systemd, init — systemd system and service manager
systemd [OPTIONS...] 
init [OPTIONS...]  {COMMAND} 
systemd is a system and service manager for Linux operating systems. When run as first process on boot (as PID 1), it acts as init system that brings up and maintains userspace services.
For compatibility with SysV, if systemd is called as init and a PID that is not 1, it will execute telinit and pass all command line arguments unmodified. That means init and telinit are mostly equivalent when invoked from normal login sessions. See telinit(8) for more information.
When run as system instance, systemd interprets
                the configuration file
                system.conf, otherwise
                user.conf. See
                systemd-system.conf(5)
                for more information.
The following options are understood:
-h, --help¶Prints a short help text and exits.
--version¶Prints a systemd version identifier and exits.
--test¶Determine startup sequence, dump it and exit. This is an option useful for debugging only.
--dump-configuration-items¶Dump understood unit configuration items. This outputs a terse but complete list of configuration items understood in unit definition files.
--introspect=¶Extract D-Bus interface introspection data. This is mostly useful at install time to generate data suitable for the D-Bus interfaces repository. Optionally the interface name for the introspection data may be specified. If omitted, the introspection data for all interfaces is dumped.
--unit=¶Set default unit to
                                activate on startup. If not specified
                                defaults to
                                default.target.
--system, --user¶For --system,
                                tell systemd to run a
                                system instance, even if the process ID is
                                not 1, i.e. systemd is not run as init process.
                                --user does the opposite,
                                running a user instance even if the process
                                ID is 1.
                                Normally it should not be necessary to
                                pass these options, as systemd
                                automatically detects the mode it is
                                started in. These options are hence of
                                little use except for debugging. Note
                                that it is not supported booting and
                                maintaining a full system with systemd
                                running in --system
                                mode, but PID not 1. In practice,
                                passing --system explicitly is
                                only useful in conjunction with
                                --test.
--dump-core¶Dump core on crash. This switch has no effect when run as user instance.
--crash-shell¶Run shell on crash. This switch has no effect when run as user instance.
--confirm-spawn¶Ask for confirmation when spawning processes. This switch has no effect when run as user instance.
--show-status=¶Show terse service
                                status information while booting. This
                                switch has no effect when run as user
                                instance. Takes a boolean argument
                                which may be omitted which is
                                interpreted as
                                true.
--log-target=¶Set log
                                target. Argument must be one of
                                console,
                                journal,
                                syslog,
                                kmsg,
                                journal-or-kmsg,
                                syslog-or-kmsg,
                                null.
--log-level=¶Set log level. As
                                argument this accepts a numerical log
                                level or the well-known syslog(3)
                                symbolic names (lowercase):
                                emerg,
                                alert,
                                crit,
                                err,
                                warning,
                                notice,
                                info,
                                debug.
--log-color=¶Highlight important
                                log messages. Argument is a boolean
                                value. If the argument is omitted it
                                defaults to
                                true.
--log-location=¶Include code location
                                in log messages. This is mostly
                                relevant for debugging
                                purposes. Argument is a boolean
                                value. If the argument is omitted
                                it defaults to
                                true.
--default-standard-output=, --default-standard-error=¶Sets the default
                                output or error output for all
                                services and sockets, respectively. That is, controls
                                the default for
                                StandardOutput=
                                and StandardError=
                                (see
                                systemd.exec(5)
                                for details). Takes one of
                                inherit,
                                null,
                                tty,
                                journal,
                                journal+console,
                                syslog,
                                syslog+console,
                                kmsg,
                                kmsg+console.  If the
                                argument is omitted
                                --default-standard-output=
                                defaults to journal
                                and
                                --default-standard-error=
                                to
                                inherit.
systemd provides a dependency system between various entities called "units". Units encapsulate various objects that are relevant for system boot-up and maintenance. The majority of units are configured in unit configuration files, whose syntax and basic set of options is described in systemd.unit(5), however some are created automatically from other configuration or dynamically from system state. Units may be 'active' (meaning started, bound, plugged in, ... depending on the unit type, see below), or 'inactive' (meaning stopped, unbound, unplugged, ...), as well as in the process of being activated or deactivated, i.e. between the two states (these states are called 'activating', 'deactivating'). A special 'failed' state is available as well which is very similar to 'inactive' and is entered when the service failed in some way (process returned error code on exit, or crashed, or an operation timed out). If this state is entered the cause will be logged, for later reference. Note that the various unit types may have a number of additional substates, which are mapped to the five generalized unit states described here.
