Ubuntu Manpage: systemd, init - systemd system and service manager

Provided by: systemd_255.4-1ubuntu8.5_amd64

NAME

       systemd, init - systemd system and service manager

SYNOPSIS

       /usr/lib/systemd/systemd [OPTIONS...]

       init [OPTIONS...] {COMMAND}

DESCRIPTION

       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. Separate instances
       are started for logged-in users to start their services.

       systemd is usually not invoked directly by the user, but is installed as the /sbin/init symlink and
       started during early boot. The user manager instances are started automatically through the
       user@.service(5) service.

       For compatibility with SysV, if the binary is called as init and is not the first process on the machine
       (PID 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 a system instance, systemd interprets the configuration file system.conf and the files in
       system.conf.d directories; when run as a user instance, systemd interprets the configuration file
       user.conf and the files in user.conf.d directories. See systemd-system.conf(5) for more information.

CONCEPTS

systemd provides a dependency system between various entities called "units" of 11 different types. 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 files, dynamically from
system state or programmatically at runtime. 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, an operation timed out, or after too many restarts). 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:

1. Service units, which start and control daemons and the processes they consist of. For details, see
systemd.service(5).

2. 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).

3. Target units are useful to group units, or provide well-known synchronization points during boot-up,
see systemd.target(5).

4. Device units expose kernel devices in systemd and may be used to implement device-based activation.
For details, see systemd.device(5).

5. Mount units control mount points in the file system, for details see systemd.mount(5).

6. Automount units provide automount capabilities, for on-demand mounting of file systems as well as
parallelized boot-up. See systemd.automount(5).

7. Timer units are useful for triggering activation of other units based on timers. You may find details
in systemd.timer(5).

8. 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).

9. Path units may be used to activate other services when file system objects change or are modified.
See systemd.path(5).

10. Slice units may be used to group units which manage system processes (such as service and scope
units) in a hierarchical tree for resource management purposes. See systemd.slice(5).

11. Scope units are similar to service units, but manage foreign processes instead of starting them as
well. See systemd.scope(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.

On first boot, systemd will enable or disable units according to preset policy. See systemd.preset(5) and
"First Boot Semantics" in machine-id(5).

systemd only keeps a minimal set of units loaded into memory. Specifically, the only units that are kept
loaded into memory are those for which at least one of the following conditions is true:

1. It is in an active, activating, deactivating or failed state (i.e. in any unit state except for
"inactive")

2. It has a job queued for it

3. It is a dependency of at least one other unit that is loaded into memory

4. It has some form of resource still allocated (e.g. a service unit that is inactive but for which a
process is still lingering that ignored the request to be terminated)

5. It has been pinned into memory programmatically by a D-Bus call

systemd will automatically and implicitly load units from disk — if they are not loaded yet — as soon as
operations are requested for them. Thus, in many respects, the fact whether a unit is loaded or not is
invisible to clients. Use systemctl list-units --all to comprehensively list all units currently loaded.
Any unit for which none of the conditions above applies is promptly unloaded. Note that when a unit is
unloaded from memory its accounting data is flushed out too. However, this data is generally not lost, as
a journal log record is generated declaring the consumed resources whenever a unit shuts down.

Processes systemd spawns are placed in individual Linux control groups named after the unit which they
belong to in the private systemd hierarchy. (see Control Groups v2[1] 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/), or in tools such as systemd-cgls(1) or 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.

Note that transactions are generated independently of a unit's state at runtime, hence, for example, if a
start job is requested on an already started unit, it will still generate a transaction and wake up any
inactive dependencies (and cause propagation of other jobs as per the defined relationships). This is
because the enqueued job is at the time of execution compared to the target unit's state and is marked
successful and complete when both satisfy. However, this job also pulls in other dependencies due to the
defined relationships and thus leads to, in our example, start jobs for any of those inactive units
getting queued as well.

systemd contains native implementations of various tasks that need to be executed as part of the boot
process. For example, it sets the hostname 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[2].

Note that some but not all interfaces provided by systemd are covered by the Interface Portability and
Stability Promise[3].

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
systemd.generator(7).

