systemd, init — systemd System and Session Manager
systemd [OPTIONS...]
init [OPTIONS...] {COMMAND}
systemd is a system and session 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 system instance, systemd interprets
the configuration file
system.conf
, otherwise
session.conf
. See
systemd.conf(5)
for more information.
The following options are understood:
-h
, --help
Prints a short help text and exits.
--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.
--introspect=
Extract D-Bus interface introspection data. This is mostly useful at install time to generate data suitable for the D-Bus interfaces repository. Optionally the interface name for the introspection data may be specified. If omitted, the introspection data for all interfaces is dumped.
--unit=
Set default unit to
activate on startup. If not specified
defaults to
default.target
.
--running-as=
Tell systemd to run in
a particular mode. Argument is one of
system
,
session
. Normally it
should not be necessary to pass this
option, as systemd automatically
detects the mode it is started
in. This call is hence of little use
except for
debugging.
--dump-core
Dump core on crash. This switch has no effect when run as session instance.
--crash-shell
Run shell on crash. This switch has no effect when run as session instance.
--confirm-spawn
Ask for confirmation when spawning processes. This switch has no effect when run as session instance.
--show-status
Show terse service status information while booting. This switch has no effect when run as session instance.
--log-target=
Set log
target. Argument must be one of
console
,
syslog
,
kmsg
,
syslog-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
.
systemd provides a dependency system between various entities called "units". Units encapsulate various objects that are relevant for system boot-up and maintainance. 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 or dynamically from system state. Units may be active (meaning started, bound, plugged in, ... depending on the unit type), or inactive (meaning stopped, unbound, unplugged, ...), as well as in the process of being activated or deactivated, i.e. between the two states. The following unit types are available:
Service units, which 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.automount(5).
Timer units are useful for triggering activation of other units based on timers. You may find details in systemd.timer(5).
Swap units are very similar to mount units and encapsulated memory swap partitions or files of the operating systemd. 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).
Units are named as their configuration files. Some units have special semantics. A detailed list you may find in systemd.special(7).
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
/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 host name 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.
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/share/systemd/system
and
/usr/share/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
systemd-install(1)
tool.
Similar rules apply for the session 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=systemdsessionunitdir. Global configuration is done in the directory reported by pkg-config systemd --variable=systemdsessionconfdir. The systemd-install(1) tool can handle both global (i.e. for all users) and private (for one user) enabling/disabling of units.
The location of the
SysV init script directory varies
between distributions. If systemd
cannot find a native unit file for a
requested service, it will look for a
SysV init script of the same name
(with the
.service
suffix
removed).
The location of the SysV runlevel link farm directory varies between distributions. systemd will take the link farm into account when figuring out whether a service shall be enabled. Note that a service unit with a native unit configuration file can be started by activating it in the SysV runlevel link farm.
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 session managers will
start the
exit.target
unit
when this signal is received. This is
mostly equivalent to
systemctl --session start
exit.target.
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 session managers treat this signal the same way as SIGTERM.
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 session managers.
When this signal is
received the systemd manager
will start the
sigpwr.target
unit. This is mostly equivalent to
systemctl start
sigpwr.target.
When this signal is received the systemd manager will try to reconnect to the D-Bus bus.
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 systemctl dump.
Reloads the complete daemon configuration. This is mostly equivalent to systemctl daemon-reload.
Enters default mode, starts the
default.target
unit. This is mostly equivalent to
systemctl start
default.target.
Enters rescue mode,
starts the
rescue.target
unit. This is mostly equivalent to
systemctl isolate
rescue.target.
Enters emergency mode,
starts the
emergency.service
unit. This is mostly equivalent to
systemctl isolate
emergency.service.
Halts the machine,
starts the
halt.target
unit. This is mostly equivalent to
systemctl start
halt.target.
Powers off the machine,
starts the
poweroff.target
unit. This is mostly equivalent to
systemctl start
poweroff.target.
Reboots the machine,
starts the
reboot.target
unit. This is mostly equivalent to
systemctl start
reboot.target.
$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 session 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.
When run as system instance systemd parses a few kernel command line arguments:
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.
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.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 core dump is
created. 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.show_status=
Takes a boolean
argument. If true
shows terse service status updates on
the console during bootup. Defaults to
true
.
@/org/freedesktop/systemd1/notify
Daemon status notification socket. This is an AF_UNIX datagram socket in the Linux abstract namespace, and is used to implement the daemon notification logic as implemented by sd_notify(3).
@/org/freedesktop/systemd1/logger
Used internally by the
systemd-logger.service
unit to connect STDOUT and/or STDERR
of spawned processes to
syslog(3)
or the kernel log buffer. This is an
AF_UNIX stream socket in the Linux
abstract namespace.
@/org/freedesktop/systemd1/private
Used internally as communication channel between systemctl(1) and the systemd process. This is an AF_UNIX stream socket in the Linux abstract namespace. 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.