1 <?xml version='
1.0'
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2 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
6 SPDX-License-Identifier: LGPL-2.1+
8 This file is part of systemd.
10 Copyright 2010 Lennart Poettering
12 systemd is free software; you can redistribute it and/or modify it
13 under the terms of the GNU Lesser General Public License as published by
14 the Free Software Foundation; either version 2.1 of the License, or
15 (at your option) any later version.
17 systemd is distributed in the hope that it will be useful, but
18 WITHOUT ANY WARRANTY; without even the implied warranty of
19 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
20 Lesser General Public License for more details.
22 You should have received a copy of the GNU Lesser General Public License
23 along with systemd; If not, see <http://www.gnu.org/licenses/>.
26 <refentry id=
"systemd"
27 xmlns:
xi=
"http://www.w3.org/2001/XInclude">
30 <title>systemd
</title>
31 <productname>systemd
</productname>
35 <contrib>Developer
</contrib>
36 <firstname>Lennart
</firstname>
37 <surname>Poettering
</surname>
38 <email>lennart@poettering.net
</email>
44 <refentrytitle>systemd
</refentrytitle>
45 <manvolnum>1</manvolnum>
49 <refname>systemd
</refname>
50 <refname>init
</refname>
51 <refpurpose>systemd system and service manager
</refpurpose>
56 <command>systemd
</command>
57 <arg choice=
"opt" rep=
"repeat">OPTIONS
</arg>
60 <command>init
</command>
61 <arg choice=
"opt" rep=
"repeat">OPTIONS
</arg>
62 <arg choice=
"req">COMMAND
</arg>
67 <title>Description
</title>
69 <para>systemd is a system and service manager for Linux operating
70 systems. When run as first process on boot (as PID
1), it acts as
71 init system that brings up and maintains userspace
74 <para>For compatibility with SysV, if systemd is called as
75 <command>init
</command> and a PID that is not
1, it will execute
76 <command>telinit
</command> and pass all command line arguments
77 unmodified. That means
<command>init
</command> and
78 <command>telinit
</command> are mostly equivalent when invoked from
79 normal login sessions. See
80 <citerefentry><refentrytitle>telinit
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
81 for more information.
</para>
83 <para>When run as a system instance, systemd interprets the
84 configuration file
<filename>system.conf
</filename> and the files
85 in
<filename>system.conf.d
</filename> directories; when run as a
86 user instance, systemd interprets the configuration file
87 <filename>user.conf
</filename> and the files in
88 <filename>user.conf.d
</filename> directories. See
89 <citerefentry><refentrytitle>systemd-system.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
90 for more information.
</para>
94 <title>Options
</title>
96 <para>The following options are understood:
</para>
100 <term><option>--test
</option></term>
102 <listitem><para>Determine startup sequence, dump it and exit.
103 This is an option useful for debugging only.
</para></listitem>
106 <term><option>--dump-configuration-items
</option></term>
108 <listitem><para>Dump understood unit configuration items. This
109 outputs a terse but complete list of configuration items
110 understood in unit definition files.
</para></listitem>
113 <term><option>--unit=
</option></term>
115 <listitem><para>Set default unit to activate on startup. If
116 not specified, defaults to
117 <filename>default.target
</filename>.
</para></listitem>
120 <term><option>--system
</option></term>
121 <term><option>--user
</option></term>
123 <listitem><para>For
<option>--system
</option>, tell systemd to
124 run a system instance, even if the process ID is not
1, i.e.
125 systemd is not run as init process.
<option>--user
</option>
126 does the opposite, running a user instance even if the process
127 ID is
1. Normally, it should not be necessary to pass these
128 options, as systemd automatically detects the mode it is
129 started in. These options are hence of little use except for
130 debugging. Note that it is not supported booting and
131 maintaining a full system with systemd running in
132 <option>--system
</option> mode, but PID not
1. In practice,
133 passing
<option>--system
</option> explicitly is only useful in
134 conjunction with
<option>--test
</option>.
</para></listitem>
137 <term><option>--dump-core
</option></term>
139 <listitem><para>Enable core dumping on crash. This switch has
140 no effect when running as user instance. This setting may also
141 be enabled during boot on the kernel command line via the
142 <varname>systemd.dump_core=
</varname> option, see
143 below.
</para></listitem>
147 <term><option>--crash-vt=
</option><replaceable>VT
</replaceable></term>
149 <listitem><para>Switch to a specific virtual console (VT) on
150 crash. Takes a positive integer in the range
1–
63, or a
151 boolean argument. If an integer is passed, selects which VT to
152 switch to. If
<constant>yes
</constant>, the VT kernel messages
153 are written to is selected. If
<constant>no
</constant>, no VT
154 switch is attempted. This switch has no effect when running as
155 user instance. This setting may also be enabled during boot,
156 on the kernel command line via the
157 <varname>systemd.crash_vt=
</varname> option, see
158 <!-- FIXME: there is no crash_vt command line option? -->
159 below.
</para></listitem>
163 <term><option>--crash-shell
</option></term>
165 <listitem><para>Run a shell on crash. This switch has no
166 effect when running as user instance. This setting may also be
167 enabled during boot, on the kernel command line via the
168 <varname>systemd.crash_shell=
</varname> option, see
169 below.
</para></listitem>
173 <term><option>--crash-reboot
</option></term>
175 <listitem><para>Automatically reboot the system on crash. This
176 switch has no effect when running as user instance. This
177 setting may also be enabled during boot, on the kernel command
178 line via the
<varname>systemd.crash_reboot=
</varname> option,
179 see below.
