systemd, init — systemd system and service manager
/usr/lib/systemd/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. 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.
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/,
/proc/, and /dev/.
Note that some but not all interfaces provided by systemd are covered by the Interface Portability and Stability Promise.
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 or initrd Interface specifications, respectively.
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 in a number of states, described in the following table. Note that the various unit types may have a number of additional substates, which are mapped to the generalized unit states described here.
Table 1. Unit ACTIVE states
| State | Description |
|---|---|
active | Started, bound, plugged in, …, depending on the unit type. |
inactive | Stopped, unbound, unplugged, …, depending on the unit type. |
failed | Similar to inactive, but the unit failed in some way (process returned error code on exit, crashed, an operation timed out, or after too many restarts).
|
activating | Changing from inactive to active. |
deactivating | Changing from active to inactive. |
maintenance | Unit is inactive and a maintenance operation is in progress. |
reloading | Unit is active and it is reloading its configuration. |
The following unit types are available:
Service units, which start and 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).
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).
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).
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:
It is in an active, activating, deactivating or failed state (i.e. in any unit state except for "inactive")
It has a job queued for it
It is a dependency of at least one other unit that is loaded into memory
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)
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 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.
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 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.
The service listens to various UNIX process signals that can be used to request various actions
asynchronously. The signal handling is enabled very early during boot, before any further processes are
invoked. However, a supervising container manager or similar that intends to request these operations via
this mechanism must take into consideration that this functionality is not available during the earliest
initialization phase. An sd_notify() notification message carrying the
X_SYSTEMD_SIGNALS_LEVEL=2 field is emitted once the signal handlers are enabled, see
below. This may be used to schedule submission of these signals correctly.
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.
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.
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".
SIGRTMIN+24¶Immediately exits the manager (only available for --user instances).
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.
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.
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.
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). Takes a comma-separated list of values. A
value may be 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. Each value may optionally be prefixed with one of console,
syslog, kmsg or journal followed by a
colon to set the maximum log level for that specific log target (e.g.
SYSTEMD_LOG_LEVEL=debug,console:info specifies to log at debug level except when
logging to the console which should be at info level). Note that the global maximum log level takes
priority over any per target maximum log levels.
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=.
$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).
$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.
$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, $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.
Note that setting the regular $LESS environment variable has no effect
for less invocations by systemd tools.
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).
Note that setting the regular $LESSCHARSET environment variable has no effect
for less invocations by systemd tools.
$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 service manager for its services for status and readiness notifications. Also consumed by service manager for notifying supervising container managers or service managers up the stack about its own progress. See sd_notify(3) and the relevant section below for more information.
For further environment variables understood by systemd and its various components, see Known Environment Variables.
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.
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.
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.
systemd.crash_action=¶Takes one of "freeze", "reboot" or
"poweroff". Defaults to "freeze". If set to
"freeze", the system will hang indefinitely when the system manager (PID 1) crashes.
If set to "reboot", the system manager (PID 1) will reboot the machine automatically
when it crashes, after a 10s delay. If set to "poweroff", the system manager (PID 1)
will power off the machine immediately when it crashes. If combined with
systemd.crash_shell, the configured crash action is executed after the shell
exits.
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.
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.
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).
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).
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.
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 documentation.
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.
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.
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.
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.
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.
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).
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:,
and
name=valueio.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 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 message when the service manager has completed
booting. See
sd_notify(3) and
the next section 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 a string in the form
"vsock:CID:PORT". "vsock-stream", "vsock-dgram"
and "vsock-seqpacket" can be used instead of "vsock" to force
usage of the corresponding socket type.
This feature is useful for machine managers or other processes on the host to receive a notification via VSOCK when a virtual machine has finished booting.
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.
For a list of system credentials various other components of systemd consume, see systemd.system-credentials(7).
The service manager implements a readiness notification protocol both between the manager and its services (i.e. down the stack), and between the manager and a potential supervisor further up the stack (the latter could be a machine or container manager, or in case of a per-user service manager the system service manager instance). The basic protocol (and the suggested API for it) is described in sd_notify(3).
