systemd

Description¶

systemd is a system and service manager for Linux operating systems. When run as first process on boot (as PID 1), it acts as init system that brings up and maintains userspace services.

For compatibility with SysV, if systemd is called as init and a PID that is not 1, it will execute telinit and pass all command line arguments unmodified. That means init and telinit are mostly equivalent when invoked from normal login sessions. See telinit(8) for more information.

When run as 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.

Options¶

The following options are understood:

--test¶: Determine startup sequence, dump it and exit. This is an option useful for debugging only.
--dump-configuration-items¶: Dump understood unit configuration items. This outputs a terse but complete list of configuration items understood in unit definition files.
--unit=¶: Set default unit to activate on startup. If not specified, defaults to default.target.
--system, --user¶: For --system, tell systemd to run a system instance, even if the process ID is not 1, i.e. systemd is not run as init process. --user does the opposite, running a user instance even if the process ID is 1. Normally it should not be necessary to pass these options, as systemd automatically detects the mode it is started in. These options are hence of little use except for debugging. Note that it is not supported booting and maintaining a full system with systemd running in --system mode, but PID not 1. In practice, passing --system explicitly is only useful in conjunction with --test.
--dump-core¶: Dump core on crash. This switch has no effect when run as user instance.
--crash-shell¶: Run shell on crash. This switch has no effect when run as user instance.
--confirm-spawn¶: Ask for confirmation when spawning processes. This switch has no effect when run as user instance.
--show-status=¶: Show terse service status information while booting. This switch has no effect when run as user instance. Takes a boolean argument which may be omitted which is interpreted as true.
--log-target=¶: Set log target. Argument must be one of console, journal, kmsg, journal-or-kmsg, null.
--log-level=¶: Set log level. As argument this accepts a numerical log level or the well-known syslog(3) symbolic names (lowercase): emerg, alert, crit, err, warning, notice, info, debug.
--log-color=¶: Highlight important log messages. Argument is a boolean value. If the argument is omitted, it defaults to true.
--log-location=¶: Include code location in log messages. This is mostly relevant for debugging purposes. Argument is a boolean value. If the argument is omitted it defaults to true.
--default-standard-output=, --default-standard-error=¶: Sets the default output or error output for all services and sockets, respectively. That is, controls the default for StandardOutput= and StandardError= (see systemd.exec(5) for details). Takes one of inherit, null, tty, journal, journal+console, syslog, syslog+console, kmsg, kmsg+console. If the argument is omitted --default-standard-output= defaults to journal and --default-standard-error= to inherit.
-h, --help¶: Print a short help text and exit.
--version¶: Print a short version string and exit.

Concepts¶

systemd provides a dependency system between various entities called "units" of 12 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, dynamically from system state or programmatically at runtime. Units may be "active" (meaning started, bound, plugged in, ..., depending on the unit type, see below), or "inactive" (meaning stopped, unbound, unplugged, ...), as well as in the process of being activated or deactivated, i.e. between the two states (these states are called "activating", "deactivating"). A special "failed" state is available as well, which is very similar to "inactive" and is entered when the service failed in some way (process returned error code on exit, or crashed, or an operation timed out). If this state is entered, the cause will be logged, for later reference. Note that the various unit types may have a number of additional substates, which are mapped to the five generalized unit states described here.

The following unit types are available:

Service units, which 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).
Snapshot units can be used to temporarily save the state of the set of systemd units, which later may be restored by activating the saved snapshot unit. For more information see systemd.snapshot(5).
Timer units are useful for triggering activation of other units based on timers. You may find details in systemd.timer(5).
Swap units are very similar to mount units and encapsulate memory swap partitions or files of the operating system. They are described in systemd.swap(5).
Path units may be used to activate other services when file system objects change or are modified. See systemd.path(5).
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.

Processes systemd spawns are placed in individual Linux control groups named after the unit which they belong to in the private systemd hierarchy. (see cgroups.txt for more information about control groups, or short "cgroups"). systemd uses this to effectively keep track of processes. Control group information is maintained in the kernel, and is accessible via the file system hierarchy (beneath /sys/fs/cgroup/systemd/), or in tools such as ps(1) (ps xawf -eo pid,user,cgroup,args is particularly useful to list all processes and the systemd units they belong to.).

systemd is compatible with the SysV init system to a large degree: SysV init scripts are supported and simply read as an alternative (though limited) configuration file format. The SysV /dev/initctl interface is provided, and compatibility implementations of the various SysV client tools are available. In addition to that, various established Unix functionality such as /etc/fstab or the utmp database are supported.

systemd has a minimal transaction system: if a unit is requested to start up or shut down it will add it and all its dependencies to a temporary transaction. Then, it will verify if the transaction is consistent (i.e. whether the ordering of all units is cycle-free). If it is not, systemd will try to fix it up, and removes non-essential jobs from the transaction that might remove the loop. Also, systemd tries to suppress non-essential jobs in the transaction that would stop a running service. Finally it is checked whether the jobs of the transaction contradict jobs that have already been queued, and optionally the transaction is aborted then. If all worked out and the transaction is consistent and minimized in its impact it is merged with all already outstanding jobs and added to the run queue. Effectively this means that before executing a requested operation, systemd will verify that it makes sense, fixing it if possible, and only failing if it really cannot work.

