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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/.

systemd will also reset the system clock during early boot if it appears to be set incorrectly. See "System clock epoch" section below.

Note that some but not all interfaces provided by systemd are covered by the Interface Portability and Stability Promise^[1].

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^[2] or initrd Interface^[3] 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

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^[4] 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).

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^[5] 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.

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.

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.

You may want to use SetShowStatus() instead of SIGRTMIN+20 in order to prevent race conditions. See org.freedesktop.systemd1(5).

SIGRTMIN+21

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

You may want to use SetShowStatus() instead of SIGRTMIN+21 in order to prevent race conditions. See org.freedesktop.systemd1(5).

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.

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=.

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^[5] 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^[6].

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. 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_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.

Added in version 256.

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^[7] 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).

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^[7] 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.

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.

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.

  Added in version 256.

• 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.

  Added in version 256.

• 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 that indicates 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.

  Added in version 256.

• 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.

  Added in version 256.

• 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.

  Added in version 256.

• 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=....

  Added in version 256.

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.

When systemd is started or restarted, it may set the system clock to the "epoch". This mechanism is used to ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across reboots, in case no battery-backed local RTC is available or it does not work correctly.

The epoch is the lowest date above which the system clock time is assumed to be set correctly. When initializing, the local clock is advanced to the epoch if it was set to a lower value. As a special case, if the local clock is sufficiently far in the future (by default 15 years, but this can be configured at build time), the hardware clock is assumed to be broken, and the system clock is rewound to the epoch.

The epoch is set to the highest of: the build time of systemd, the modification time ("mtime") of /usr/lib/clock-epoch, and the modification time of /var/lib/systemd/timesync/clock.

/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.

/usr/lib/clock-epoch

The modification time ("mtime") of this file is used for the time epoch, see previous section.

Added in version 247.

/var/lib/systemd/timesync/clock

The modification time ("mtime") of this file is updated by systemd-timesyncd.service(8). If present, the modification time of file is used for the epoch, see previous section.

Added in version 257.

systemd 252

The systemd Homepage^[8], 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^[9].

1. Interface Portability and Stability Promise
   https://systemd.io/PORTABILITY_AND_STABILITY/
2. Container Interface
   https://systemd.io/CONTAINER_INTERFACE
3. initrd Interface
   https://systemd.io/INITRD_INTERFACE/
4. Control Groups v2
   https://docs.kernel.org/admin-guide/cgroup-v2.html
5. XDG Base Directory specification
   https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
6. Known Environment Variables
   https://systemd.io/ENVIRONMENT
7. System and Service Credentials
   https://systemd.io/CREDENTIALS
8. systemd Homepage
   https://systemd.io/
9. Original Design Document
   https://0pointer.de/blog/projects/systemd.html