</para></listitem>
183 <term><option>--confirm-spawn
</option></term>
185 <listitem><para>Ask for confirmation when spawning processes.
186 This switch has no effect when run as user
187 instance.
</para></listitem>
190 <term><option>--show-status=
</option></term>
192 <listitem><para>Takes a boolean argument or the special value
<constant>auto
</constant>. If on, terse unit
193 status information is shown on the console during boot-up and shutdown. If off, no such status information is
194 shown. If set to
<constant>auto
</constant> behavior is similar to off, except that it is automatically switched
195 to on, as soon as the first unit failure or significant boot delay is encountered. This switch has no effect
196 when invoked as user instance. If specified, overrides both the kernel command line setting
197 <varname>systemd.show_status=
</varname> (see below) and the configuration file option
198 <option>ShowStatus=
</option>, see
199 <citerefentry><refentrytitle>systemd-system.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
202 <term><option>--log-target=
</option></term>
204 <listitem><para>Set log target. Argument must be one of
205 <option>console
</option>,
206 <option>journal
</option>,
207 <option>kmsg
</option>,
208 <option>journal-or-kmsg
</option>,
209 <option>null
</option>.
</para></listitem>
212 <term><option>--log-level=
</option></term>
214 <listitem><para>Set log level. As
215 argument this accepts a numerical log
216 level or the well-known
<citerefentry project='man-pages'
><refentrytitle>syslog
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
217 symbolic names (lowercase):
218 <option>emerg
</option>,
219 <option>alert
</option>,
220 <option>crit
</option>,
221 <option>err
</option>,
222 <option>warning
</option>,
223 <option>notice
</option>,
224 <option>info
</option>,
225 <option>debug
</option>.
</para></listitem>
228 <term><option>--log-color=
</option></term>
230 <listitem><para>Highlight important log messages. Argument is
231 a boolean value. If the argument is omitted, it defaults to
232 <option>true
</option>.
</para></listitem>
235 <term><option>--log-location=
</option></term>
237 <listitem><para>Include code location in log messages. This is
238 mostly relevant for debugging purposes. Argument is a boolean
239 value. If the argument is omitted it defaults to
240 <option>true
</option>.
</para></listitem>
243 <term><option>--default-standard-output=
</option></term>
244 <term><option>--default-standard-error=
</option></term>
246 <listitem><para>Sets the default output or error output for
247 all services and sockets, respectively. That is, controls the
248 default for
<option>StandardOutput=
</option> and
249 <option>StandardError=
</option> (see
250 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
251 for details). Takes one of
252 <option>inherit
</option>,
253 <option>null
</option>,
254 <option>tty
</option>,
255 <option>journal
</option>,
256 <option>journal+console
</option>,
257 <option>syslog
</option>,
258 <option>syslog+console
</option>,
259 <option>kmsg
</option>,
260 <option>kmsg+console
</option>. If the
262 <option>--default-standard-output=
</option> defaults to
263 <option>journal
</option> and
264 <option>--default-standard-error=
</option> to
265 <option>inherit
</option>.
</para></listitem>
269 <term><option>--machine-id=
</option></term>
271 <listitem><para>Override the machine-id set on the hard drive,
272 useful for network booting or for containers. May not be set
273 to all zeros.
</para></listitem>
276 <xi:include href=
"standard-options.xml" xpointer=
"help" />
277 <xi:include href=
"standard-options.xml" xpointer=
"version" />
282 <title>Concepts
</title>
284 <para>systemd provides a dependency system between various
285 entities called
"units" of
11 different types. Units encapsulate
286 various objects that are relevant for system boot-up and
287 maintenance. The majority of units are configured in unit
288 configuration files, whose syntax and basic set of options is
290 <citerefentry><refentrytitle>systemd.unit
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
291 however some are created automatically from other configuration,
292 dynamically from system state or programmatically at runtime.
293 Units may be
"active" (meaning started, bound, plugged in, …,
294 depending on the unit type, see below), or
"inactive" (meaning
295 stopped, unbound, unplugged, …), as well as in the process of
296 being activated or deactivated, i.e. between the two states (these
297 states are called
"activating",
"deactivating"). A special
298 "failed" state is available as well, which is very similar to
299 "inactive" and is entered when the service failed in some way
300 (process returned error code on exit, or crashed, or an operation
301 timed out). If this state is entered, the cause will be logged,
302 for later reference. Note that the various unit types may have a
303 number of additional substates, which are mapped to the five
304 generalized unit states described here.