The D-Bus API of systemd is described in org.freedesktop.systemd1(5) and org.freedesktop.LogControl1(5).

Systems which invoke systemd in a container or initrd environment should implement the Container
Interface[4] or initrd Interface[5] specifications, respectively.

DIRECTORIES

System unit directories
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).

User unit directories
Similar rules apply for the user unit directories. However, here the XDG Base Directory
specification[6] 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).

SysV init scripts directory
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).

SysV runlevel link farm directory
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.

SIGNALS

SIGTERM
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 --job-mode=replace-irreversibly.

SIGINT
Upon receiving this signal the systemd system manager will start the ctrl-alt-del.target unit. This
is mostly equivalent to systemctl start ctrl-alt-del.target --job-mode=replace-irreversibly. If this
signal is received more than 7 times per 2s, an immediate reboot is triggered. Note that pressing
Ctrl+Alt+Del on the console will trigger this signal. Hence, if a reboot is hanging, pressing
Ctrl+Alt+Del more than 7 times in 2 seconds is a relatively safe way to trigger an immediate reboot.

systemd user managers treat this signal the same way as SIGTERM.

SIGWINCH
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.

SIGPWR
When this signal is received the systemd manager will start the sigpwr.target unit. This is mostly
equivalent to systemctl start sigpwr.target.

SIGUSR1
When this signal is received the systemd manager will try to reconnect to the D-Bus bus.

SIGUSR2
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 systemd-analyze dump.

SIGHUP
Reloads the complete daemon configuration. This is mostly equivalent to systemctl daemon-reload.

SIGRTMIN+0
Enters default mode, starts the default.target unit. This is mostly equivalent to systemctl isolate
default.target.

SIGRTMIN+1
Enters rescue mode, starts the rescue.target unit. This is mostly equivalent to systemctl isolate
rescue.target.

SIGRTMIN+2
Enters emergency mode, starts the emergency.service unit. This is mostly equivalent to systemctl
isolate emergency.service.

SIGRTMIN+3
Halts the machine, starts the halt.target unit. This is mostly equivalent to systemctl start
halt.target --job-mode=replace-irreversibly.

SIGRTMIN+4
Powers off the machine, starts the poweroff.target unit. This is mostly equivalent to systemctl start
poweroff.target --job-mode=replace-irreversibly.

SIGRTMIN+5
Reboots the machine, starts the reboot.target unit. This is mostly equivalent to systemctl start
reboot.target --job-mode=replace-irreversibly.

SIGRTMIN+6
Reboots the machine via kexec, starts the kexec.target unit. This is mostly equivalent to systemctl
start kexec.target --job-mode=replace-irreversibly.

SIGRTMIN+7
Reboots userspace, starts the soft-reboot.target unit. This is mostly equivalent to systemctl start
soft-reboot.target --job-mode=replace-irreversibly.

Added in version 254.

SIGRTMIN+13
Immediately halts the machine.

SIGRTMIN+14
Immediately powers off the machine.

SIGRTMIN+15
Immediately reboots the machine.

SIGRTMIN+16
Immediately reboots the machine with kexec.

SIGRTMIN+17
Immediately reboots the userspace.

Added in version 254.

SIGRTMIN+20
Enables display of status messages on the console, as controlled via systemd.show_status=1 on the
kernel command line.

SIGRTMIN+21
Disables display of status messages on the console, as controlled via systemd.show_status=0 on the
kernel command line.

SIGRTMIN+22
Sets the service manager's log level to "debug", in a fashion equivalent to systemd.log_level=debug
on the kernel command line.

SIGRTMIN+23
Restores the log level to its configured value. The configured value is derived from – in order of
priority – the value specified with systemd.log-level= on the kernel command line, or the value
specified with LogLevel= in the configuration file, or the built-in default of "info".

Added in version 239.

SIGRTMIN+24
Immediately exits the manager (only available for --user instances).

Added in version 195.

SIGRTMIN+25
Upon receiving this signal the systemd manager will reexecute itself. This is mostly equivalent to
systemctl daemon-reexec except that it will be done asynchronously.

The systemd system manager treats this signal the same way as SIGTERM.