acpid(8), agetty(8), apachectl(8), apcupsd(8), beanstalkd(1), binfmt.d(5), bootparam(7), bootup(7), bpftool-map(8), bpftool-prog(8), bundle(1), busctl(1), capget(2), capsule@.service(5), cgroups(7), cockpit(1), cockpit-tls(8), cockpit-ws(8), conntrackd(8), conntrackd.conf(5), core(5), crypttab(5), ctrlaltdel(8), daemon(7), dbus-update-activation-environment(1), dconf-service(1), dnf4.conf(5), dnf5.conf(5), dnssec-trust-anchors.d(5), environment.d(5), _exit(2), file-hierarchy(7), function::pstrace.3stap(3), gamemoded(8), getpid(2), homectl(1), homed.conf(5), hostname(5), hostnamectl(1), httpd.service(8), importctl(1), in.fingerd(8), integritytab(5), ipsec-pluto(8), irtt-server(1), journalctl(1), journald.conf(5), journal-remote.conf(5), journal-upload.conf(5), kacpimon(8), kafs-client.conf(5), kernel-command-line(7), keyctl(2), libsystemd(3), libudev(3), locale.conf(5), localectl(1), localtime(5), logger(1), loginctl(1), logind.conf(5), logrotate(8), machinectl(1), machine-id(5), machine-info(5), modules-load.d(5), mount_namespaces(7), MQTTAsync.h(3), nbdkit-client(1), nbdkit-service(1), networkd.conf(5), nsca-ng(8), nsca-ng.cfg(5), nss-myhostname(8), nss-mymachines(8), nss-resolve(8), nss-systemd(8), oidentd(8), oomctl(1), oomd.conf(5), org.freedesktop.home1(5), org.freedesktop.hostname1(5), org.freedesktop.import1(5), org.freedesktop.locale1(5), org.freedesktop.LogControl1(5), org.freedesktop.login1(5), org.freedesktop.machine1(5), org.freedesktop.network1(5), org.freedesktop.oom1(5), org.freedesktop.portable1(5), org.freedesktop.resolve1(5), org.freedesktop.systemd1(5), org.freedesktop.timedate1(5), org.freedesktop.timesync1(5), os-release(5), pam_systemd(8), pam_systemd_home(8), pcp-check(1), pcp-geolocate(1), pcp-reboot-init(1), pid_namespaces(7), pivot_root(2), pmfind_check(1), pmie(1), pmie_check(1), pmiectl(1), pmlogctl(1), pmlogger(1), pmlogger_check(1), pmlogger.control(5), pmlogger_daily(1), pmproxy(1), __pmServerNotifyServiceManagerReady(3), polkitd(8), portablectl(1), poweroff(8), proc(5), proc_sys_kernel(5), ptrace(2), public-inbox-daemon(8), public-inbox-watch(1), qm(8), reboot(2), repart.d(5), resolvectl(1), resolved.conf(5), run0(1), runlevel(8), saned(8), saslauthd(8), sd_booted(3), sd-bus(3), sd_bus_add_match(3), sd_bus_attach_event(3), sd_bus_call(3), sd_bus_call_method(3), sd_bus_can_send(3), sd_bus_close(3), sd_bus_creds_get_pid(3), sd_bus_creds_new_from_pid(3), sd_bus_default(3), sd_bus_enqueue_for_read(3), sd_bus_error(3), sd_bus_error_add_map(3), sd-bus-errors(3), sd_bus_get_current_handler(3), sd_bus_get_fd(3), sd_bus_get_name_creds(3), sd_bus_get_name_machine_id(3), sd_bus_get_n_queued_read(3), sd_bus_interface_name_is_valid(3), sd_bus_is_open(3), sd_bus_list_names(3), sd_bus_message_append(3), sd_bus_message_append_array(3), sd_bus_message_append_basic(3), sd_bus_message_append_string_memfd(3), sd_bus_message_append_strv(3), sd_bus_message_at_end(3), sd_bus_message_copy(3), sd_bus_message_dump(3), sd_bus_message_get_cookie(3), sd_bus_message_get_monotonic_usec(3), sd_bus_message_get_signature(3), sd_bus_message_get_type(3), sd_bus_message_new(3), sd_bus_message_new_method_call(3), sd_bus_message_new_method_error(3), sd_bus_message_new_signal(3), sd_bus_message_open_container(3), sd_bus_message_read(3), sd_bus_message_read_array(3), sd_bus_message_read_basic(3), sd_bus_message_read_strv(3), sd_bus_message