</para>
306 <para>The following unit types are available:
</para>
309 <listitem><para>Service units, which start and control daemons
310 and the processes they consist of. For details, see
311 <citerefentry><refentrytitle>systemd.service
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
313 <listitem><para>Socket units, which encapsulate local IPC or
314 network sockets in the system, useful for socket-based
315 activation. For details about socket units, see
316 <citerefentry><refentrytitle>systemd.socket
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
317 for details on socket-based activation and other forms of
319 <citerefentry><refentrytitle>daemon
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
</para></listitem>
321 <listitem><para>Target units are useful to group units, or
322 provide well-known synchronization points during boot-up, see
323 <citerefentry><refentrytitle>systemd.target
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
325 <listitem><para>Device units expose kernel devices in systemd
326 and may be used to implement device-based activation. For
328 <citerefentry><refentrytitle>systemd.device
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
330 <listitem><para>Mount units control mount points in the file
331 system, for details see
332 <citerefentry><refentrytitle>systemd.mount
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
334 <listitem><para>Automount units provide automount capabilities,
335 for on-demand mounting of file systems as well as parallelized
337 <citerefentry><refentrytitle>systemd.automount
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
339 <listitem><para>Timer units are useful for triggering activation
340 of other units based on timers. You may find details in
341 <citerefentry><refentrytitle>systemd.timer
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
343 <listitem><para>Swap units are very similar to mount units and
344 encapsulate memory swap partitions or files of the operating
345 system. They are described in
346 <citerefentry><refentrytitle>systemd.swap
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
348 <listitem><para>Path units may be used to activate other
349 services when file system objects change or are modified. See
350 <citerefentry><refentrytitle>systemd.path
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
352 <listitem><para>Slice units may be used to group units which
353 manage system processes (such as service and scope units) in a
354 hierarchical tree for resource management purposes. See
355 <citerefentry><refentrytitle>systemd.slice
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
357 <listitem><para>Scope units are similar to service units, but
358 manage foreign processes instead of starting them as well. See
359 <citerefentry><refentrytitle>systemd.scope
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para></listitem>
363 <para>Units are named as their configuration files. Some units
364 have special semantics. A detailed list is available in
365 <citerefentry><refentrytitle>systemd.special
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
</para>
367 <para>systemd knows various kinds of dependencies, including
368 positive and negative requirement dependencies (i.e.
369 <varname>Requires=
</varname> and
<varname>Conflicts=
</varname>) as
370 well as ordering dependencies (
<varname>After=
</varname> and
371 <varname>Before=
</varname>). NB: ordering and requirement
372 dependencies are orthogonal. If only a requirement dependency
373 exists between two units (e.g.
<filename>foo.service
</filename>
374 requires
<filename>bar.service
</filename>), but no ordering
375 dependency (e.g.
<filename>foo.service
</filename> after
376 <filename>bar.service
</filename>) and both are requested to start,
377 they will be started in parallel. It is a common pattern that both
378 requirement and ordering dependencies are placed between two
379 units. Also note that the majority of dependencies are implicitly
380 created and maintained by systemd. In most cases, it should be
381 unnecessary to declare additional dependencies manually, however
382 it is possible to do this.
</para>
384 <para>Application programs and units (via dependencies) may
385 request state changes of units. In systemd, these requests are
386 encapsulated as 'jobs' and maintained in a job queue. Jobs may
387 succeed or can fail, their execution is ordered based on the
388 ordering dependencies of the units they have been scheduled
391 <para>On boot systemd activates the target unit
392 <filename>default.target
</filename> whose job is to activate
393 on-boot services and other on-boot units by pulling them in via
394 dependencies. Usually, the unit name is just an alias (symlink) for
395 either
<filename>graphical.target
</filename> (for fully-featured
396 boots into the UI) or
<filename>multi-user.target
</filename> (for
397 limited console-only boots for use in embedded or server
398 environments, or similar; a subset of graphical.target). However,
399 it is at the discretion of the administrator to configure it as an
400 alias to any other target unit. See
401 <citerefentry><refentrytitle>systemd.special
</refentrytitle><manvolnum>7</manvolnum></citerefentry>
402 for details about these target units.
</para>
404 <para>Processes systemd spawns are placed in individual Linux
405 control groups named after the unit which they belong to in the
406 private systemd hierarchy. (see
<ulink
407 url=
"https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt
</ulink>
408 for more information about control groups, or short
"cgroups").
409 systemd uses this to effectively keep track of processes. Control
410 group information is maintained in the kernel, and is accessible
411 via the file system hierarchy (beneath
412 <filename>/sys/fs/cgroup/systemd/
</filename>), or in tools such as
413 <citerefentry project='man-pages'
><refentrytitle>systemd-cgls
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
415 <citerefentry project='man-pages'
><refentrytitle>ps
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
416 (
<command>ps xawf -eo pid,user,cgroup,args
</command> is
417 particularly useful to list all processes and the systemd units
418 they belong to.).
</para>
420 <para>systemd is compatible with the SysV init system to a large
421 degree: SysV init scripts are supported and simply read as an
422 alternative (though limited) configuration file format. The SysV
423 <filename>/dev/initctl
</filename> interface is provided, and
424 compatibility implementations of the various SysV client tools are
425 available. In addition to that, various established Unix
426 functionality such as
<filename>/etc/fstab
</filename> or the
427 <filename>utmp
</filename> database are supported.
</para>
429 <para>systemd has a minimal transaction system: if a unit is
430 requested to start up or shut down it will add it and all its
431 dependencies to a temporary transaction. Then, it will verify if
432 the transaction is consistent (i.e. whether the ordering of all
433 units is cycle-free). If it is not, systemd will try to fix it up,
434 and removes non-essential jobs from the transaction that might
435 remove the loop. Also, systemd tries to suppress non-essential
436 jobs in the transaction that would stop a running service. Finally
437 it is checked whether the jobs of the transaction contradict jobs
438 that have already been queued, and optionally the transaction is
439 aborted then. If all worked out and the transaction is consistent
440 and minimized in its impact it is merged with all already
441 outstanding jobs and added to the run queue. Effectively this
442 means that before executing a requested operation, systemd will
443 verify that it makes sense, fixing it if possible, and only
444 failing if it really cannot work.
