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SYSTEMD(1) | systemd | SYSTEMD(1) |
NAME¶
systemd, init - systemd System and Service ManagerSYNOPSIS¶
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.conf(5) for more information.OPTIONS¶
The following options are understood: -h, --helpPrints 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.
--system, --user
Tell systemd to run a system instance (resp.
user instance), even if the process ID is not 1 (resp. is 1), i.e. systemd is
not (resp. is) run as init process. 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.
--sysv-console=
Controls whether output of SysV init scripts
will be directed to the console. 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 resp. error output for
all services and sockets, i.e. controls the default for StandardOutput=
resp. 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].
DIRECTORIES¶
System unit directoriesThe 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.
User unit directories
Similar rules apply for the user unit
directories. However, here the XDG Base Directory specification[4] 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.
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¶
SIGTERMUpon 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 (resp. info on
SIGRTMIN+23), as controlled via systemd.log_level=debug (resp.
systemd.log_level=info on SIGRTMIN+23) on the kernel command
line.
SIGRTMIN+26, SIGRTMIN+27, SIGRTMIN+28, SIGRTMIN+29
Sets the log level to journal-or-kmsg (resp.
console on SIGRTMIN+27; resp. kmsg on SIGRTMIN+28; resp. syslog-or-kmsg on
SIGRTMIN+29), as controlled via systemd.log_target=journal-or-kmsg
(resp. systemd.log_target=console on SIGRTMIN+27; resp.
systemd.log_target=kmsg on SIGRTMIN+28; resp
systemd.log_target=syslog-or-kmsg on SIGRTMIN+29) on the kernel command
line.
ENVIRONMENT¶
$SYSTEMD_LOG_LEVELsystemd 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[4] 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[5]: 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.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.
systemd.sysv_console=
Takes a boolean argument. If true
output of SysV init scripts will be directed to the console. 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/error output
for services, with the same effect as the --default-standard-output=
resp. --default-standard-error= command line arguments described
above.
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 unsets an environment
variable which might be passed in from the initial ram disk.
SOCKETS AND FIFOS¶
/run/systemd/notifyDaemon 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¶
AUTHOR¶
Lennart Poettering <lennart@poettering.net>Developer
NOTES¶
- 1.
- cgroups.txt
- 2.
- Original Design Document
- 3.
- Interface Stability Promise
- 4.
- XDG Base Directory specification
- 5.
- 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.
10/07/2013 | systemd |