The following unit types are available:
Service units, which control daemons and the processes they consist of. For details see systemd.service(5).
Socket units, which encapsulate local IPC or network sockets in the system, useful for socket-based activation. For details about socket units see systemd.socket(5), for details on socket-based activation and other forms of activation, see daemon(7).
Target units are useful to group units, or provide well-known synchronization points during boot-up, see systemd.target(5).
Device units expose kernel devices in systemd and may be used to implement device-based activation. For details see systemd.device(5).
Mount units control mount points in the file system, for details see systemd.mount(5).
Automount units provide automount capabilities, for on-demand mounting of file systems as well as parallelized boot-up. See systemd.automount(5).
Snapshot units can be used to temporarily save the state of the set of systemd units, which later may be restored by activating the saved snapshot unit. For more information see systemd.snapshot(5).
Timer units are useful for triggering activation of other units based on timers. You may find details in systemd.timer(5).
Swap units are very similar to mount units and encapsulate memory swap partitions or files of the operating system. They are described in systemd.swap(5).
Path units may be used to activate other services when file system objects change or are modified. See systemd.path(5).
Units are named as their configuration files. Some units have special semantics. A detailed list is available in systemd.special(7).
systemd knows various kinds of dependencies,
                including positive and negative requirement
                dependencies (i.e. Requires= and
                Conflicts=) as well as ordering
                dependencies (After= and
                Before=). NB: ordering and
                requirement dependencies are orthogonal. If only a
                requirement dependency exists between two units
                (e.g. foo.service requires
                bar.service), but no ordering
                dependency (e.g. foo.service
                after bar.service) and both are
                requested to start, they will be started in
                parallel. It is a common pattern that both requirement
                and ordering dependencies are placed between two
                units. Also note that the majority of dependencies are
                implicitly created and maintained by systemd. In most
                cases it should be unnecessary to declare additional
                dependencies manually, however it is possible to do
                this.
Application programs and units (via dependencies) may request state changes of units. In systemd, these requests are encapsulated as 'jobs' and maintained in a job queue. Jobs may succeed or can fail, their execution is ordered based on the ordering dependencies of the units they have been scheduled for.
On boot systemd activates the target unit
                default.target whose job is to
                activate on-boot services and other on-boot units by
                pulling them in via dependencies. Usually the unit
                name is just an alias (symlink) for either
                graphical.target (for
                fully-featured boots into the UI) or
                multi-user.target (for limited
                console-only boots for use in embedded or server
                environments, or similar; a subset of
                graphical.target). However it is at the discretion of
                the administrator to configure it as an alias to any
                other target unit. See
                systemd.special(7)
                for details about these target units.
Processes systemd spawns are placed in
                individual Linux control groups named after the unit
                which they belong to in the private systemd
                hierarchy. (see cgroups.txt
                for more information about control groups, or short
                "cgroups"). systemd uses this to effectively keep
                track of processes. Control group information is
                maintained in the kernel, and is accessible via the
                file system hierarchy (beneath
                /sys/fs/cgroup/systemd/), or in tools
                such as
                ps(1)
                (ps xawf -eo pid,user,cgroup,args
                is particularly useful to list all processes and the
                systemd units they belong to.).
systemd is compatible with the SysV init system
                to a large degree: SysV init scripts are supported and
                simply read as an alternative (though limited)
                configuration file format. The SysV
                /dev/initctl interface is
                provided, and compatibility implementations of the
                various SysV client tools are available. In addition to
                that, various established Unix functionality such as
                /etc/fstab or the
                utmp database are
                supported.
systemd has a minimal transaction system: if a unit is requested to start up or shut down it will add it and all its dependencies to a temporary transaction. Then, it will verify if the transaction is consistent (i.e. whether the ordering of all units is cycle-free). If it is not, systemd will try to fix it up, and removes non-essential jobs from the transaction that might remove the loop. Also, systemd tries to suppress non-essential jobs in the transaction that would stop a running service. Finally it is checked whether the jobs of the transaction contradict jobs that have already been queued, and optionally the transaction is aborted then. If all worked out and the transaction is consistent and minimized in its impact it is merged with all already outstanding jobs and added to the run queue. Effectively this means that before executing a requested operation, systemd will verify that it makes sense, fixing it if possible, and only failing if it really cannot work.