</para>
446 <para>Systemd contains native implementations of various tasks
447 that need to be executed as part of the boot process. For example,
448 it sets the hostname or configures the loopback network device. It
449 also sets up and mounts various API file systems, such as
450 <filename>/sys
</filename> or
<filename>/proc
</filename>.
</para>
452 <para>For more information about the concepts and
453 ideas behind systemd, please refer to the
454 <ulink url=
"http://0pointer.de/blog/projects/systemd.html">Original Design Document
</ulink>.
</para>
456 <para>Note that some but not all interfaces provided
457 by systemd are covered by the
458 <ulink url=
"https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
459 Stability Promise
</ulink>.
</para>
461 <para>Units may be generated dynamically at boot and system
462 manager reload time, for example based on other configuration
463 files or parameters passed on the kernel command line. For details, see
464 <citerefentry><refentrytitle>systemd.generator
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
</para>
466 <para>Systems which invoke systemd in a container or initrd
467 environment should implement the
468 <ulink url=
"https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface
</ulink> or
469 <ulink url=
"https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface
</ulink>
470 specifications, respectively.
</para>
474 <title>Directories
</title>
478 <term>System unit directories
</term>
480 <listitem><para>The systemd system manager reads unit
481 configuration from various directories. Packages that want to
482 install unit files shall place them in the directory returned
483 by
<command>pkg-config systemd
484 --variable=systemdsystemunitdir
</command>. Other directories
485 checked are
<filename>/usr/local/lib/systemd/system
</filename>
486 and
<filename>/usr/lib/systemd/system
</filename>. User
487 configuration always takes precedence.
<command>pkg-config
488 systemd --variable=systemdsystemconfdir
</command> returns the
489 path of the system configuration directory. Packages should
490 alter the content of these directories only with the
491 <command>enable
</command> and
<command>disable
</command>
493 <citerefentry><refentrytitle>systemctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
494 tool. Full list of directories is provided in
495 <citerefentry><refentrytitle>systemd.unit
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
502 <term>User unit directories
</term>
504 <listitem><para>Similar rules apply for the user unit
505 directories. However, here the
506 <ulink url=
"http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
507 Base Directory specification
</ulink> is followed to find
508 units. Applications should place their unit files in the
509 directory returned by
<command>pkg-config systemd
510 --variable=systemduserunitdir
</command>. Global configuration
511 is done in the directory reported by
<command>pkg-config
512 systemd --variable=systemduserconfdir
</command>. The
513 <command>enable
</command> and
<command>disable
</command>
515 <citerefentry><refentrytitle>systemctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
516 tool can handle both global (i.e. for all users) and private
517 (for one user) enabling/disabling of units. Full list of
518 directories is provided in
519 <citerefentry><refentrytitle>systemd.unit
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
526 <term>SysV init scripts directory
</term>
528 <listitem><para>The location of the SysV init script directory
529 varies between distributions. If systemd cannot find a native
530 unit file for a requested service, it will look for a SysV
531 init script of the same name (with the
532 <filename>.service
</filename> suffix
533 removed).
</para></listitem>
539 <term>SysV runlevel link farm directory
</term>
541 <listitem><para>The location of the SysV runlevel link farm
542 directory varies between distributions. systemd will take the
543 link farm into account when figuring out whether a service
544 shall be enabled. Note that a service unit with a native unit
545 configuration file cannot be started by activating it in the
546 SysV runlevel link farm.
</para></listitem>
552 <title>Signals
</title>
556 <term><constant>SIGTERM
</constant></term>
558 <listitem><para>Upon receiving this signal the systemd system
559 manager serializes its state, reexecutes itself and
560 deserializes the saved state again. This is mostly equivalent
561 to
<command>systemctl daemon-reexec
</command>.
</para>
563 <para>systemd user managers will start the
564 <filename>exit.target
</filename> unit when this signal is
565 received. This is mostly equivalent to
<command>systemctl
566 --user start exit.target
567 --job-mode=replace-irreversible
</command>.
</para></listitem>
571 <term><constant>SIGINT
</constant></term>
573 <listitem><para>Upon receiving this signal the systemd system
574 manager will start the
575 <filename>ctrl-alt-del.target
</filename> unit. This is mostly
576 equivalent to
<command>systemctl start ctl-alt-del.target
577 --job-mode=replace-irreversible
</command>. If this signal is
578 received more than
7 times per
2s, an immediate reboot is
579 triggered. Note that pressing Ctrl-Alt-Del on the console
580 will trigger this signal. Hence, if a reboot is hanging,
581 pressing Ctrl-Alt-Del more than
7 times in
2s is a relatively
582 safe way to trigger an immediate reboot.
</para>
584 <para>systemd user managers treat this signal the same way as
585 <constant>SIGTERM
</constant>.
</para></listitem>
589 <term><constant>SIGWINCH
</constant></term>
591 <listitem><para>When this signal is received the systemd
592 system manager will start the
593 <filename>kbrequest.target
</filename> unit. This is mostly
594 equivalent to
<command>systemctl start
595 kbrequest.target
</command>.
</para>
597 <para>This signal is ignored by systemd user
598 managers.
</para></listitem>
602 <term><constant>SIGPWR
</constant></term>
604 <listitem><para>When this signal is received the systemd
605 manager will start the
<filename>sigpwr.target
</filename>
606 unit. This is mostly equivalent to
<command>systemctl start
607 sigpwr.target
</command>.
