summaryrefslogtreecommitdiff
path: root/man/systemd.xml
blob: 4f4a588a98e677e306dbe4313c19d3abb2b618a5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2010 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

<refentry id="systemd">

        <refentryinfo>
                <title>systemd</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>systemd</refentrytitle>
                <manvolnum>1</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>systemd</refname>
                <refname>init</refname>
                <refpurpose>systemd System and Session Manager</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <cmdsynopsis>
                        <command>systemd <arg choice="opt" rep="repeat">OPTIONS</arg></command>
                </cmdsynopsis>
                <cmdsynopsis>
                        <command>init <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="req">COMMAND</arg></command>
                </cmdsynopsis>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para>systemd is a system and session manager for
                Linux operating systems. When run as first process on
                boot (as PID 1), it acts as init system that brings
                up and maintains userspace services.</para>

                <para>For compatibility with SysV, if systemd is called
                as <command>init</command> and a PID that is not
                1, it will execute <command>telinit</command> and pass
                all command line arguments unmodified. That means
                <command>init</command> and <command>telinit</command>
                are mostly equivalent when invoked from normal login sessions. See
                <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                for more information.</para>

                <para>When run as system instance, systemd interprets
                the configuration file
                <filename>system.conf</filename>, otherwise
                <filename>session.conf</filename>. See
                <citerefentry><refentrytitle>systemd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                for more information.</para>
        </refsect1>

        <refsect1>
                <title>Options</title>

                <para>The following options are understood:</para>

                <variablelist>
                        <varlistentry>
                                <term><option>-h</option></term>
                                <term><option>--help</option></term>

                                <listitem><para>Prints a short help
                                text and exits.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--test</option></term>

                                <listitem><para>Determine startup
                                sequence, dump it and exit. This is an
                                option useful for debugging
                                only.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--dump-configuration-items</option></term>

                                <listitem><para>Dump understood unit
                                configuration items. This outputs a
                                terse but complete list of
                                configuration items understood in unit
                                definition files.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--introspect=</option></term>

                                <listitem><para>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.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--unit=</option></term>

                                <listitem><para>Set default unit to
                                activate on startup. If not specified
                                defaults to
                                <filename>default.target</filename>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--running-as=</option></term>

                                <listitem><para>Tell systemd to run in
                                a particular mode. Argument is one of
                                <option>system</option>,
                                <option>session</option>. Normally it
                                should not be necessary to pass this
                                option, as systemd automatically
                                detects the mode it is started
                                in. This call is hence of little use
                                except for
                                debugging.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--dump-core</option></term>

                                <listitem><para>Dump core on crash. This switch has no effect when run as session instance.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--crash-shell</option></term>

                                <listitem><para>Run shell on crash. This switch has no effect when run as session instance.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--confirm-spawn</option></term>

                                <listitem><para>Ask for confirmation when spawning processes. This switch has no effect when run as session instance.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--show-status</option></term>

                                <listitem><para>Show terse service status information while booting. This switch has no effect when run as session instance.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--log-target=</option></term>

                                <listitem><para>Set log
                                target. Argument must be one of
                                <option>console</option>,
                                <option>syslog</option>,
                                <option>kmsg</option>,
                                <option>syslog-or-kmsg</option>,
                                <option>null</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--log-level=</option></term>

                                <listitem><para>Set log level. As
                                argument this accepts a numerical log
                                level or the well-known <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                symbolic names (lowercase):
                                <option>emerg</option>,
                                <option>alert</option>,
                                <option>crit</option>,
                                <option>err</option>,
                                <option>warning</option>,
                                <option>notice</option>,
                                <option>info</option>,
                                <option>debug</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--log-color=</option></term>

                                <listitem><para>Highlight important
                                log messages. Argument is a boolean
                                value. If the argument is omitted it
                                defaults to
                                <option>true</option>.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--log-location=</option></term>

                                <listitem><para>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
                                <option>true</option>.</para></listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Concepts</title>