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

For more information about the concepts and ideas behind systemd, please refer to the Original Design Document.

Note that some but not all interfaces provided by systemd are covered by the Interface Stability Promise.

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

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

Directories¶

System unit directories¶: The systemd system manager reads unit configuration from various directories. Packages that want to install unit files shall place them in the directory returned by pkg-config systemd --variable=systemdsystemunitdir. Other directories checked are /usr/local/lib/systemd/system and /usr/lib/systemd/system. User configuration always takes precedence. pkg-config systemd --variable=systemdsystemconfdir returns the path of the system configuration directory. Packages should alter the content of these directories only with the enable and disable commands of the systemctl(1) tool. Full list of directories is provided in systemd.unit(5).

User unit directories¶: Similar rules apply for the user unit directories. However, here the XDG Base Directory specification is followed to find units. Applications should place their unit files in the directory returned by pkg-config systemd --variable=systemduserunitdir. Global configuration is done in the directory reported by pkg-config systemd --variable=systemduserconfdir. The enable and disable commands of the systemctl(1) tool can handle both global (i.e. for all users) and private (for one user) enabling/disabling of units. Full list of directories is provided in systemd.unit(5).

SysV init scripts directory¶: The location of the SysV init script directory varies between distributions. If systemd cannot find a native unit file for a requested service, it will look for a SysV init script of the same name (with the .service suffix removed).

SysV runlevel link farm directory¶: The location of the SysV runlevel link farm directory varies between distributions. systemd will take the link farm into account when figuring out whether a service shall be enabled. Note that a service unit with a native unit configuration file cannot be started by activating it in the SysV runlevel link farm.

Signals¶

SIGTERM¶

Upon receiving this signal the systemd system manager serializes its state, reexecutes itself and deserializes the saved state again. This is mostly equivalent to systemctl daemon-reexec.

systemd user managers will start the exit.target unit when this signal is received. This is mostly equivalent to systemctl --user start exit.target.

SIGINT¶

Upon receiving this signal the systemd system manager will start the ctrl-alt-del.target unit. This is mostly equivalent to systemctl start ctl-alt-del.target.

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

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 start 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.

SIGRTMIN+4¶

Powers off the machine, starts the poweroff.target unit. This is mostly equivalent to systemctl start poweroff.target.

SIGRTMIN+5¶

Reboots the machine, starts the reboot.target unit. This is mostly equivalent to systemctl start reboot.target.

SIGRTMIN+6¶

Reboots the machine via kexec, starts the kexec.target unit. This is mostly equivalent to systemctl start kexec.target.

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+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, SIGRTMIN+23¶

Sets the log level to "debug" (or "info" on SIGRTMIN+23), as controlled via systemd.log_level=debug (or systemd.log_level=info on SIGRTMIN+23) on the kernel command line.

SIGRTMIN+24¶

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

SIGRTMIN+26, SIGRTMIN+27, SIGRTMIN+28¶

Sets the log level to "journal-or-kmsg" (or "console" on SIGRTMIN+27, "kmsg" on SIGRTMIN+28), as controlled via systemd.log_target=journal-or-kmsg (or systemd.log_target=console on SIGRTMIN+27 or systemd.log_target=kmsg on SIGRTMIN+28) on the kernel command line.

Environment¶

$SYSTEMD_LOG_LEVEL¶: systemd reads the log level from this environment variable. This can be overridden with --log-level=.
$SYSTEMD_LOG_TARGET¶: systemd reads the log target from this environment variable. This can be overridden with --log-target=.
$SYSTEMD_LOG_COLOR¶: Controls whether systemd highlights important log messages. This can be overridden with --log-color=.
$SYSTEMD_LOG_LOCATION¶: Controls whether systemd prints the code location along with log messages. This can be overridden with --log-location=.
$XDG_CONFIG_HOME, $XDG_CONFIG_DIRS, $XDG_DATA_HOME, $XDG_DATA_DIRS¶: The systemd user manager uses these variables in accordance to the XDG Base Directory specification to find its configuration.
$SYSTEMD_UNIT_PATH¶: Controls where systemd looks for unit files.
$SYSTEMD_SYSVINIT_PATH¶: Controls where systemd looks for SysV init scripts.
$SYSTEMD_SYSVRCND_PATH¶: Controls where systemd looks for SysV init script runlevel link farms.
$LISTEN_PID, $LISTEN_FDS¶: Set by systemd for supervised processes during socket-based activation. See sd_listen_fds(3) for more information.
$NOTIFY_SOCKET¶: Set by systemd for supervised processes for status and start-up completion notification. See sd_notify(3) for more information.