_rewind(3), sd_bus_message_seal(3), sd_bus_message_sensitive(3), sd_bus_message_set_destination(3), sd_bus_message_set_expect_reply(3), sd_bus_message_skip(3), sd_bus_message_verify_type(3), sd_bus_negotiate_fds(3), sd_bus_new(3), sd_bus_path_encode(3), sd_bus_pending_method_calls(3), sd_bus_process(3), sd_bus_query_sender_creds(3), sd_bus_reply_method_error(3), sd_bus_reply_method_return(3), sd_bus_request_name(3), sd_bus_send(3), sd_bus_set_address(3), sd_bus_set_close_on_exit(3), sd_bus_set_connected_signal(3), sd_bus_set_description(3), sd_bus_set_exit_on_disconnect(3), sd_bus_set_fd(3), sd_bus_set_method_call_timeout(3), sd_bus_set_property(3), sd_bus_set_sender(3), sd_bus_set_server(3), sd_bus_set_watch_bind(3), sd_bus_slot_get_bus(3), sd_bus_slot_ref(3), sd_bus_slot_set_description(3), sd_bus_slot_set_destroy_callback(3), sd_bus_slot_set_floating(3), sd_bus_slot_set_userdata(3), sd_bus_start(3), sd_bus_track_add_name(3), sd_bus_track_new(3), sd_bus_wait(3), sd-daemon(3), sd-device(3), sd_device_get_syspath(3), sd-event(3), sd_event_add_child(3), sd_event_add_defer(3), sd_event_add_inotify(3), sd_event_add_io(3), sd_event_add_memory_pressure(3), sd_event_add_signal(3), sd_event_add_time(3), sd_event_exit(3), sd_event_new(3), sd_event_now(3), sd_event_run(3), sd_event_set_signal_exit(3), sd_event_set_watchdog(3), sd_event_source_set_destroy_callback(3), sd_event_wait(3), sd_get_seats(3), sd-hwdb(3), sd_hwdb_get(3), sd_hwdb_new(3), sd-id128(3), sd_id128_get_machine(3), sd_id128_randomize(3), sd_id128_to_string(3), sd_is_fifo(3), sd-journal(3), sd_journal_add_match(3), sd_journal_enumerate_fields(3), sd_journal_get_catalog(3), sd_journal_get_cursor(3), sd_journal_get_cutoff_realtime_usec(3), sd_journal_get_data(3), sd_journal_get_fd(3), sd_journal_get_realtime_usec(3), sd_journal_get_seqnum(3), sd_journal_get_usage(3), sd_journal_has_runtime_files(3), sd_journal_next(3), sd_journal_open(3), sd_journal_print(3), sd_journal_query_unique(3), sd_journal_seek_head(3), sd_journal_stream_fd(3), sd-json(3), sd_listen_fds(3), sd-login(3), sd_login_monitor_new(3), sd_machine_get_class(3), sd_notify(3), sd_path_lookup(3), sd_pid_get_owner_uid(3), sd_seat_get_active(3), sd_session_is_active(3), sd_uid_get_state(3), sd-varlink(3), sd_watchdog_enabled(3), service(8), shutdown(8), smartd(8), smbios-type-11(7), smcrouted(8), sssd-kcm(8), sysctl.d(5), systemctl(1), systemd-ac-power(1), systemd-analyze(1), systemd-ask-password(1), systemd-ask-password-console.service(8), systemd.automount(5), systemd-backlight@.service(8), systemd-battery-check.service(8), systemd-binfmt.service(8), systemd-bless-boot-generator(8), systemd-bless-boot.service(8), systemd-boot-check-no-failures.service(8), systemd-boot-random-seed.service(8), systemd-bsod.service(8), systemd-cat(1), systemd-cgls(1), systemd-cgtop(1), systemd-creds(1), systemd-cryptenroll(1), systemd-cryptsetup(8), systemd-cryptsetup-generator(8), systemd-debug-generator(8), systemd-delta(1), systemd-detect-virt(1), systemd.device(5), systemd.directives(7), systemd-dissect(1), systemd.dnssd(5), systemd-environment-d-generator(8), systemd.environment-generator(7), systemd-escape(1), systemd.exec(5), systemd-firstboot(1), systemd-fsck@.service(8), systemd-fstab-generator(8), systemd.generator(7), systemd-getty-generator(8), systemd-gpt-auto-generator(8), systemd-hibernate-resume-generator(8), systemd-hibernate-resume.service(8), systemd-homed.service(8), systemd-hostnamed.service(8), systemd-hwdb(8), systemd-id128(1), systemd.image-policy(7), systemd-importd.service(8), systemd-import-generator(8), systemd.index(7), systemd-inhibit(1), systemd-initctl.service(8), systemd-integritysetup-generator(8), systemd-integritysetup@.service(8), systemd-journald.service(8), systemd.journal-fields(7), systemd-journal-gatewayd.service(8), systemd.kill(5), systemd-localed.service(8), systemd-logind.service(8), systemd-machined.service(8), systemd-machine-id-commit.service(8), systemd-machine-id-setup(1), systemd-makefs@.service(8), systemd-measure(1), systemd-modules-load.service(8), systemd-mount(1), systemd.mount(5), systemd-mountfsd.service(8), systemd.netdev(5), systemd.network(5), systemd-networkd.service(8), systemd-networkd-wait-online.service(8), systemd-network-generator.service(8), systemd-notify(1), systemd-nspawn(1), systemd.nspawn(5), systemd-nsresourced.service(8), systemd.offline-updates(7), systemd-oomd.service(8), systemd-path(1), systemd.path(5), systemd-pcrlock(8), systemd.pcrlock(5), systemd-pcrphase.service(8), systemd-portabled.service(8), systemd-poweroff.service(8), systemd.preset(5), systemd-quotacheck.service(8), systemd-random-seed.service(8), systemd-rc-local-generator(8), systemd-remount-fs.service(8), systemd-repart(8), systemd-resolved.service(8), systemd.resource-control(5), systemd-rfkill.service(8), systemd-run(1), systemd-run-generator(8), systemd.scope(5), systemd.service(5), systemd-sleep.conf(5), systemd.slice(5), systemd.socket(5), systemd-socket-activate(1), systemd-socket-proxyd(8), systemd-soft-reboot.service(8), systemd.special(7), systemd-ssh-generator(8), systemd-ssh-proxy(1), systemd-stdio-bridge(1), systemd-storagetm.service(8), systemd-suspend.service(8), systemd.swap(5), systemd.syntax(7), systemd-sysctl.service(8), systemd-sysext(8), systemd-system.conf(5), systemd.system-credentials(7), systemd-system-update-generator(8), systemd-sysupdate(8), systemd-sysusers(8), systemd-sysv-generator(8), systemd.target(5), systemd.time(7), systemd-timedated.service(8), systemd.timer(5), systemd-timesyncd.service(8), systemd-time-wait-sync.service(8), systemd-tmpfiles(8), systemd-tpm2-generator(8), systemd-tpm2-setup.service(8), systemd-tty-ask-password-agent(1), systemd.unit(5), systemd-update-done.service(8), systemd-update-utmp.service(8), systemd-userdbd.service(8), systemd-user-sessions.service(8), systemd.v(7), systemd-vconsole-setup.service(8), systemd-veritysetup-generator(8), systemd-veritysetup@.service(8), systemd-vmspawn(1), systemd-volatile-root.service(8), systemd-vpick(1), systemd-xdg-autostart-generator(8), sysupdate.d(5), sysupdate.features(5), sysusers.d(5), tapset::pstrace.3stap(3), telinit(8), timedatectl(1), timesyncd.conf(5), tmpfiles.d(5), ttyslot(3), udev_device_get_syspath(3), udev_device_has_tag(3), udev_device_new_from_syspath(3), udev_enumerate_add_match_subsystem(3), udev_enumerate_new(3), udev_enumerate_scan_devices(3), udev_list_entry(3), udev_monitor_filter_update(3), udev_monitor_new_from_netlink(3), udev_monitor_receive_device(3), udev_new(3), udisksd(8), ukify(1), unshare(2), userdbctl(1), user@.service(5), utmp(5), uuidd(8), vconsole.conf(5), veritytab(5), vhangup(2), wait(2), xscreensaver(1), xscreensaver-systemd(6).

The man page init(1) is an alias of systemd(1).