                <para>systemd provides a dependency system between
                various entities called "units". Units encapsulate
                various objects that are relevant for system boot-up
                and maintainance. The majority of units are configured
                in unit configuration files, whose syntax and basic
                set of options is described in
                <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                however some are created automatically from other
                configuration or dynamically from system state. Units
                may be active (meaning started, bound, plugged in, ...
                depending on the unit type), or inactive (meaning
                stopped, unbound, unplugged, ...), as well is in the
                process of being activated or deactivated,
                i.e. between the two states. The following unit types
                are available:</para>

                <orderedlist>
                        <listitem><para>Service units, which control
                        daemons and the processes they consist of. For
                        details see
                        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Socket units, which
                        encapsulate local IPC or network sockets in
                        the system, useful for socket-based
                        activation. For details about socket units see
                        <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        for details on socket-based activation and
                        other forms of activation, see
                        <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Target units are useful to
                        group units, or provide well-known
                        synchronization points during boot-up, see
                        <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Device units expose kernel
                        devices in systemd and may be used to
                        implement device-based activation. For details
                        see
                        <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Mount units control mount
                        points in the file system, for details see
                        <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Automount units provide
                        automount capabilities, for on-demand mounting
                        of file systems as well as parallelized
                        boot-up. See
                        <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>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
                        <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Timer units are useful for
                        triggering activation of other units based on
                        timers. You may find details in
                        <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Swap units are very similar to
                        mount units and encapsulated memory swap
                        partitions or files of the operating
                        systemd. They are described in <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                        <listitem><para>Path units may be used
                        activate other services when file system
                        objects change or are modified. See
                        <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

                </orderedlist>

                <para>Units are named as their configuration
                files. Some units have special semantics. A detailed
                list you may find in
                <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

                <para>On boot systemd activates the target unit
                <filename>default.target</filename> whose job it 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
                <filename>graphical.target</filename> (for
                fully-featured boots into the UI) or
                <filename>multi-user.target</filename> (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
                <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                for details about these target units.</para>

                <para>Processes systemd spawns ared placed in
                individual Linux control groups named after the unit
                which they belong to in the private systemd
                hierarchy. (see <ulink
                url="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
                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
                <filename>/cgroup/systemd/</filename>), or in tools
                such as
                <citerefentry><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                (<command>ps xawf -eo pid,user,cgroup,args</command>
                is particularly useful to list all processes and the
                systemd units they belong to.).</para>

                <para>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
                <filename>/dev/initctl</filename> interface is
                provided, and comaptibility implementations of the
                various SysV client tools available. In addition to
                that various established Unix functionality such as
                <filename>/etc/fstab</filename> or the
                <filename>utmp</filename> database are
                supported.</para>

                <para>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.</para>

                <para>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
                <filename>/sys</filename> or
                <filename>/proc</filename>.</para>

                <para>For more information about the concepts and
                ideas behind systemd please refer to the <ulink
                url="http://0pointer.de/blog/projects/systemd.html">Original
                Design Document</ulink>.</para>
        </refsect1>

        <refsect1>
                <title>Directories</title>

                <variablelist>
                        <varlistentry>
                                <term>System unit directories</term>

                                <listitem><para>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
                                <command>pkg-config systemd
                                --variable=systemdsystemunitdir</command>. Other
                                directories checked are
                                <filename>/usr/local/share/systemd/system</filename>
                                and
                                <filename>/usr/share/systemd/system</filename>. User
                                configuration always takes
                                precedence. <command>pkg-config
                                systemd
                                --variable=systemdsystemconfdir</command>
                                returns the path of the system
                                configuration directory. Packages
                                should alter the content of these directories
                                only with the
                                <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                                tool.</para></listitem>
                        </varlistentry>
                </variablelist>

                <variablelist>
                        <varlistentry>
                                <term>Session unit directories</term>

                                <listitem><para>Similar rules apply
                                for the session unit
                                directories. However, here the <ulink
                                url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
                                Base Directory specification</ulink>
                                is followed to find
                                units. Applications should place their
                                unit files in the directory returned
                                by <command>pkg-config systemd
                                --variable=systemdsessionunitdir</command>. Global
                                configuration is done in the
                                directory reported by
                                <command>pkg-config systemd
                                --variable=systemdsessionconfdir</command>. The
                                <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                                tool can handle both global (i.e. for
                                all users) and private (for one user)
                                enabling/disabling of
                                units.</para></listitem>
                        </varlistentry>
                </variablelist>

