systemd

system and service manager for Linux

systemd version252

/usr/lib/systemd/systemd [OPTIONS...]
init [OPTIONS...] {COMMAND}

A system and service manager for Linux. When run as first process on boot (as PID 1), it acts as init system that brings up and maintains userspace 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.

When invoked 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. init and telinit are similar when invoked from normal login sessions. See telinit (8)

When run as a system instance, systemd interprets system.conf and the files in system.conf.d directories.
When run as a user instance, systemd interprets user.conf and the files in user.conf.d directories. See systemd-system.conf (5)

Concepts

systemd provides a dependency system between various entities called "units" of 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)
Some are created automatically from other configuration files, dynamically from system state or programmatically at runtime.
Units may be "active" (meaning started, bound, plugged in, …, depending on the unit type, see below), or "inactive" (meaning stopped, unbound, unplugged, …), as well as in the process of being activated or deactivated, i.e. between the two states (these states are called "activating", "deactivating"). A special "failed" state which is very similar to "inactive" and is entered when the service failed in some way (process returned a non-zero error code on exit, or crashed, an operation timed out, or after too many restarts). If this state is entered, the cause will be logged. Unit types may have a number of additional substates, which are mapped to the five generalized unit states described here.

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= ). 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. The majority of dependencies are implicitly created and maintained by systemd. It should be unnecessary to declare additional dependencies manually,

Application programs and units (via dependencies) may request state changes of units. 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

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. The unit name is just an 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). It is at the discretion of the administrator to configure it as an alias to any other target unit. See systemd.special (7)

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 which are:

systemd will automatically load units from disk when operations are requested for them. Use systemctl list-units --all to list units currently loaded. Any unit for which none of the conditions above applies is unloaded and its accounting data is flushed, 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 systemd uses this to 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 useful to list processes and the systemd units they belong to.).

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. Before executing a requested operation, systemd will verify that it makes sense, fixing it if possible, and only

that transactions are generated independently of a unit's state at runtime. 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

systemd contains native implementations of various tasks that need to be executed as part of the boot process. For example, it sets the hostname or configures the loopback network device. It also sets up and mounts various API file systems, such as /sys/ or /proc/ . Original Design Document

Interface Portability and Stability Promise .

Units may be generated dynamically at boot and system manager reload time, for example based on other configuration files or parameters passed on the kernel command line. For details, see systemd.generator (7) .

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

Systems which invoke systemd in a container or initrd environment should implement the Container Interface or initrd Interface

Directories

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. 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)

Signals

SIGTERM manager serializes its state, reexecutes itself and deserializes the saved state again. Similar to systemctl daemon-reexec
systemd user managers will start the exit.target unit . Similar to systemctl --user start exit.target --job-mode=replace-irreversibly
SIGINT manager will start the ctrl-alt-del.target unit. Similar 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. on the console will trigger this signal. if a reboot is hanging, pressing Ctrl + Alt + Del more than 7 times in 2 seconds is a safe way to trigger an immediate reboot. Systemd user managers treat this signal the same way as SIGTERM
SIGWINCH manager will start the kbrequest.target unit. Similar to systemctl start kbrequest.target
This signal is ignored by systemd user??? (sic)
SIGPWR manager will start the sigpwr.target unit. Similar to systemctl start sigpwr.target
SIGUSR1 manager will reconnect to the D-Bus
SIGUSR2 manager will log its complete state in human-readable form. the same as printed by systemd-analyze dump
SIGHUP Reloads the complete daemon configuration. Similar to systemctl daemon-reload
SIGRTMIN+0 Enters default mode, starts the default.target unit. Similar to systemctl isolate default.target
SIGRTMIN+1 Enters rescue mode, starts the rescue.target unit. This is similar to systemctl isolate rescue.target
SIGRTMIN+2 Enters emergency mode, starts the emergency.service unit. This is similar to systemctl isolate emergency.service
SIGRTMIN+3 Halts the machine, starts the halt.target unit. This is similar to systemctl start halt.target --job-mode=replace-irreversibly
SIGRTMIN+4 starts the poweroff.target unit. This is similar to systemctl start poweroff.target --job-mode=replace-irreversibly
SIGRTMIN+5 Reboots the machine, starts the reboot.target unit. This is similar to systemctl start reboot.target --job-mode=replace-irreversibly
SIGRTMIN+6 Reboots the machine via kexec, starts the kexec.target unit. This is similar to systemctl start kexec.target --job-mode=replace-irreversibly
SIGRTMIN+13 SIGRTMIN+14
SIGRTMIN+15
SIGRTMIN+16
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. 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 SIGRTMIN+25

manager will reexecute itself. This is similar 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, 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.

Environment

The environment block for the system manager is initially set by the kernel. (In particular, " key=value " assignments on the kernel command line are turned into environment variables for PID 1). For the user manager, the system manager sets the environment as described in the "Environment Variables in Spawned Processes" section of (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 (5) ). Additional environment variables may be set through the ManagerEnvironment= setting in (5) and (5) .

Some of the variables understood by systemd :

Kernel Command Line

When run as the system instance, systemd arguments are parsed from /proc/cmdline and from the " SystemdOptions " EFI variable (on EFI systems) instead. Options from /proc/cmdline have higher priority.

variables

  • 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 (7) 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

  • 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

  • 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 <

  • code>systemd.crash_reboot

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

  • systemd.crash_shell , the system is rebooted after the shell exits.
  • 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 (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 (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

  • (5) systemd.log_color , systemd.log_level= , systemd.log_location , systemd.log_target= , systemd.log_time , systemd.log_tid

    Controls log output, with the same effect as the $SYSTEMD_LOG_COLOR , $SYSTEMD_LOG_LEVEL $SYSTEMD_LOG_LOCATION , $SYSTEMD_LOG_TARGET $SYSTEMD_LOG_TIME , and $SYSTEMD_LOG_TID environment variables described above. systemd.log_color , systemd.log_location systemd.log_time , and systemd.log_tid= 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 (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

  • 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

  • systemd.set_credential=

    Sets a system credential, which can then be propagated to system services using the LoadCredential= setting, see

  • (5) for details. Takes a pair of credential name and value, separated by a colon. 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 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 (5) and (7)

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

Options

systemd is only very rarely invoked directly, since it is started early and is already running by the time users may interact with it. Normally, tools like (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.