</para></listitem>
611 <term><constant>SIGUSR1
</constant></term>
613 <listitem><para>When this signal is received the systemd
614 manager will try to reconnect to the D-Bus
615 bus.
</para></listitem>
619 <term><constant>SIGUSR2
</constant></term>
621 <listitem><para>When this signal is received the systemd
622 manager will log its complete state in human-readable form.
623 The data logged is the same as printed by
624 <command>systemd-analyze dump
</command>.
</para></listitem>
628 <term><constant>SIGHUP
</constant></term>
630 <listitem><para>Reloads the complete daemon configuration.
631 This is mostly equivalent to
<command>systemctl
632 daemon-reload
</command>.
</para></listitem>
636 <term><constant>SIGRTMIN+
0</constant></term>
638 <listitem><para>Enters default mode, starts the
639 <filename>default.target
</filename> unit. This is mostly
640 equivalent to
<command>systemctl isolate
641 default.target
</command>.
</para></listitem>
645 <term><constant>SIGRTMIN+
1</constant></term>
647 <listitem><para>Enters rescue mode, starts the
648 <filename>rescue.target
</filename> unit. This is mostly
649 equivalent to
<command>systemctl isolate
650 rescue.target
</command>.
</para></listitem>
654 <term><constant>SIGRTMIN+
2</constant></term>
656 <listitem><para>Enters emergency mode, starts the
657 <filename>emergency.service
</filename> unit. This is mostly
658 equivalent to
<command>systemctl isolate
659 emergency.service
</command>.
</para></listitem>
663 <term><constant>SIGRTMIN+
3</constant></term>
665 <listitem><para>Halts the machine, starts the
666 <filename>halt.target
</filename> unit. This is mostly
667 equivalent to
<command>systemctl start halt.target
668 --job-mode=replace-irreversible
</command>.
</para>
673 <term><constant>SIGRTMIN+
4</constant></term>
675 <listitem><para>Powers off the machine, starts the
676 <filename>poweroff.target
</filename> unit. This is mostly
677 equivalent to
<command>systemctl start poweroff.target
678 --job-mode=replace-irreversible
</command>.
</para>
683 <term><constant>SIGRTMIN+
5</constant></term>
685 <listitem><para>Reboots the machine, starts the
686 <filename>reboot.target
</filename> unit. This is mostly
687 equivalent to
<command>systemctl start reboot.target
688 --job-mode=replace-irreversible
</command>.
</para>
693 <term><constant>SIGRTMIN+
6</constant></term>
695 <listitem><para>Reboots the machine via kexec, starts the
696 <filename>kexec.target
</filename> unit. This is mostly
697 equivalent to
<command>systemctl start kexec.target
698 --job-mode=replace-irreversible
</command>.
</para>
703 <term><constant>SIGRTMIN+
13</constant></term>
705 <listitem><para>Immediately halts the machine.
</para></listitem>
709 <term><constant>SIGRTMIN+
14</constant></term>
711 <listitem><para>Immediately powers off the machine.
</para></listitem>
715 <term><constant>SIGRTMIN+
15</constant></term>
717 <listitem><para>Immediately reboots the machine.
</para></listitem>
721 <term><constant>SIGRTMIN+
16</constant></term>
723 <listitem><para>Immediately reboots the machine with kexec.
</para></listitem>
727 <term><constant>SIGRTMIN+
20</constant></term>
729 <listitem><para>Enables display of status messages on the
730 console, as controlled via
731 <varname>systemd.show_status=
1</varname> on the kernel command
732 line.
</para></listitem>
736 <term><constant>SIGRTMIN+
21</constant></term>
738 <listitem><para>Disables display of
739 status messages on the console, as
741 <varname>systemd.show_status=
0</varname>
742 on the kernel command
743 line.
</para></listitem>
747 <term><constant>SIGRTMIN+
22</constant></term>
748 <term><constant>SIGRTMIN+
23</constant></term>
750 <listitem><para>Sets the log level to
<literal>debug
</literal>
751 (or
<literal>info
</literal> on
752 <constant>SIGRTMIN+
23</constant>), as controlled via
753 <varname>systemd.log_level=debug
</varname> (or
754 <varname>systemd.log_level=info
</varname> on
755 <constant>SIGRTMIN+
23</constant>) on the kernel command
756 line.
</para></listitem>
760 <term><constant>SIGRTMIN+
24</constant></term>
762 <listitem><para>Immediately exits the manager (only available
763 for --user instances).
</para></listitem>
767 <term><constant>SIGRTMIN+
26</constant></term>
768 <term><constant>SIGRTMIN+
27</constant></term>
769 <term><constant>SIGRTMIN+
28</constant></term>
771 <listitem><para>Sets the log target to
772 <literal>journal-or-kmsg
</literal> (or
773 <literal>console
</literal> on
774 <constant>SIGRTMIN+
27</constant>,
<literal>kmsg
</literal> on
775 <constant>SIGRTMIN+
28</constant>), as controlled via
776 <varname>systemd.log_target=journal-or-kmsg
</varname> (or
777 <varname>systemd.log_target=console
</varname> on
778 <constant>SIGRTMIN+
27</constant> or
779 <varname>systemd.log_target=kmsg
</varname> on
780 <constant>SIGRTMIN+
28</constant>) on the kernel command
781 line.
</para></listitem>
787 <title>Environment
</title>
789 <variablelist class='environment-variables'
>
791 <term><varname>$SYSTEMD_LOG_LEVEL
</varname></term>
792 <listitem><para>systemd reads the log level from this
793 environment variable. This can be overridden with
794 <option>--log-level=
</option>.