                <variablelist>
                        <varlistentry>
                                <term>SysV init scripts directory</term>

                                <listitem><para>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
                                <filename>.service</filename> suffix
                                removed).</para></listitem>
                        </varlistentry>
                </variablelist>

                <variablelist>
                        <varlistentry>
                                <term>SysV runlevel link farm directory</term>

                                <listitem><para>The location of the
                                SysV runlevel link farm directory
                                varies between distributions. systemd
                                will take the link farm into account
                                when figuring out whether a service
                                shall be enabled. Note that a service
                                unit with a native unit configuration
                                file can be started by activating it
                                in the SysV runlevel link
                                farm.</para></listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Signals</title>

                <variablelist>
                        <varlistentry>
                                <term>SIGTERM</term>

                                <listitem><para>Upon receiving this
                                signal the systemd system manager
                                serializes its state, reexecutes
                                itself and deserializes the saved
                                state again. This is mostly equivalent
                                to <command>systemctl
                                daemon-reexec</command>.</para>

                                <para>systemd session managers will
                                start the
                                <filename>exit.target</filename> unit
                                when this signal is received. This is
                                mostly equivalent to
                                <command>systemctl --session start
                                exit.target</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGINT</term>

                                <listitem><para>Upon receiving this
                                signal the systemd system manager will
                                start the
                                <filename>ctrl-alt-del.target</filename> unit. This
                                is mostly equivalent to
                                <command>systemctl start
                                ctl-alt-del.target</command>.</para>

                                <para>systemd session managers
                                treat this signal the same way as
                                SIGTERM.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGWINCH</term>

                                <listitem><para>When this signal is
                                received the systemd system manager
                                will start the
                                <filename>kbrequest.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl start
                                kbrequest.target</command>.</para>

                                <para>This signal is ignored by
                                systemd session
                                managers.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGPWR</term>

                                <listitem><para>When this signal is
                                received the systemd manager
                                will start the
                                <filename>sigpwr.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl start
                                sigpwr.target</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGUSR1</term>

                                <listitem><para>When this signal is
                                received the systemd manager will try
                                to reconnect to the D-Bus
                                bus.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGUSR2</term>

                                <listitem><para>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 <command>systemctl
                                dump</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGHUP</term>

                                <listitem><para>Reloads the complete
                                daemon configuration. This is mostly
                                equivalent to <command>systemctl
                                daemon-reload</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGRTMIN+0</term>

                                <listitem><para>Enters default mode, starts the
                                <filename>default.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl start
                                default.target</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGRTMIN+1</term>

                                <listitem><para>Enters rescue mode,
                                starts the
                                <filename>rescue.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl isolate
                                rescue.target</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGRTMIN+2</term>

                                <listitem><para>Enters emergency mode,
                                starts the
                                <filename>emergency.service</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl isolate
                                emergency.service</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGRTMIN+3</term>

                                <listitem><para>Halts the machine,
                                starts the
                                <filename>halt.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl start
                                halt.target</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGRTMIN+4</term>

                                <listitem><para>Powers off the machine,
                                starts the
                                <filename>poweroff.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl start
                                poweroff.target</command>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>SIGRTMIN+5</term>

                                <listitem><para>Reboots the machine,
                                starts the
                                <filename>reboot.target</filename>
                                unit. This is mostly equivalent to
                                <command>systemctl start
                                reboot.target</command>.</para></listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Environment</title>

                <variablelist>
                        <varlistentry>
                                <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
                                <listitem><para>systemd reads the
                                log level from this environment
                                variable. This can be overridden with
                                <option>--log-level=</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
                                <listitem><para>systemd reads the
                                log target from this environment
                                variable. This can be overridden with
                                <option>--log-target=</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
                                <listitem><para>Controls whether
                                systemd highlights important log
                                messages. This can be overridden with
                                <option>--log-color=</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
                                <listitem><para>Controls whether
                                systemd prints the code location along
                                with log messages. This can be
                                overridden with
                                <option>--log-location=</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$XDG_CONFIG_HOME</varname></term>
                                <term><varname>$XDG_CONFIG_DIRS</varname></term>
                                <term><varname>$XDG_DATA_HOME</varname></term>
                                <term><varname>$XDG_DATA_DIRS</varname></term>

