfor this kernel command line option is "emergency", for compatibility with SysV.In many ways booting into emergency.target is similar to the effect of booting with "init=/bin/sh" on thekernel command line, except that emergency mode provides you with the full system and service manager, andallows starting individual units in order to continue the boot process in steps.exit.targetA special service unit for shutting down the system or user service manager. It is equivalent topoweroff.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 servicedaemon.Normally, this (indirectly) pulls in shutdown.target, which in turn should be conflicted by all units thatwant to be scheduled for shutdown when the service manager starts to exit.final.targetA special target unit that is used during the shutdown logic and may be used to pull in late servicesafter all normal services are already terminated and all mounts unmounted.getty.targetA special target unit that pulls in statically configured local TTY getty instances.graphical.targetA 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 (ormulti-user.target) during installation. This is best configured via WantedBy=graphical.target in theunit's "[Install]" section.hibernate.targetA special target unit for hibernating the system. This pulls in sleep.target.hybrid-sleep.targetA special target unit for hibernating and suspending the system at the same time. This pulls insleep.target.suspend-then-hibernate.targetA special target unit for suspending the system for a period of time, waking it and putting it intohibernate. This pulls in sleep.target.halt.targetA special target unit for shutting down and halting the system. Note that this target is distinct frompoweroff.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 executesystemctl halt (possibly with the --no-block option) or call systemd(1)'sorg.freedesktop.systemd1.Manager.Halt D-Bus method directly.init.scopeThis scope unit is where the system and service manager (PID 1) itself resides. It is active as long asthe system is running.initrd-fs.targetsystemd-fstab-generator(3) automatically adds dependencies of type Before= to sysroot-usr.mount and allmount points found in /etc/fstab that have x-initrd.mount and not have noauto mount options set.initrd-root-device.targetA special initrd target unit that is reached when the root filesystem device is available, but before ithas been mounted. systemd-fstab-generator(3) and systemd-gpt-auto-generator(3) automatically setup theappropriate dependencies to make this happen.initrd-root-fs.targetsystemd-fstab-generator(3) automatically adds dependencies of type Before= to the sysroot.mount unit,which is generated from the kernel command line.kbrequest.targetsystemd starts this target whenever Alt+ArrowUp is pressed on the console. Note that any user withphysical access to the machine will be able to do this, without authentication, so this should be usedcarefully.kexec.targetA 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 executesystemctl kexec (possibly with the --no-block option) or call systemd(1)'sorg.freedesktop.systemd1.Manager.KExec D-Bus method directly.local-fs.targetsystemd-fstab-generator(3) automatically adds dependencies of type Before= to all mount units that referto local mount points for this target unit. In addition, it adds dependencies of type Wants= to thistarget unit for those mounts listed in /etc/fstab that have the auto mount option set.machines.targetA standard target unit for starting all the containers and other virtual machines. Seesystemd-nspawn@.service for an example.multi-user.targetA special target unit for setting up a multi-user system (non-graphical). This is pulled in bygraphical.target.Units that are needed for a multi-user system shall add Wants= dependencies for their unit to this unitduring installation. This is best configured via WantedBy=multi-user.target in the unit's "[Install]"section.network-online.targetUnits that strictly require a configured network connection should pull in network-online.target (via aWants= type dependency) and order themselves after it. This target unit is intended to pull in a servicethat delays further execution until the network is sufficiently set up. What precisely this requires isleft 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 bythe consumer rather than the provider of this functionality) and pulls in a service which possibly addssubstantial delays to further execution. In contrast, network.target is a passive unit (i.e. pulled in bythe 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, exceptwhen at least one unit requires it. Also see Running Services After the Network is up for moreinformation.All mount units for remote network file systems automatically pull in this unit, and order themselvesafter it. Note that networking daemons that simply provide functionality to other hosts generally do notneed to pull this in.Note that this unit is only useful during the original system start-up logic. After the system hascompleted booting up, it will not track the online state of the system anymore. Due to this it cannot beused as a network connection monitor concept, it is purely a one-time system start-up concept.paths.targetA special target unit that sets up all path units (see systemd.path(5) for details) that shall be activeafter boot.It is recommended that path units installed by applications get pulled in via Wants= dependencies fromthis unit. This is best configured via a WantedBy=paths.target in the path unit's "[Install]" section.poweroff.targetA special target unit for shutting down and powering off the system.Applications wanting to reboot the system should not start this unit directly, but should instead executesystemctl poweroff (possibly with the --no-block option) or call systemd-logind(8)'sorg.freedesktop.login1.Manager.PowerOff D-Bus method directly.runlevel0.target is an alias for this target unit, for compatibility with SysV.reboot.targetA 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 executesystemctl reboot (possibly with the --no-block option) or call