</para></listitem>
798 <term><varname>$SYSTEMD_LOG_TARGET
</varname></term>
799 <listitem><para>systemd reads the log target from this
800 environment variable. This can be overridden with
801 <option>--log-target=
</option>.
</para></listitem>
805 <term><varname>$SYSTEMD_LOG_COLOR
</varname></term>
806 <listitem><para>Controls whether systemd highlights important
807 log messages. This can be overridden with
808 <option>--log-color=
</option>.
</para></listitem>
812 <term><varname>$SYSTEMD_LOG_LOCATION
</varname></term>
813 <listitem><para>Controls whether systemd prints the code
814 location along with log messages. This can be overridden with
815 <option>--log-location=
</option>.
</para></listitem>
819 <term><varname>$XDG_CONFIG_HOME
</varname></term>
820 <term><varname>$XDG_CONFIG_DIRS
</varname></term>
821 <term><varname>$XDG_DATA_HOME
</varname></term>
822 <term><varname>$XDG_DATA_DIRS
</varname></term>
824 <listitem><para>The systemd user manager uses these variables
825 in accordance to the
<ulink
826 url=
"http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
827 Base Directory specification
</ulink> to find its
828 configuration.
</para></listitem>
832 <term><varname>$SYSTEMD_UNIT_PATH
</varname></term>
834 <listitem><para>Controls where systemd looks for unit
835 files.
</para></listitem>
839 <term><varname>$SYSTEMD_SYSVINIT_PATH
</varname></term>
841 <listitem><para>Controls where systemd looks for SysV init
842 scripts.
</para></listitem>
846 <term><varname>$SYSTEMD_SYSVRCND_PATH
</varname></term>
848 <listitem><para>Controls where systemd looks for SysV init
849 script runlevel link farms.
</para></listitem>
853 <term><varname>$SYSTEMD_COLORS
</varname></term>
855 <listitem><para>The value must be a boolean. Controls whether colorized output should be
856 generated. This can be specified to override the decision that
<command>systemd
</command>
857 makes based on
<varname>$TERM
</varname> and what the console is connected to.
</para>
862 <term><varname>$LISTEN_PID
</varname></term>
863 <term><varname>$LISTEN_FDS
</varname></term>
864 <term><varname>$LISTEN_FDNAMES
</varname></term>
866 <listitem><para>Set by systemd for supervised processes during
867 socket-based activation. See
868 <citerefentry><refentrytitle>sd_listen_fds
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
869 for more information.
</para></listitem>
873 <term><varname>$NOTIFY_SOCKET
</varname></term>
875 <listitem><para>Set by systemd for supervised processes for
876 status and start-up completion notification. See
877 <citerefentry><refentrytitle>sd_notify
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
878 for more information.
</para></listitem>
884 <title>Kernel Command Line
</title>
886 <para>When run as system instance systemd parses a number of
887 kernel command line arguments
<footnote><para>If run inside a Linux
888 container these arguments may be passed as command line arguments
889 to systemd itself, next to any of the command line options listed
890 in the Options section above. If run outside of Linux containers,
891 these arguments are parsed from
<filename>/proc/cmdline
</filename>
892 instead.
</para></footnote>:
</para>
894 <variablelist class='kernel-commandline-options'
>
896 <term><varname>systemd.unit=
</varname></term>
897 <term><varname>rd.systemd.unit=
</varname></term>
899 <listitem><para>Overrides the unit to activate on boot.
900 Defaults to
<filename>default.target
</filename>. This may be
901 used to temporarily boot into a different boot unit, for
902 example
<filename>rescue.target
</filename> or
903 <filename>emergency.service
</filename>. See
904 <citerefentry><refentrytitle>systemd.special
</refentrytitle><manvolnum>7</manvolnum></citerefentry>
905 for details about these units. The option prefixed with
906 <literal>rd.
</literal> is honored only in the initial RAM disk
907 (initrd), while the one that is not prefixed only in the main
908 system.
</para></listitem>
912 <term><varname>systemd.dump_core
</varname></term>
914 <listitem><para>Takes a boolean argument or enables the option if specified
915 without an argument. If enabled, the systemd manager (PID
1) dumps core when
916 it crashes. Otherwise, no core dump is created. Defaults to enabled.
</para>
921 <term><varname>systemd.crash_chvt
</varname></term>
923 <listitem><para>Takes a positive integer, or a boolean argument. Can be also
924 specified without an argument, with the same effect as a positive boolean. If
925 a positive integer (in the range
1–
63) is specified, the system manager (PID
926 1) will activate the specified virtual terminal (VT) when it
927 crashes. Defaults to disabled, meaning that no such switch is attempted. If
928 set to enabled, the VT the kernel messages are written to is selected.
933 <term><varname>systemd.crash_shell
</varname></term>
935 <listitem><para>Takes a boolean argument or enables the option if specified
936 without an argument. If enabled, the system manager (PID
1) spawns a shell
937 when it crashes, after a
10s delay. Otherwise, no shell is spawned. Defaults
938 to disabled, for security reasons, as the shell is not protected by password
939 authentication.