Systemd contains native implementations of
                various tasks that need to be executed as part of the
                boot process. For example, it sets the host name or
                configures the loopback network device. It also sets
                up and mounts various API file systems, such as
                /sys or
                /proc.
For more information about the concepts and ideas behind systemd please refer to the Original Design Document.
Note that some but not all interfaces provided by systemd are covered by the Interface Stability Promise.
Units may be generated dynamically at boot and system manager reload time, for example based on other configuration files or parameters passed on the kernel command line. For details see the Generators Specification.
Systems which invoke systemd in a container or initrd environment should implement the Container Interface or initrd Interface specifications, respectively.
The systemd system
                                manager reads unit configuration from
                                various directories. Packages that
                                want to install unit files shall place
                                them in the directory returned by
                                pkg-config systemd
                                --variable=systemdsystemunitdir. Other
                                directories checked are
                                /usr/local/lib/systemd/system
                                and
                                /usr/lib/systemd/system. User
                                configuration always takes
                                precedence. pkg-config
                                systemd
                                --variable=systemdsystemconfdir
                                returns the path of the system
                                configuration directory. Packages
                                should alter the content of these
                                directories only with the
                                enable and
                                disable commands of
                                the
                                systemctl(1)
                                tool. Full list of directories is provided in
                                systemd.unit(5).
                                
Similar rules apply for the user unit directories. However, here the XDG Base Directory specification is followed to find units. Applications should place their unit files in the directory returned by pkg-config systemd --variable=systemduserunitdir. Global configuration is done in the directory reported by pkg-config systemd --variable=systemduserconfdir. The enable and disable commands of the systemctl(1) tool can handle both global (i.e. for all users) and private (for one user) enabling/disabling of units. Full list of directories is provided in systemd.unit(5).
The location of the
                                SysV init script directory varies
                                between distributions. If systemd
                                cannot find a native unit file for a
                                requested service, it will look for a
                                SysV init script of the same name
                                (with the
                                .service suffix
                                removed).
The location of the SysV runlevel link farm directory varies between distributions. systemd will take the link farm into account when figuring out whether a service shall be enabled. Note that a service unit with a native unit configuration file cannot be started by activating it in the SysV runlevel link farm.
Upon receiving this signal the systemd system manager serializes its state, reexecutes itself and deserializes the saved state again. This is mostly equivalent to systemctl daemon-reexec.
systemd user managers will
                                start the
                                exit.target unit
                                when this signal is received. This is
                                mostly equivalent to
                                systemctl --user start
                                exit.target.
Upon receiving this
                                signal the systemd system manager will
                                start the
                                ctrl-alt-del.target unit. This
                                is mostly equivalent to
                                systemctl start
                                ctl-alt-del.target.
systemd user managers treat this signal the same way as SIGTERM.
When this signal is
                                received the systemd system manager
                                will start the
                                kbrequest.target
                                unit. This is mostly equivalent to
                                systemctl start
                                kbrequest.target.
This signal is ignored by systemd user managers.
When this signal is
                                received the systemd manager
                                will start the
                                sigpwr.target
                                unit. This is mostly equivalent to
                                systemctl start
                                sigpwr.target.
When this signal is received the systemd manager will try to reconnect to the D-Bus bus.
When this signal is received the systemd manager will log its complete state in human readable form. The data logged is the same as printed by systemctl dump.
Reloads the complete daemon configuration. This is mostly equivalent to systemctl daemon-reload.
Enters default mode, starts the
                                default.target
                                unit. This is mostly equivalent to
                                systemctl start
                                default.target.
Enters rescue mode,
                                starts the
                                rescue.target
                                unit. This is mostly equivalent to
                                systemctl isolate
                                rescue.target.
Enters emergency mode,
                                starts the
                                emergency.service
                                unit. This is mostly equivalent to
                                systemctl isolate
                                emergency.service.