Kernel Command Line¶

When run as system instance systemd parses a number of kernel command line arguments^[1]:

systemd.unit=, rd.systemd.unit=¶: Overrides the unit to activate on boot. Defaults to default.target. This may be used to temporarily boot into a different boot unit, for example rescue.target or emergency.service. See systemd.special(7) for details about these units. The option prefixed with "rd." is honored only in the initial RAM disk (initrd), while the one that is not prefixed only in the main system.
systemd.dump_core=¶: Takes a boolean argument. If true, systemd dumps core when it crashes. Otherwise, no core dump is created. Defaults to true.
systemd.crash_shell=¶: Takes a boolean argument. If true, systemd spawns a shell when it crashes. Otherwise, no shell is spawned. Defaults to false, for security reasons, as the shell is not protected by any password authentication.
systemd.crash_chvt=¶: Takes an integer argument. If positive systemd activates the specified virtual terminal when it crashes. Defaults to -1.
systemd.confirm_spawn=¶: Takes a boolean argument. If true, asks for confirmation when spawning processes. Defaults to false.
systemd.show_status=¶: Takes a boolean argument or the constant auto. If true, shows terse service status updates on the console during bootup. auto behaves like false until a service fails or there is a significant delay in boot. Defaults to true, unless quiet is passed as kernel command line option in which case it defaults to auto.
systemd.log_target=, systemd.log_level=, systemd.log_color=, systemd.log_location=¶: Controls log output, with the same effect as the $SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_COLOR, $SYSTEMD_LOG_LOCATION environment variables described above.
systemd.default_standard_output=, systemd.default_standard_error=¶: Controls default standard output and error output for services, with the same effect as the --default-standard-output= and --default-standard-error= command line arguments described above, respectively.
systemd.setenv=¶: Takes a string argument in the form VARIABLE=VALUE. May be used to set default environment variables to add to forked child processes. May be used more than once to set multiple variables.
quiet¶: Turn off status output at boot, much like systemd.show_status=false would. Note that this option is also read by the kernel itself and disables kernel log output. 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, -b¶: Boot into emergency mode. This is equivalent to systemd.unit=emergency.target and provided for compatibility reasons and to be easier to type.
rescue, single, s, S, 1¶: Boot into rescue mode. This is equivalent to systemd.unit=rescue.target and provided for compatibility reasons and to be easier to type.
2, 3, 4, 5¶: Boot into the specified legacy SysV runlevel. These are equivalent to systemd.unit=runlevel2.target, systemd.unit=runlevel3.target, systemd.unit=runlevel4.target, and systemd.unit=runlevel5.target, respectively, and provided for compatibility reasons and to be easier to type.
locale.LANG=, locale.LANGUAGE=, locale.LC_CTYPE=, locale.LC_NUMERIC=, locale.LC_TIME=, locale.LC_COLLATE=, locale.LC_MONETARY=, locale.LC_MESSAGES=, locale.LC_PAPER=, locale.LC_NAME=, locale.LC_ADDRESS=, locale.LC_TELEPHONE=, locale.LC_MEASUREMENT=, locale.LC_IDENTIFICATION=¶: Set the system locale to use. This overrides the settings in /etc/locale.conf. For more information see locale.conf(5) and locale(7).

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

Sockets and FIFOs¶

/run/systemd/notify¶: Daemon status notification socket. This is an AF_UNIX datagram socket and is used to implement the daemon notification logic as implemented by sd_notify(3).
/run/systemd/shutdownd¶: Used internally by the shutdown(8) tool to implement delayed shutdowns. This is an AF_UNIX datagram socket.
/run/systemd/private¶: Used internally as communication channel between systemctl(1) and the systemd process. This is an AF_UNIX stream socket. This interface is private to systemd and should not be used in external projects.
/dev/initctl¶: Limited compatibility support for the SysV client interface, as implemented by the systemd-initctl.service unit. This is a named pipe in the file system. This interface is obsolete and should not be used in new applications.

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