Added in version 250.

SIGRTMIN+26
Restores the log target to its configured value. The configured value is derived from – in order of
priority – the value specified with systemd.log-target= on the kernel command line, or the value
specified with LogTarget= in the configuration file, or the built-in default.

Added in version 239.

SIGRTMIN+27, SIGRTMIN+28
Sets the log target to "console" on SIGRTMIN+27 (or "kmsg" on SIGRTMIN+28), in a fashion equivalent
to systemd.log_target=console (or systemd.log_target=kmsg on SIGRTMIN+28) on the kernel command line.

Added in version 239.

ENVIRONMENT

The environment block for the system manager is initially set by the kernel. (In particular, "key=value"
assignments on the kernel command line are turned into environment variables for PID 1). For the user
manager, the system manager sets the environment as described in the "Environment Variables in Spawned
Processes" section of systemd.exec(5). The DefaultEnvironment= setting in the system manager applies to
all services including user@.service. Additional entries may be configured (as for any other service)
through the Environment= and EnvironmentFile= settings for user@.service (see systemd.exec(5)). Also,
additional environment variables may be set through the ManagerEnvironment= setting in systemd-
system.conf(5) and systemd-user.conf(5).

Some of the variables understood by systemd:

$SYSTEMD_LOG_LEVEL
The maximum log level of emitted messages (messages with a higher log level, i.e. less important
ones, will be suppressed). Either one of (in order of decreasing importance) emerg, alert, crit, err,
warning, notice, info, debug, or an integer in the range 0...7. See syslog(3) for more information.

This can be overridden with --log-level=.

$SYSTEMD_LOG_COLOR
A boolean. If true, messages written to the tty will be colored according to priority.

This can be overridden with --log-color=.

$SYSTEMD_LOG_TIME
A boolean. If true, console log messages will be prefixed with a timestamp.

This can be overridden with --log-time=.

Added in version 246.

$SYSTEMD_LOG_LOCATION
A boolean. If true, messages will be prefixed with a filename and line number in the source code
where the message originates.

This can be overridden with --log-location=.

$SYSTEMD_LOG_TID
A boolean. If true, messages will be prefixed with the current numerical thread ID (TID).

Added in version 247.

$SYSTEMD_LOG_TARGET
The destination for log messages. One of console (log to the attached tty), console-prefixed (log to
the attached tty but with prefixes encoding the log level and "facility", see syslog(3), kmsg (log to
the kernel circular log buffer), journal (log to the journal), journal-or-kmsg (log to the journal if
available, and to kmsg otherwise), auto (determine the appropriate log target automatically, the
default), null (disable log output).

This can be overridden with --log-target=.

$SYSTEMD_LOG_RATELIMIT_KMSG
Whether to ratelimit kmsg or not. Takes a boolean. Defaults to "true". If disabled, systemd will not
ratelimit messages written to kmsg.

Added in version 254.

$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[6] to find its configuration.

$SYSTEMD_UNIT_PATH, $SYSTEMD_GENERATOR_PATH, $SYSTEMD_ENVIRONMENT_GENERATOR_PATH
Controls where systemd looks for unit files and generators.

These variables may contain a list of paths, separated by colons (":"). When set, if the list ends
with an empty component ("...:"), this list is prepended to the usual set of paths. Otherwise, the
specified list replaces the usual set of paths.

$SYSTEMD_PAGER
Pager to use when --no-pager is not given; overrides $PAGER. If neither $SYSTEMD_PAGER nor $PAGER are
set, a set of well-known pager implementations are tried in turn, including less(1) and more(1),
until one is found. If no pager implementation is discovered no pager is invoked. Setting this
environment variable to an empty string or the value "cat" is equivalent to passing --no-pager.

Note: if $SYSTEMD_PAGERSECURE is not set, $SYSTEMD_PAGER (as well as $PAGER) will be silently
ignored.

$SYSTEMD_LESS
Override the options passed to less (by default "FRSXMK").

Users might want to change two options in particular:

K
This option instructs the pager to exit immediately when Ctrl+C is pressed. To allow less to
handle Ctrl+C itself to switch back to the pager command prompt, unset this option.