Halts the machine,
                                starts the
                                halt.target
                                unit. This is mostly equivalent to
                                systemctl start
                                halt.target.
Powers off the machine,
                                starts the
                                poweroff.target
                                unit. This is mostly equivalent to
                                systemctl start
                                poweroff.target.
Reboots the machine,
                                starts the
                                reboot.target
                                unit. This is mostly equivalent to
                                systemctl start
                                reboot.target.
Reboots the machine via kexec,
                                starts the
                                kexec.target
                                unit. This is mostly equivalent to
                                systemctl start
                                kexec.target.
Immediately halts the machine.
Immediately powers off the machine.
Immediately reboots the machine.
Immediately reboots the machine with kexec.
Enables display of
                                status messages on the console, as
                                controlled via
                                systemd.show_status=1
                                on the kernel command
                                line.
Disables display of
                                status messages on the console, as
                                controlled via
                                systemd.show_status=0
                                on the kernel command
                                line.
Sets the log level to
                                debug
                                (or info on
                                SIGRTMIN+23), as
                                controlled via
                                systemd.log_level=debug
                                (or systemd.log_level=info
                                on SIGRTMIN+23) on
                                the kernel command
                                line.
Immediately exits the manager (only available for --user instances).
Sets the log level to
                                journal-or-kmsg
                                (or console on
                                SIGRTMIN+27,
                                kmsg on
                                SIGRTMIN+28,
                                or syslog-or-kmsg
                                on SIGRTMIN+29), as
                                controlled via
                                systemd.log_target=journal-or-kmsg
                                (or systemd.log_target=console
                                on SIGRTMIN+27,
                                systemd.log_target=kmsg
                                on SIGRTMIN+28,
                                or
                                systemd.log_target=syslog-or-kmsg
                                on SIGRTMIN+29) on
                                the kernel command
                                line.
$SYSTEMD_LOG_LEVEL¶systemd reads the
                                log level from this environment
                                variable. This can be overridden with
                                --log-level=.
$SYSTEMD_LOG_TARGET¶systemd reads the
                                log target from this environment
                                variable. This can be overridden with
                                --log-target=.
$SYSTEMD_LOG_COLOR¶Controls whether
                                systemd highlights important log
                                messages. This can be overridden with
                                --log-color=.
$SYSTEMD_LOG_LOCATION¶Controls whether
                                systemd prints the code location along
                                with log messages. This can be
                                overridden with
                                --log-location=.
$XDG_CONFIG_HOME, $XDG_CONFIG_DIRS, $XDG_DATA_HOME, $XDG_DATA_DIRS¶The systemd user manager uses these variables in accordance to the XDG Base Directory specification to find its configuration.
$SYSTEMD_UNIT_PATH¶Controls where systemd looks for unit files.
$SYSTEMD_SYSVINIT_PATH¶Controls where systemd looks for SysV init scripts.
$SYSTEMD_SYSVRCND_PATH¶Controls where systemd looks for SysV init script runlevel link farms.
$LISTEN_PID, $LISTEN_FDS¶Set by systemd for supervised processes during socket-based activation. See sd_listen_fds(3) for more information.
$NOTIFY_SOCKET¶Set by systemd for supervised processes for status and start-up completion notification. See sd_notify(3) for more information.
When run as system instance systemd parses a number of kernel command line arguments[1]:
systemd.unit=, rd.systemd.unit=¶Overrides the unit to
                                activate on boot. Defaults to
                                default.target. This
                                may be used to temporarily boot into a
                                different boot unit, for example
                                rescue.target or
                                emergency.service. See
                                systemd.special(7)
                                for details about these units. The
                                option prefixed with
                                rd. is honored
                                only in the initial RAM disk (initrd),
                                while the one that isn't prefixed only
                                in the main system.
systemd.dump_core=¶Takes a boolean
                                argument. If true
                                systemd dumps core when it
                                crashes. Otherwise no core dump is
                                created. Defaults to
                                true.
systemd.crash_shell=¶Takes a boolean
                                argument. If true
                                systemd spawns a shell when it
                                crashes. Otherwise no shell is
                                spawned. Defaults to
                                false, for security
                                reasons, as the shell is not protected
                                by any password
                                authentication.
