Ubuntu Manpage: systemd, init - systemd system and service manager

name
synopsis
description
options
concepts
directories
signals
environment
kernel command line
sockets and fifos
see also
notes

Provided by: systemd_204-5ubuntu20.31_amd64

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 system instance, systemd interprets the configuration file system.conf, otherwise user.conf.
       See systemd-system.conf(5) for more information.

OPTIONS

The following options are understood:

-h, --help
Prints a short help text and exits.

--version
Prints a systemd version identifier 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.

--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, syslog, kmsg, journal-or-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.

--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.

CONCEPTS

systemd provides a dependency system between various entities called "units". 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 or dynamically from system state. 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 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).

10. 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 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[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 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[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 the Generators
Specification[4].

Systems which invoke systemd in a container or initrd environment should implement the Container
Interface[5] or initrd Interface[6] 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[7] 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 systemctl 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, SIGRTMIN+29
Sets the log level to journal-or-kmsg (or console on SIGRTMIN+27, kmsg on SIGRTMIN+28, or
syslog-or-kmsg on SIGRTMIN+29), as controlled via systemd.log_target=journal-or-kmsg (or
systemd.log_target=console on SIGRTMIN+27, systemd.log_target=kmsg on SIGRTMIN+28, or
systemd.log_target=syslog-or-kmsg on SIGRTMIN+29) 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[7] 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[8]:

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 isn't 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. If true shows terse service status updates on the console during bootup.
Defaults to true, unless quiet is passed as kernel command line option in which case it defaults to
false.

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 environment variables for the
init process and all its children at boot time. May be used more than once to set multiple variables.
If the equal sign and variable are missing it unsets an environment variable which might be passed in
from the initial ram disk.

quiet
If passed turns 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 to the kernel. Passing this
option hence turns off the usual output from both the system manager and the kernel.

emergency
Boot into emergency mode. This is equivalent to systemd.unit=emergency.target and provided for
compatibility reasons and to be easier to type.

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.

NOTES

        1. cgroups.txt
           http://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. Generators Specification
           http://www.freedesktop.org/wiki/Software/systemd/Generators

        5. Container Interface
           http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface

        6. initrd Interface
           http://www.freedesktop.org/wiki/Software/systemd/InitrdInterface

        7. XDG Base Directory specification
           http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

        8. If run inside a Linux container these arguments may be passed as command line arguments 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 instead.