systemd-logind(8)'sorg.freedesktop.login1.Manager.Reboot D-Bus method directly.runlevel6.target is an alias for this target unit, for compatibility with SysV.remote-cryptsetup.targetSimilar to cryptsetup.target, but for encrypted devices which are accessed over the network. It is usedfor crypttab(8) entries marked with _netdev.remote-fs.targetSimilar to local-fs.target, but for remote mount points.systemd automatically adds dependencies of type After= for this target unit to all SysV init scriptservice units with an LSB header referring to the "$remote_fs" facility.rescue.targetA 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 mountedbut with no services running, except for the most basic. Compare with emergency.target, which is much morereduced and does not provide the file systems or most basic services. Compare with multi-user.target, thistarget 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 forthis kernel command line option is "1", for compatibility with SysV.runlevel2.target, runlevel3.target, runlevel4.target, runlevel5.targetThese 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 runlevel5) or multi-user.target (the others).shutdown.targetA 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 thisunit for their service unit, which is implicitly done when DefaultDependencies=yes is set (the default).sigpwr.targetA special target that is started when systemd receives the SIGPWR process signal, which is normally sentby the kernel or UPS daemons when power fails.sleep.targetA special target unit that is pulled in by suspend.target, hibernate.target and hybrid-sleep.target andmay be used to hook units into the sleep state logic.slices.targetA special target unit that sets up all slice units (see systemd.slice(5) for details) that shall be activeafter boot. By default the generic system.slice slice unit, as well as the root slice unit -.slice, ispulled in and ordered before this unit (see below).It's a good idea to add WantedBy=slices.target lines to the "[Install]" section of all slices units thatmay be installed dynamically.sockets.targetA special target unit that sets up all socket units (see systemd.socket(5) for details) that shall beactive after boot.Services that can be socket-activated shall add Wants= dependencies to this unit for their socket unitduring installation. This is best configured via a WantedBy=sockets.target in the socket unit's"[Install]" section.suspend.targetA special target unit for suspending the system. This pulls in sleep.target.swap.targetSimilar to local-fs.target, but for swap partitions and swap files.sysinit.targetsystemd automatically adds dependencies of the types Requires= and After= for this target unit to allservices (except for those with DefaultDependencies=no).This target pulls in the services required for system initialization. System services pulled in by thistarget should declare DefaultDependencies=no and specify all their dependencies manually, including accessto anything more than a read only root filesystem. For details on the dependencies of this target, referto bootup(7).syslog.socketThe socket unit syslog implementations should listen on. All userspace log messages will be made availableon this socket. For more information about syslog integration, please consult the Syslog Interfacedocument.system-update.target, system-update-cleanup.serviceA special target unit that is used for offline system updates. systemd-system-update-generator(8) willredirect the boot process to this target if /system-update exists. For more information seesystemd.offline-updates(7).Updates should happen before the system-update.target is reached, and the services which implement themshould cause the machine to reboot. As a safety measure, if this does not happen, and /system-update stillexists after system-update.target is reached, system-update-cleanup.service will remove this symlink andreboot the machine.timers.targetA special target unit that sets up all timer units (see systemd.timer(5) for details) that shall be activeafter boot.It is recommended that timer units installed by applications get pulled in via Wants= dependencies fromthis unit. This is best configured via WantedBy=timers.target in the timer unit's "[Install]" section.umount.targetA 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 theirmount unit, which is implicitly done when DefaultDependencies=yes is set (the default).SPECIAL SYSTEM UNITS FOR DEVICESSome target units are automatically pulled in as devices of certain kinds show up in the system. These may beused to automatically activate various services based on the specific type of the available hardware.bluetooth.targetThis target is started automatically as soon as a Bluetooth controller is plugged in or becomes availableat boot.This may be used to pull in Bluetooth management daemons dynamically when Bluetooth hardware is found.printer.targetThis 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.targetThis target is started automatically as soon as a smartcard controller is plugged in or becomes availableat boot.This may be used to pull in smartcard management daemons dynamically when smartcard hardware is found.sound.targetThis 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.SPECIAL PASSIVE SYSTEM UNITSA number of special system targets are defined that can be used to properly order boot-up of optionalservices. These targets are generally not part of the initial boot transaction, unless they are explicitlypulled in by one of the implementing services. Note specifically that these passive target units are generallynot pulled in by the consumer of a service, but by the provider of the service. This means: a consumingservice should order itself after these targets (as appropriate), but not pull it in. A providing serviceshould 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 failwith an error. They can only be pulled in by dependency. This is enforced since they exist for orderingpurposes only and thus are not useful as only unit within a transaction.cryptsetup-pre.targetThis passive target unit may be pulled in by services that want to run before any encrypted