systemd.crash_chvt=¶Takes an integer
                                argument. If positive systemd
                                activates the specified virtual
                                terminal when it crashes. Defaults to
                                -1.
systemd.confirm_spawn=¶Takes a boolean
                                argument. If true
                                asks for confirmation when spawning
                                processes. Defaults to
                                false.
systemd.show_status=¶Takes a boolean
                                argument. If true
                                shows terse service status updates on
                                the console during bootup. Defaults to
                                true, unless
                                quiet is passed as
                                kernel command line option in which
                                case it defaults to
                                false.
systemd.log_target=, systemd.log_level=, systemd.log_color=, systemd.log_location=¶Controls log output,
                                with the same effect as the
                                $SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_COLOR, $SYSTEMD_LOG_LOCATION
                                environment variables described above.
systemd.default_standard_output=, systemd.default_standard_error=¶Controls default
                                standard output and error output for
                                services, with the same effect as the
                                --default-standard-output=
                                and --default-standard-error=
                                command line arguments described
                                above, respectively.
systemd.setenv=¶Takes a string argument in the form VARIABLE=VALUE. May be used to set environment variables for the init process and all its children at boot time. May be used more than once to set multiple variables. If the equal sign and variable are missing it unsets an environment variable which might be passed in from the initial ram disk.
quiet¶If passed turns off
                                status output at boot, much like
                                systemd.show_status=false
                                would. Note that this option is also
                                read by the kernel itself and disables
                                kernel log output to the
                                kernel. Passing this option hence
                                turns off the usual output from both
                                the system manager and the
                                kernel.
emergency¶Boot into emergency
                                mode. This is equivalent to
                                systemd.unit=emergency.target
                                and provided for compatibility
                                reasons and to be easier to type.
single, s, S, 1¶Boot into rescue
                                mode. This is equivalent to
                                systemd.unit=rescue.target
                                and provided for compatibility reasons
                                and to be easier to
                                type.
2, 3, 4, 5¶Boot into the
                                specified legacy SysV runlevel. These
                                are equivalent to
                                systemd.unit=runlevel2.target,
                                systemd.unit=runlevel3.target,
                                systemd.unit=runlevel4.target,
                                and systemd.unit=runlevel5.target, respectively,
                                and provided for compatibility reasons
                                and to be easier to
                                type.
locale.LANG=, locale.LANGUAGE=, locale.LC_CTYPE=, locale.LC_NUMERIC=, locale.LC_TIME=, locale.LC_COLLATE=, locale.LC_MONETARY=, locale.LC_MESSAGES=, locale.LC_PAPER=, locale.LC_NAME=, locale.LC_ADDRESS=, locale.LC_TELEPHONE=, locale.LC_MEASUREMENT=, locale.LC_IDENTIFICATION=¶Set the system locale
                                to use. This overrides the settings in
                                /etc/locale.conf. For
                                more information see
                                locale.conf(5)
                                and
                                locale(7).
                                
For other kernel command line parameters understood by components of the core OS, please refer to kernel-command-line(7).
/run/systemd/notify¶Daemon status notification socket. This is an AF_UNIX datagram socket and is used to implement the daemon notification logic as implemented by sd_notify(3).
/run/systemd/shutdownd¶Used internally by the shutdown(8) tool to implement delayed shutdowns. This is an AF_UNIX datagram socket.
/run/systemd/private¶Used internally as communication channel between systemctl(1) and the systemd process. This is an AF_UNIX stream socket. This interface is private to systemd and should not be used in external projects.
/dev/initctl¶Limited compatibility
                                support for the SysV client interface,
                                as implemented by the
                                systemd-initctl.service
                                unit. This is a named pipe in the file
                                system. This interface is obsolete and
                                should not be used in new
                                applications.
systemd-system.conf(5), locale.conf(5), systemctl(1), journalctl(1), systemd-notify(1), daemon(7), sd-daemon(3), systemd.unit(5), systemd.special(5), pkg-config(1), kernel-command-line(7), bootup(7), systemd.directives(7)
[1] If run inside a Linux
                container these arguments may be passed as command
                line arguments to systemd itself, next to any of the
                command line options listed in the Options section
                above. If run outside of Linux containers, these
                arguments are parsed from
                /proc/cmdline
                instead.