If the value of $SYSTEMD_LESS does not include "K", and the pager that is invoked is less, Ctrl+C
will be ignored by the executable, and needs to be handled by the pager.

X
This option instructs the pager to not send termcap initialization and deinitialization strings
to the terminal. It is set by default to allow command output to remain visible in the terminal
even after the pager exits. Nevertheless, this prevents some pager functionality from working, in
particular paged output cannot be scrolled with the mouse.

See less(1) for more discussion.

$SYSTEMD_LESSCHARSET
Override the charset passed to less (by default "utf-8", if the invoking terminal is determined to be
UTF-8 compatible).

$SYSTEMD_PAGERSECURE
Takes a boolean argument. When true, the "secure" mode of the pager is enabled; if false, disabled.
If $SYSTEMD_PAGERSECURE is not set at all, secure mode is enabled if the effective UID is not the
same as the owner of the login session, see geteuid(2) and sd_pid_get_owner_uid(3). In secure mode,
LESSSECURE=1 will be set when invoking the pager, and the pager shall disable commands that open or
create new files or start new subprocesses. When $SYSTEMD_PAGERSECURE is not set at all, pagers which
are not known to implement secure mode will not be used. (Currently only less(1) implements secure
mode.)

Note: when commands are invoked with elevated privileges, for example under sudo(8) or pkexec(1),
care must be taken to ensure that unintended interactive features are not enabled. "Secure" mode for
the pager may be enabled automatically as describe above. Setting SYSTEMD_PAGERSECURE=0 or not
removing it from the inherited environment allows the user to invoke arbitrary commands. Note that if
the $SYSTEMD_PAGER or $PAGER variables are to be honoured, $SYSTEMD_PAGERSECURE must be set too. It
might be reasonable to completely disable the pager using --no-pager instead.

$SYSTEMD_COLORS
Takes a boolean argument. When true, systemd and related utilities will use colors in their output,
otherwise the output will be monochrome. Additionally, the variable can take one of the following
special values: "16", "256" to restrict the use of colors to the base 16 or 256 ANSI colors,
respectively. This can be specified to override the automatic decision based on $TERM and what the
console is connected to.

$SYSTEMD_URLIFY
The value must be a boolean. Controls whether clickable links should be generated in the output for
terminal emulators supporting this. This can be specified to override the decision that systemd makes
based on $TERM and other conditions.

$LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
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.

For further environment variables understood by systemd and its various components, see Known Environment
Variables[7].

KERNEL COMMAND LINE

When run as the system instance, systemd parses a number of options listed below. They can be specified
as kernel command line arguments which are parsed from a number of sources depending on the environment
in which systemd is executed. If run inside a Linux container, these options are parsed from the command
line arguments passed 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 and from
the "SystemdOptions" EFI variable (on EFI systems) instead. Options from /proc/cmdline have higher
priority.

Note: use of "SystemdOptions" is deprecated.

The following variables are understood:

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 initrd, while the one that is not prefixed only in the main system.

systemd.dump_core
Takes a boolean argument or enables the option if specified without an argument. If enabled, the
systemd manager (PID 1) dumps core when it crashes. Otherwise, no core dump is created. Defaults to
enabled.

Added in version 233.

systemd.crash_chvt
Takes a positive integer, or a boolean argument. Can be also specified without an argument, with the
same effect as a positive boolean. If a positive integer (in the range 1–63) is specified, the system
manager (PID 1) will activate the specified virtual terminal when it crashes. Defaults to disabled,
meaning that no such switch is attempted. If set to enabled, the virtual terminal the kernel messages
are written to is used instead.

Added in version 233.

systemd.crash_shell
Takes a boolean argument or enables the option if specified without an argument. If enabled, the
system manager (PID 1) spawns a shell when it crashes, after a 10s delay. Otherwise, no shell is
spawned. Defaults to disabled, for security reasons, as the shell is not protected by password
authentication.