</para></listitem>
943 <term><varname>systemd.crash_reboot
</varname></term>
945 <listitem><para>Takes a boolean argument or enables the option if specified
946 without an argument. If enabled, the system manager (PID
1) will reboot the
947 machine automatically when it crashes, after a
10s delay. Otherwise, the
948 system will hang indefinitely. Defaults to disabled, in order to avoid a
949 reboot loop. If combined with
<varname>systemd.crash_shell
</varname>, the
950 system is rebooted after the shell exits.
</para></listitem>
954 <term><varname>systemd.confirm_spawn
</varname></term>
956 <listitem><para>Takes a boolean argument or a path to the virtual console
957 where the confirmation messages should be emitted. Can be also specified
958 without an argument, with the same effect as a positive boolean. If enabled,
959 the system manager (PID
1) asks for confirmation when spawning processes
960 using
<option>/dev/console
</option>. If a path or a console name (such as
961 <literal>ttyS0
</literal>) is provided, the virtual console pointed to by this
962 path or described by the give name will be used instead. Defaults to disabled.
967 <term><varname>systemd.show_status
</varname></term>
969 <listitem><para>Takes a boolean argument or the constant
970 <constant>auto
</constant>. Can be also specified without an argument, with
971 the same effect as a positive boolean. If enabled, the systemd manager (PID
972 1) shows terse service status updates on the console during bootup.
973 <constant>auto
</constant> behaves like
<option>false
</option> until a unit
974 fails or there is a significant delay in boot. Defaults to enabled, unless
975 <option>quiet
</option> is passed as kernel command line option, in which case
976 it defaults to
<constant>auto
</constant>. If specified overrides the system
977 manager configuration file option
<option>ShowStatus=
</option>, see
978 <citerefentry><refentrytitle>systemd-system.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
979 However, the process command line option
<option>--show-status=
</option>
980 takes precedence over both this kernel command line option and the
981 configuration file option.
</para></listitem>
985 <term><varname>systemd.log_target=
</varname></term>
986 <term><varname>systemd.log_level=
</varname></term>
987 <term><varname>systemd.log_location=
</varname></term>
988 <term><varname>systemd.log_color
</varname></term>
990 <listitem><para>Controls log output, with the same effect as the
991 <varname>$SYSTEMD_LOG_TARGET
</varname>,
992 <varname>$SYSTEMD_LOG_LEVEL
</varname>,
993 <varname>$SYSTEMD_LOG_LOCATION
</varname>,
994 <varname>$SYSTEMD_LOG_COLOR
</varname> environment variables described above.
995 <varname>systemd.log_color
</varname> can be specified without an argument,
996 with the same effect as a positive boolean.
</para></listitem>
1000 <term><varname>systemd.default_standard_output=
</varname></term>
1001 <term><varname>systemd.default_standard_error=
</varname></term>
1002 <listitem><para>Controls default standard output and error
1003 output for services, with the same effect as the
1004 <option>--default-standard-output=
</option> and
1005 <option>--default-standard-error=
</option> command line
1006 arguments described above, respectively.
</para></listitem>
1010 <term><varname>systemd.setenv=
</varname></term>
1012 <listitem><para>Takes a string argument in the form
1013 VARIABLE=VALUE. May be used to set default environment
1014 variables to add to forked child processes. May be used more
1015 than once to set multiple variables.
</para></listitem>
1019 <term><varname>systemd.machine_id=
</varname></term>
1021 <listitem><para>Takes a
32 character hex value to be
1022 used for setting the machine-id. Intended mostly for
1023 network booting where the same machine-id is desired
1024 for every boot.
</para></listitem>
1028 <term><varname>systemd.unified_cgroup_hierarchy
</varname></term>
1030 <listitem><para>When specified without an argument or with a true argument,
1031 enables the usage of
1032 <ulink url=
"https://www.kernel.org/doc/Documentation/cgroup-v2.txt">unified cgroup hierarchy
</ulink>
1033 (a.k.a. cgroups-v2). When specified with a false argument, fall back to
1034 hybrid or full legacy cgroup hierarchy.
</para>
1036 <para>If this option is not specified, the default behaviour is determined
1037 during compilation (the
<option>--with-default-hierarchy=
</option>
1038 option). If the kernel does not support unified cgroup hierarchy, the legacy
1039 hierarchy will be used even if this option is specified.
</para>
1044 <term><varname>systemd.legacy_systemd_cgroup_controller
</varname></term>
1046 <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
1047 (see previous option). When specified without an argument or with a true
1048 argument, disables the use of
"hybrid" cgroup hierarchy (i.e. a cgroups-v2
1049 tree used for systemd, and
1050 <ulink url=
"https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
1051 cgroup hierarchy
</ulink>, a.k.a. cgroups-v1, for other controllers), and
1052 forces a full
"legacy" mode. When specified with a false argument, enables
1053 the use of
"hybrid" hierarchy.
</para>
1055 <para>If this option is not specified, the default behaviour is determined
1056 during compilation (the
<option>--with-default-hierarchy=
</option>
1057 option). If the kernel does not support unified cgroup hierarchy, the legacy
1058 hierarchy will be used even if this option is specified.
</para>
1063 <term><varname>quiet
</varname></term>
1065 <listitem><para>Turn off status output at boot, much like
1066 <varname>systemd.show_status=false
</varname> would. Note that
1067 this option is also read by the kernel itself and disables
1068 kernel log output. Passing this option hence turns off the
1069 usual output from both the system manager and the kernel.