The notification socket the service manager (including PID 1) uses for reporting readiness to its
own supervisor is set via the usual $NOTIFY_SOCKET environment variable (see
above). Since this is directly settable only for container managers and for the per-user instance of the
service manager, an additional mechanism to configure this is available, in particular intended for use
in VM environments: the vmm.notify_socket system credential (see above) may be set to
a suitable socket (typically an AF_VSOCK one) via SMBIOS Type 11 vendor strings. For
details see above.
The notification protocol from the service manager up the stack towards a supervisor supports a number of extension fields that allow a supervisor to learn about specific properties of the system and track its boot progress. Specifically the following fields are sent:
An X_SYSTEMD_HOSTNAME=… message will be sent out once the initial
hostname for the system has been determined. Note that during later runtime the hostname might be
changed again programmatically, and (currently) no further notifications are sent out in that case.
An X_SYSTEMD_MACHINE_ID=… message will be sent out once the machine
ID of the system has been determined. See
machine-id(5) for
details.
An X_SYSTEMD_SIGNALS_LEVEL=… message will be sent out once the
service manager installed the various UNIX process signal handlers described above. The field's value
is an unsigned integer formatted as decimal string, and indicates the supported UNIX process signal
feature level of the service manager. Currently, only a single feature level is defined:
X_SYSTEMD_SIGNALS_LEVEL=2 covers the various UNIX process signals
documented above – which are a superset of those supported by the historical SysV init
system.
Signals sent to PID 1 before this message is sent might not be handled correctly yet. A consumer of these messages should parse the value as an unsigned integer indication the level of support. For now only the mentioned level 2 is defined, but later on additional levels might be defined with higher integers, that will implement a superset of the currently defined behaviour.
X_SYSTEMD_UNIT_ACTIVE=… and
X_SYSTEMD_UNIT_INACTIVE=… messages will be sent out for each target unit as it
becomes active or stops being active. This is useful to track boot progress and functionality. For
example, once the ssh-access.target unit is reported started SSH access is
typically available, see
systemd.special(7) for
details.
An X_SYSTEMD_SHUTDOWN=… message will be sent out very shortly before
the system shuts down. The value is one of the strings "reboot",
"halt", "poweroff", "kexec" and indicates which kind
of shutdown is being executed.
An X_SYSTEMD_REBOOT_PARAMETER=… message will also be sent out very
shortly before the system shuts down. Its value is the reboot argument as configured with
systemctl --reboot-argument=….
Note that these extension fields are sent in addition to the regular "READY=1" and
"RELOADING=1" notifications.
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.
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.
--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¶--version¶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!).
--crash-shell¶Run a shell on crash. This switch has no effect when running as user instance. See
systemd.crash_shell= above.
--crash-action=¶Specify what to do when the system manager (PID 1) crashes. This switch has no
effect when systemd is running as user instance. See systemd.crash_action=
above.
--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.
--log-color¶Highlight important log messages. See systemd.log_color above.
--log-level=¶Set log level. See systemd.log_level above.
--log-location¶Include code location in log messages. See systemd.log_location
above.
--log-target=¶Set log target. See systemd.log_target above.
--log-time=¶Prefix console messages with timestamp. See systemd.log_time above.
--machine-id=¶Override the machine-id set on the hard drive. See
systemd.machine_id= above.
--service-watchdogs¶Globally enable/disable all service watchdog timeouts and emergency actions. See
systemd.service_watchdogs above.
--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.
/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.
Kernel command-line arguments systemd.unified_cgroup_hierarchy
and systemd.legacy_systemd_cgroup_controller were deprecated. Please switch to
the unified cgroup hierarchy.
The systemd Homepage, systemd-system.conf(5), locale.conf(5), systemctl(1), journalctl(1), systemd-notify(1), daemon(7), sd-daemon(3), org.freedesktop.systemd1(5), systemd.unit(5), systemd.special(7), pkg-config(1), kernel-command-line(7), bootup(7), systemd.directives(7), org.freedesktop.systemd1(5)
For more information about the concepts and ideas behind systemd, please refer to the Original Design Document.