Added in version 233.

systemd.crash_reboot
Takes a boolean argument or enables the option if specified without an argument. If enabled, the
system manager (PID 1) will reboot the machine automatically when it crashes, after a 10s delay.
Otherwise, the system will hang indefinitely. Defaults to disabled, in order to avoid a reboot loop.
If combined with systemd.crash_shell, the system is rebooted after the shell exits.

Added in version 227.

systemd.confirm_spawn
Takes a boolean argument or a path to the virtual console where the confirmation messages should be
emitted. Can be also specified without an argument, with the same effect as a positive boolean. If
enabled, the system manager (PID 1) asks for confirmation when spawning processes using /dev/console.
If a path or a console name (such as "ttyS0") is provided, the virtual console pointed to by this
path or described by the give name will be used instead. Defaults to disabled.

Added in version 233.

systemd.service_watchdogs=
Takes a boolean argument. If disabled, all service runtime watchdogs (WatchdogSec=) and emergency
actions (e.g. OnFailure= or StartLimitAction=) are ignored by the system manager (PID 1); see
systemd.service(5). Defaults to enabled, i.e. watchdogs and failure actions are processed normally.
The hardware watchdog is not affected by this option.

Added in version 237.

systemd.show_status
Takes a boolean argument or the constants error and auto. Can be also specified without an argument,
with the same effect as a positive boolean. If enabled, the systemd manager (PID 1) shows terse
service status updates on the console during bootup. With error, only messages about failures are
shown, but boot is otherwise quiet. auto behaves like false until there is a significant delay in
boot. Defaults to enabled, unless quiet is passed as kernel command line option, in which case it
defaults to error. If specified overrides the system manager configuration file option ShowStatus=,
see systemd-system.conf(5).

Added in version 233.

systemd.status_unit_format=
Takes name, description or combined as the value. If name, the system manager will use unit names in
status messages. If combined, the system manager will use unit names and description in status
messages. When specified, overrides the system manager configuration file option StatusUnitFormat=,
see systemd-system.conf(5).

Added in version 243.

systemd.log_color, systemd.log_level=, systemd.log_location, systemd.log_target=, systemd.log_time,
systemd.log_tid, systemd.log_ratelimit_kmsg
Controls log output, with the same effect as the $SYSTEMD_LOG_COLOR, $SYSTEMD_LOG_LEVEL,
$SYSTEMD_LOG_LOCATION, $SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_TIME, $SYSTEMD_LOG_TID and
$SYSTEMD_LOG_RATELIMIT_KMSG environment variables described above. systemd.log_color,
systemd.log_location, systemd.log_time, systemd.log_tid and systemd.log_ratelimit_kmsg can be
specified without an argument, with the same effect as a positive boolean.

systemd.default_standard_output=, systemd.default_standard_error=
Controls default standard output and error output for services and sockets. That is, controls the
default for StandardOutput= and StandardError= (see systemd.exec(5) for details). Takes one of
inherit, null, tty, journal, journal+console, kmsg, kmsg+console. If the argument is omitted
systemd.default-standard-output= defaults to journal and systemd.default-standard-error= to inherit.

systemd.setenv=
Takes a string argument in the form VARIABLE=VALUE. May be used to set default environment variables
to add to forked child processes. May be used more than once to set multiple variables.

systemd.machine_id=
Takes a 32 character hex value to be used for setting the machine-id. Intended mostly for network
booting where the same machine-id is desired for every boot.

Added in version 229.

systemd.set_credential=, systemd.set_credential_binary=
Sets a system credential, which can then be propagated to system services using the ImportCredential=
or LoadCredential= setting, see systemd.exec(5) for details. Takes a pair of credential name and
value, separated by a colon. The systemd.set_credential= parameter expects the credential value in
literal text form, the systemd.set_credential_binary= parameter takes binary data encoded in Base64.
Note that the kernel command line is typically accessible by unprivileged programs in /proc/cmdline.
Thus, this mechanism is not suitable for transferring sensitive data. Use it only for data that is
not sensitive (e.g. public keys/certificates, rather than private keys), or in testing/debugging
environments.

For further information see System and Service Credentials[8] documentation.

Added in version 251.

systemd.import_credentials=
Takes a boolean argument. If false disables importing credentials from the kernel command line, the
DMI/SMBIOS OEM string table, the qemu_fw_cfg subsystem or the EFI kernel stub.