block deviceis set up. All encrypted block devices are set up after this target has been reached. Since the shutdownorder is implicitly the reverse start-up order between units, this target is particularly useful to ensurethat a service is shut down only after all encrypted block devices are fully stopped.getty-pre.targetA special passive target unit. Users of this target are expected to pull it in the boot transaction via adependency (e.g. Wants=). Order your unit before this unit if you want to make use of the console justbefore getty is started.local-fs-pre.targetThis target unit is automatically ordered before all local mount points marked with auto (see above). Itcan be used to execute certain units before all local mounts.network.targetThis unit is supposed to indicate when network functionality is available, but it is only very weaklydefined what that is supposed to mean, with one exception: at shutdown, a unit that is ordered afternetwork.target will be stopped before the network — to whatever level it might be set up then — is shutdown. It is hence useful when writing service files that require network access on shutdown, which shouldorder themselves after this target, but not pull it in. Also see Running Services After the Network isup for more information. Also see network-online.target described above.systemd automatically adds dependencies of type After= for this target unit to all SysV init scriptservice units with an LSB header referring to the "$network" facility.network-pre.targetThis passive target unit may be pulled in by services that want to run before any network is set up, forexample for the purpose of setting up a firewall. All network management software orders itself after thistarget, but does not pull it in.nss-lookup.targetA target that should be used as synchronization point for all host/network name service lookups. Note thatthis is independent of user/group name lookups for which nss-user-lookup.target should be used. Allservices for which the availability of full host/network name resolution is essential should be orderedafter this target, but not pull it in. systemd automatically adds dependencies of type After= for thistarget unit to all SysV init script service units with an LSB header referring to the "$named" facility.nss-user-lookup.targetA target that should be used as synchronization point for all user/group name service lookups. Note thatthis is independent of host/network name lookups for which nss-lookup.target should be used. All servicesfor which the availability of the full user/group database is essential should be ordered after thistarget, but not pull it in. Note that system users are always resolvable, and hence do not require anyspecial ordering against this target.remote-fs-pre.targetThis target unit is automatically ordered before all mount point units (see above) and cryptsetup devicesmarked with the _netdev. It can be used to run certain units before remote encrypted devices and mountsare established. Note that this unit is generally not part of the initial transaction, unless the unitthat wants to be ordered before all remote mounts pulls it in via a Wants= type dependency. If the unitwants to be pulled in by the first remote mount showing up, it should use network-online.target (seeabove).rpcbind.targetThe portmapper/rpcbind pulls in this target and orders itself before it, to indicate its availability.systemd automatically adds dependencies of type After= for this target unit to all SysV init scriptservice units with an LSB header referring to the "$portmap" facility.time-sync.targetServices responsible for synchronizing the system clock from a remote source (such as NTP clientimplementations) should pull in this target and order themselves before it. All services where correcttime is essential should be ordered after this unit, but not pull it in. systemd automatically addsdependencies of type After= for this target unit to all SysV init script service units with an LSB headerreferring to the "$time" facility.SPECIAL USER UNITSWhen systemd runs as a user instance, the following special units are available, which have similardefinitions as their system counterparts: exit.target, default.target, shutdown.target, sockets.target,timers.target, paths.target, bluetooth.target, printer.target, smartcard.target, sound.target.SPECIAL PASSIVE USER UNITSgraphical-session.targetThis target is active whenever any graphical session is running. It is used to stop user services whichonly apply to a graphical (X, Wayland, etc.) session when the session is terminated. Such services shouldhave "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, seesystemd.unit(5). Those symlinks should either be shipped in packages, or should be added dynamically afterinstallation, 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-levelservice:[Unit]Description=User systemd services for GNOME graphical sessionWants=nautilus.serviceBindsTo=graphical-session.target"nautilus.service" gets stopped when the session stops:[Unit]Description=Render the desktop icons with NautilusPartOf=graphical-session.target[Service]...graphical-session-pre.targetThis 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) ormigration of obsolete d-conf keys after an OS upgrade (which needs to happen before starting any processthat might use them). This target must be started before starting a graphical session likegnome-session.target.SPECIAL SLICE UNITSThere 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.
systemd.special -.mount ...
The root slice is the root of the slice hierarchy. It usually does not contain units directly, but may beused to set defaults for the whole tree.system.sliceBy default, all system services started by systemd are found in this slice.user.sliceBy default, all user processes and services started on behalf of the user, including the per-user systemdinstance are found in this slice. This is pulled in by systemd-logind.servicemachine.sliceBy default, all virtual machines and containers registered with systemd-machined are found in this slice.This is pulled in by systemd-machined.service