                                <listitem><para>The systemd session
                                manager uses these variables in
                                accordance to the <ulink
                                url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
                                Base Directory specification</ulink>
                                to find its configuration.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$SYSTEMD_UNIT_PATH</varname></term>

                                <listitem><para>Controls where systemd
                                looks for unit
                                files.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>

                                <listitem><para>Controls where systemd
                                looks for SysV init scripts.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>

                                <listitem><para>Controls where systemd
                                looks for SysV init script runlevel link
                                farms.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$LISTEN_PID</varname></term>
                                <term><varname>$LISTEN_FDS</varname></term>

                                <listitem><para>Set by systemd for
                                supervised processes during
                                socket-based activation. See
                                <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                for more information.
                                </para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>$NOTIFY_SOCKET</varname></term>

                                <listitem><para>Set by systemd for
                                supervised processes for status and
                                start-up completion notification. See
                                <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                for more information.
                                </para></listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>Kernel Command Line</title>

                <para>When run as system instance systemd parses a few kernel command line arguments:</para>

                <variablelist>
                        <varlistentry>
                                <term><varname>systemd.unit=</varname></term>

                                <listitem><para>Overrides the unit to
                                activate on boot. Defaults to
                                <filename>default.target</filename>. This
                                may be used to temporarily boot into a
                                different boot unit, for example
                                <filename>rescue.target</filename> or
                                <filename>emergency.service</filename>. See
                                <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                                for details about these
                                units.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>systemd.log_target=</varname></term>
                                <term><varname>systemd.log_level=</varname></term>
                                <term><varname>systemd.log_color=</varname></term>
                                <term><varname>systemd.log_location=</varname></term>

                                <listitem><para>Controls log output,
                                with the same effect as the
                                <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
                                environment variables described above.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>systemd.dump_core=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                systemd dumps core when it
                                crashes. Otherwise no core dump is
                                created. Defaults to
                                <option>true</option>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>systemd.crash_shell=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                systemd spawns a shell when it
                                crashes. Otherwise no core dump is
                                created. Defaults to
                                <option>false</option>, for security
                                reasons, as the shell is not protected
                                by any password
                                authentication.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>systemd.crash_chvt=</varname></term>

                                <listitem><para>Takes an integer
                                argument. If positive systemd
                                activates the specified virtual
                                terminal when it crashes. Defaults to
                                <literal>-1</literal>.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><varname>systemd.show_status=</varname></term>

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>
                                shows terse service status updates on
                                the console during bootup. Defaults to
                                <option>true</option>.</para></listitem>
                        </varlistentry>

                </variablelist>
        </refsect1>

        <refsect1>
                <title>Sockets and FIFOs</title>

                <variablelist>
                        <varlistentry>
                                <term><filename>@/org/freedesktop/systemd1/notify</filename></term>

                                <listitem><para>Daemon status
                                notification socket. This is an AF_UNIX
                                datagram socket in the Linux abstract
                                namespace, and is used to implement
                                the daemon notification logic as
                                implemented by
                                <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>

                        </varlistentry>

                        <varlistentry>
                                <term><filename>@/org/freedesktop/systemd1/logger</filename></term>

                                <listitem><para>Used internally by the
                                <filename>systemd-logger.service</filename>
                                unit to connect STDOUT and/or STDERR
                                of spawned processes to
                                <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                                or the kernel log buffer. This is an
                                AF_UNIX stream socket in the Linux
                                abstract namespace.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>@/org/freedesktop/systemd1/private</filename></term>

                                <listitem><para>Used internally as
                                communication channel between
                                <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                                and the systemd process. This is an
                                AF_UNIX stream socket in the Linux
                                abstract namespace. This interface is
                                private to systemd and should not be
                                used in external
                                projects.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><filename>/dev/initctl</filename></term>

                                <listitem><para>Limited compatibility
                                support for the SysV client interface,
                                as implemented by the
                                <filename>systemd-initctl.service</filename>
                                unit. This is a named pipe in the file
                                system. This interface is obsolete and
                                should not be used in new
                                applications.</para></listitem>
                        </varlistentry>
                </variablelist>
        </refsect1>

        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemadm</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>