Added in version 251.

quiet
Turn off status output at boot, much like systemd.show_status=no would. Note that this option is also
read by the kernel itself and disables kernel log output. Passing this option hence turns off the
usual output from both the system manager and the kernel.

Added in version 186.

debug
Turn on debugging output. This is equivalent to systemd.log_level=debug. Note that this option is
also read by the kernel itself and enables kernel debug output. Passing this option hence turns on
the debug output from both the system manager and the kernel.

Added in version 205.

emergency, rd.emergency, -b
Boot into emergency mode. This is equivalent to systemd.unit=emergency.target or
rd.systemd.unit=emergency.target, respectively, and provided for compatibility reasons and to be
easier to type.

Added in version 186.

rescue, rd.rescue, single, s, S, 1
Boot into rescue mode. This is equivalent to systemd.unit=rescue.target or
rd.systemd.unit=rescue.target, respectively, and provided for compatibility reasons and to be easier
to type.

Added in version 186.

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.

Added in version 186.

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).

Added in version 186.

For other kernel command line parameters understood by components of the core OS, please refer to kernel-
command-line(7).

SYSTEM CREDENTIALS

       During initialization the service manager will import credentials from various sources into the system's
       set of credentials, which can then be propagated into services and consumed by generators:

       •   When the service manager first initializes it will read system credentials from SMBIOS Type 11 vendor
           strings io.systemd.credential:name=value, and io.systemd.credential.binary:name=value.

       •   At the same time it will import credentials from QEMU "fw_cfg". (Note that the SMBIOS mechanism is
           generally preferred, because it is faster and generic.)

       •   Credentials may be passed via the kernel command line, using the systemd.set-credential= parameter,
           see above.

       •   Credentials may be passed from the UEFI environment via systemd-stub(7).

       •   When the service manager is invoked during the initrd → host transition it will import all files in
           /run/credentials/@initrd/ as system credentials.

       Invoke systemd-creds(1) as follows to see the list of credentials passed into the system:

           # systemd-creds --system list

       For further information see System and Service Credentials[8] documentation.

       The service manager when run as PID 1 consumes the following system credentials:

       vmm.notify_socket
           Contains a AF_VSOCK or AF_UNIX address where to send a READY=1 notification datagram when the system
           has finished booting. See sd_notify(3) for more information. Note that in case the hypervisor does
           not support SOCK_DGRAM over AF_VSOCK, SOCK_SEQPACKET will be tried instead. The credential payload
           for AF_VSOCK should be in the form "vsock:CID:PORT".

           This feature is useful for hypervisors/VMMs or other processes on the host to receive a notification
           via VSOCK when a virtual machine has finished booting.

           Added in version 254.

       system.machine_id
           Takes a 128bit hexadecimal ID to initialize /etc/machine-id from, if the file is not set up yet. See
           machine-id(5) for details.

           Added in version 254.

OPTIONS

systemd is only very rarely invoked directly, since it is started early and is already running by the
time users may interact with it. Normally, tools like systemctl(1) are used to give commands to the
manager. Since systemd is usually not invoked directly, the options listed below are mostly useful for
debugging and special purposes.

Introspection and debugging options
Those options are used for testing and introspection, and systemd may be invoked with them at any time:

--dump-configuration-items
Dump understood unit configuration items. This outputs a terse but complete list of configuration
items understood in unit definition files.

--dump-bus-properties
Dump exposed bus properties. This outputs a terse but complete list of properties exposed on D-Bus.

Added in version 239.

--test
Determine the initial start-up transaction (i.e. the list of jobs enqueued at start-up), dump it and
exit — without actually executing any of the determined jobs. This option is useful for debugging
only. Note that during regular service manager start-up additional units not shown by this operation
may be started, because hardware, socket, bus or other kinds of activation might add additional jobs
as the transaction is executed. Use --system to request the initial transaction of the system service
manager (this is also the implied default), combine with --user to request the initial transaction of
the per-user service manager instead.

