SYSTEMD(1) systemd SYSTEMD(1)
NAME
systemd, init - systemd system and service manager
SYNOPSIS
systemd [OPTIONS...]
init [OPTIONS...] {COMMAND}
DESCRIPTION
systemd is a system and service manager for Linux operating systems. When run as first process on boot (as PID
1), it acts as init system that brings up and maintains userspace services.
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. 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
--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. Normally it should not be necessary to pass this options, as systemd automatically
detects the mode it is started in. This option is 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.
--crash-shell
Run shell on crash.
--confirm-spawn
Ask for confirmation when spawning processes.
--show-status=
Show terse service status information while booting. 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=
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:
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. 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).
8. Timer units are useful for triggering activation of other units based on timers. You may find details in
systemd.timer(5).
9. 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).
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[1] for more information about control groups, or short
"cgroups"). systemd uses this to effectively keep track of processes. Control group information is maintained
in the kernel, and is accessible via the file system hierarchy (beneath /sys/fs/cgroup/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[2].
Note that some but not all interfaces provided by systemd are covered by the Interface Stability Promise[3].
Units may be generated dynamically at boot and system manager reload time, for example based on other
configuration files or parameters passed on the kernel command line. For details see systemd.generator(7).
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.
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. If this signal is received more often 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 2s 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 start
default.target.
SIGRTMIN+1
Enters rescue mode, starts the rescue.target unit. This is mostly equivalent to systemctl isolate
rescue.target.
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+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[6] to
find its configuration.
$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[7]:
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.
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.
SEE ALSO
The systemd Homepage[8], systemd-system.conf(5), locale.conf(5), systemctl(1), journalctl(1), systemd-
notify(1), daemon(7), sd-daemon(3), systemd.unit(5), systemd.special(5), pkg-config(1), kernel-command-
line(7), bootup(7), systemd.directives(7)
NOTES
1. cgroups.txt
https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt
2. Original Design Document
http://0pointer.de/blog/projects/systemd.html
3. Interface Stability Promise
http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise
4. Container Interface
http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface
5. initrd Interface
http://www.freedesktop.org/wiki/Software/systemd/InitrdInterface
systemd 219 SYSTEMD(1)
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