1074 <term><varname>debug
</varname></term>
1076 <listitem><para>Turn on debugging output. This is equivalent
1077 to
<varname>systemd.log_level=debug
</varname>. Note that this
1078 option is also read by the kernel itself and enables kernel
1079 debug output. Passing this option hence turns on the debug
1080 output from both the system manager and the
1081 kernel.
</para></listitem>
1085 <term><varname>emergency
</varname></term>
1086 <term><varname>rd.emergency
</varname></term>
1087 <term><varname>-b
</varname></term>
1089 <listitem><para>Boot into emergency mode. This is equivalent
1090 to
<varname>systemd.unit=emergency.target
</varname> or
1091 <varname>rd.systemd.unit=emergency.target
</varname>, respectively, and
1092 provided for compatibility reasons and to be easier to type.
</para></listitem>
1096 <term><varname>rescue
</varname></term>
1097 <term><varname>rd.rescue
</varname></term>
1098 <term><varname>single
</varname></term>
1099 <term><varname>s
</varname></term>
1100 <term><varname>S
</varname></term>
1101 <term><varname>1</varname></term>
1103 <listitem><para>Boot into rescue mode. This is equivalent to
1104 <varname>systemd.unit=rescue.target
</varname> or
1105 <varname>rd.systemd.unit=rescue.target
</varname>, respectively, and
1106 provided for compatibility reasons and to be easier to type.
</para></listitem>
1110 <term><varname>2</varname></term>
1111 <term><varname>3</varname></term>
1112 <term><varname>4</varname></term>
1113 <term><varname>5</varname></term>
1115 <listitem><para>Boot into the specified legacy SysV runlevel.
1116 These are equivalent to
1117 <varname>systemd.unit=runlevel2.target
</varname>,
1118 <varname>systemd.unit=runlevel3.target
</varname>,
1119 <varname>systemd.unit=runlevel4.target
</varname>, and
1120 <varname>systemd.unit=runlevel5.target
</varname>,
1121 respectively, and provided for compatibility reasons and to be
1122 easier to type.
</para></listitem>
1126 <term><varname>locale.LANG=
</varname></term>
1127 <term><varname>locale.LANGUAGE=
</varname></term>
1128 <term><varname>locale.LC_CTYPE=
</varname></term>
1129 <term><varname>locale.LC_NUMERIC=
</varname></term>
1130 <term><varname>locale.LC_TIME=
</varname></term>
1131 <term><varname>locale.LC_COLLATE=
</varname></term>
1132 <term><varname>locale.LC_MONETARY=
</varname></term>
1133 <term><varname>locale.LC_MESSAGES=
</varname></term>
1134 <term><varname>locale.LC_PAPER=
</varname></term>
1135 <term><varname>locale.LC_NAME=
</varname></term>
1136 <term><varname>locale.LC_ADDRESS=
</varname></term>
1137 <term><varname>locale.LC_TELEPHONE=
</varname></term>
1138 <term><varname>locale.LC_MEASUREMENT=
</varname></term>
1139 <term><varname>locale.LC_IDENTIFICATION=
</varname></term>
1141 <listitem><para>Set the system locale to use. This overrides
1142 the settings in
<filename>/etc/locale.conf
</filename>. For
1143 more information, see
1144 <citerefentry project='man-pages'
><refentrytitle>locale.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1146 <citerefentry project='man-pages'
><refentrytitle>locale
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
1151 <para>For other kernel command line parameters understood by
1152 components of the core OS, please refer to
1153 <citerefentry><refentrytitle>kernel-command-line
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
</para>
1157 <title>Sockets and FIFOs
</title>
1161 <term><filename>/run/systemd/notify
</filename></term>
1163 <listitem><para>Daemon status notification socket. This is an
1164 <constant>AF_UNIX
</constant> datagram socket and is used to
1165 implement the daemon notification logic as implemented by
1166 <citerefentry><refentrytitle>sd_notify
</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
</para></listitem>
1171 <term><filename>/run/systemd/private
</filename></term>
1173 <listitem><para>Used internally as communication channel
1175 <citerefentry><refentrytitle>systemctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1176 and the systemd process. This is an
1177 <constant>AF_UNIX
</constant> stream socket. This interface is
1178 private to systemd and should not be used in external
1179 projects.
</para></listitem>
1183 <term><filename>/dev/initctl
</filename></term>
1185 <listitem><para>Limited compatibility support for the SysV
1186 client interface, as implemented by the
1187 <filename>systemd-initctl.service
</filename> unit. This is a
1188 named pipe in the file system. This interface is obsolete and
1189 should not be used in new applications.
</para></listitem>
1195 <title>See Also
</title>
1197 The
<ulink url=
"https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage
</ulink>,
1198 <citerefentry><refentrytitle>systemd-system.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1199 <citerefentry project='man-pages'
><refentrytitle>locale.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1200 <citerefentry><refentrytitle>systemctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1201 <citerefentry><refentrytitle>journalctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1202 <citerefentry><refentrytitle>systemd-notify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1203 <citerefentry><refentrytitle>daemon
</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1204 <citerefentry><refentrytitle>sd-daemon
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
1205 <citerefentry><refentrytitle>systemd.unit
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1206 <citerefentry><refentrytitle>systemd.special
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1207 <citerefentry project='die-net'
><refentrytitle>pkg-config
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1208 <citerefentry><refentrytitle>kernel-command-line
</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1209 <citerefentry project='man-pages'
><refentrytitle>bootup
</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1210 <citerefentry><refentrytitle>systemd.directives
</refentrytitle><manvolnum>7</manvolnum></citerefentry>