--system, --user
When used in conjunction with --test, selects whether to calculate the initial transaction for the
system instance or for a per-user instance. These options have no effect when invoked without --test,
as during regular (i.e. non---test) invocations the service manager will automatically detect whether
it shall operate in system or per-user mode, by checking whether the PID it is run as is 1 or not.
Note that it is not supported booting and maintaining a system with the service manager running in
--system mode but with a PID other than 1.

-h, --help
Print a short help text and exit.

--version
Print a short version string and exit.

Options that duplicate kernel command line settings
Those options correspond directly to options listed above in "Kernel Command Line". Both forms may be
used equivalently for the system manager, but it is recommended to use the forms listed above in this
context, because they are properly namespaced. When an option is specified both on the kernel command
line and as a normal command line argument, the latter has higher precedence.

When systemd is used as a user manager, the kernel command line is ignored and only the options described
below are understood. Nevertheless, systemd is usually started in this mode through the user@.service(5)
service, which is shared between all users. It may be more convenient to use configuration files to
modify settings (see systemd-user.conf(5)), or environment variables. See the "Environment" section above
for a discussion of how the environment block is set.

--unit=
Set default unit to activate on startup. If not specified, defaults to default.target. See
systemd.unit= above.

--dump-core
Enable core dumping on crash. This switch has no effect when running as user instance. Same as
systemd.dump_core= above.

--crash-vt=VT
Switch to a specific virtual console (VT) on crash. This switch has no effect when running as user
instance. Same as systemd.crash_chvt= above (but not the different spelling!).

Added in version 227.

--crash-shell
Run a shell on crash. This switch has no effect when running as user instance. See
systemd.crash_shell= above.

--crash-reboot
Automatically reboot the system on crash. This switch has no effect when running as user instance.
See systemd.crash_reboot above.

Added in version 227.

--confirm-spawn
Ask for confirmation when spawning processes. This switch has no effect when run as user instance.
See systemd.confirm_spawn above.

--show-status
Show terse unit status information on the console during boot-up and shutdown. See
systemd.show_status above.

Added in version 244.

--log-color
Highlight important log messages. See systemd.log_color above.

Added in version 244.

--log-level=
Set log level. See systemd.log_level above.

--log-location
Include code location in log messages. See systemd.log_location above.

Added in version 244.

--log-target=
Set log target. See systemd.log_target above.

--log-time=
Prefix console messages with timestamp. See systemd.log_time above.

Added in version 246.

--machine-id=
Override the machine-id set on the hard drive. See systemd.machine_id= above.

Added in version 229.

--service-watchdogs
Globally enable/disable all service watchdog timeouts and emergency actions. See
systemd.service_watchdogs above.

Added in version 237.

--default-standard-output=, --default-standard-error=
Sets the default output or error output for all services and sockets, respectively. See
systemd.default_standard_output= and systemd.default_standard_error= above.

SOCKETS AND FIFOS

       /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/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.

HISTORY

       systemd 252
           Kernel command-line arguments systemd.unified_cgroup_hierarchy and
           systemd.legacy_systemd_cgroup_controller were deprecated. Please switch to the unified cgroup
           hierarchy.

           Added in version 252.

NOTES

        1. Control Groups v2
           https://docs.kernel.org/admin-guide/cgroup-v2.html

        2. Original Design Document
           https://0pointer.de/blog/projects/systemd.html

        3. Interface Portability and Stability Promise
           https://systemd.io/PORTABILITY_AND_STABILITY/

        4. Container Interface
           https://systemd.io/CONTAINER_INTERFACE

        5. initrd Interface
           https://systemd.io/INITRD_INTERFACE/

        6. XDG Base Directory specification
           https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

        7. Known Environment Variables
           https://systemd.io/ENVIRONMENT

        8. System and Service Credentials
           https://systemd.io/CREDENTIALS

        9. systemd Homepage
           https://systemd.io/

NAME

SYNOPSIS

DESCRIPTION

CONCEPTS

DIRECTORIES

SIGNALS

ENVIRONMENT

KERNEL COMMAND LINE

SYSTEM CREDENTIALS

OPTIONS

SOCKETS AND FIFOS

HISTORY

SEE ALSO

NOTES