chiark / gitweb /
path: simplify recheck logic
[elogind.git] / man / systemd.unit.xml
1 <?xml version='1.0'?> <!--*-nxml-*-->
2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
3         "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4
5 <!--
6   This file is part of systemd.
7
8   Copyright 2010 Lennart Poettering
9
10   systemd is free software; you can redistribute it and/or modify it
11   under the terms of the GNU General Public License as published by
12   the Free Software Foundation; either version 2 of the License, or
13   (at your option) any later version.
14
15   systemd is distributed in the hope that it will be useful, but
16   WITHOUT ANY WARRANTY; without even the implied warranty of
17   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
18   General Public License for more details.
19
20   You should have received a copy of the GNU General Public License
21   along with systemd; If not, see <http://www.gnu.org/licenses/>.
22 -->
23
24 <refentry id="systemd.unit">
25
26         <refentryinfo>
27                 <title>systemd.unit</title>
28                 <productname>systemd</productname>
29
30                 <authorgroup>
31                         <author>
32                                 <contrib>Developer</contrib>
33                                 <firstname>Lennart</firstname>
34                                 <surname>Poettering</surname>
35                                 <email>lennart@poettering.net</email>
36                         </author>
37                 </authorgroup>
38         </refentryinfo>
39
40         <refmeta>
41                 <refentrytitle>systemd.unit</refentrytitle>
42                 <manvolnum>5</manvolnum>
43         </refmeta>
44
45         <refnamediv>
46                 <refname>systemd.unit</refname>
47                 <refpurpose>systemd unit configuration files</refpurpose>
48         </refnamediv>
49
50         <refsynopsisdiv>
51                 <para><filename>systemd.service</filename>,
52                 <filename>systemd.socket</filename>,
53                 <filename>systemd.device</filename>,
54                 <filename>systemd.mount</filename>,
55                 <filename>systemd.automount</filename>,
56                 <filename>systemd.swap</filename>,
57                 <filename>systemd.target</filename>,
58                 <filename>systemd.path</filename>,
59                 <filename>systemd.timer</filename>,
60                 <filename>systemd.snapshot</filename></para>
61         </refsynopsisdiv>
62
63         <refsect1>
64                 <title>Description</title>
65
66                 <para>A unit configuration file encodes information
67                 about a service, a socket, a device, a mount point, an
68                 automount point, a swap file or partition, a start-up
69                 target, a file system path or a timer controlled and
70                 supervised by
71                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. The
72                 syntax is inspired by <ulink
73                 url="http://standards.freedesktop.org/desktop-entry-spec/latest/">XDG
74                 Desktop Entry Specification</ulink> <filename>.desktop</filename> files, which are in turn
75                 inspired by Microsoft Windows
76                 <filename>.ini</filename> files.</para>
77
78                 <para>This man pages lists the common configuration
79                 options of all the unit types. These options need to
80                 be configured in the [Unit] resp. [Install]
81                 section of the unit files.</para>
82
83                 <para>In addition to the generic [Unit] and [Install]
84                 sections described here, each unit should have a
85                 type-specific section, e.g. [Service] for a service
86                 unit. See the respective man pages for more
87                 information.</para>
88
89                 <para>Unit files may contain additional options on top
90                 of those listed here. If systemd encounters an unknown
91                 option it will write a warning log message but
92                 continue loading the unit. If an option is prefixed
93                 with <option>X-</option> it is ignored completely by
94                 systemd. Applications may use this to include
95                 additional information in the unit files.</para>
96
97                 <para>Boolean arguments used in unit files can be
98                 written in various formats. For positive settings the
99                 strings <option>1</option>, <option>yes</option>,
100                 <option>true</option> and <option>on</option> are
101                 equivalent. For negative settings the strings
102                 <option>0</option>, <option>no</option>,
103                 <option>false</option> and <option>off</option> are
104                 equivalent.</para>
105
106                 <para>Time span values encoded in unit files can be
107                 written in various formats. A stand-alone number
108                 specifies a time in seconds. If suffixed with a time
109                 unit, the unit is honored. A concatenation of
110                 multiple values with units is supported, in which case
111                 the values are added up. Example: "50" refers to 50
112                 seconds; "2min 200ms" refers to 2 minutes plus 200
113                 milliseconds, i.e. 120200ms. The following time units
114                 are understood: s, min, h, d, w, ms, us.</para>
115
116                 <para>Empty lines and lines starting with # or ; are
117                 ignored. This may be used for commenting. Lines ending
118                 in a backslash are concatenated with the following
119                 line while reading and the backslash is replaced by a
120                 space character. This may be used to wrap long lines.</para>
121
122                 <para>If a line starts with <option>.include</option>
123                 followed by a file name, the specified file will be
124                 read as if its contents were listed in place of the
125                 <option>.include</option> directive.</para>
126
127                 <para>Along with a unit file
128                 <filename>foo.service</filename> a directory
129                 <filename>foo.service.wants/</filename> may exist. All
130                 units symlinked from such a directory are implicitly
131                 added as dependencies of type
132                 <varname>Wanted=</varname> to the unit. This is useful
133                 to hook units into the start-up of other units,
134                 without having to modify their unit configuration
135                 files. For details about the semantics of
136                 <varname>Wanted=</varname> see below. The preferred
137                 way to create symlinks in the
138                 <filename>.wants/</filename> directory of a service is
139                 with the <command>enable</command> command of the
140                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
141                 tool which reads information from the [Install]
142                 section of unit files. (See below.) A similar
143                 functionality exists for <varname>Requires=</varname>
144                 type dependencies as well, the directory suffix is
145                 <filename>.requires/</filename> in this case.</para>
146
147                 <para>Note that while systemd offers a flexible
148                 dependency system between units it is recommended to
149                 use this functionality only sparsely and instead rely
150                 on techniques such as bus-based or socket-based
151                 activation which makes dependencies implicit, which
152                 both results in a simpler and more flexible
153                 system.</para>
154
155                 <para>Some unit names reflect paths existing in the
156                 file system name space. Example: a device unit
157                 <filename>dev-sda.device</filename> refers to a device
158                 with the device node <filename>/dev/sda</filename> in
159                 the file system namespace. If this applies a special
160                 way to escape the path name is used, so that the
161                 result is usable as part of a file name. Basically,
162                 given a path, "/" is replaced by "-", and all
163                 unprintable characters and the "-" are replaced by
164                 C-style "\x20" escapes. The root directory "/" is
165                 encoded as single dash, while otherwise the initial
166                 and ending "/" is removed from all paths during
167                 transformation. This escaping is reversible.</para>
168
169                 <para>Optionally, units may be instantiated from a
170                 template file at runtime. This allows creation of
171                 multiple units from a single configuration file. If
172                 systemd looks for a unit configuration file it will
173                 first search for the literal unit name in the
174                 filesystem. If that yields no success and the unit
175                 name contains an @ character, systemd will look for a
176                 unit template that shares the same name but with the
177                 instance string (i.e. the part between the @ character
178                 and the suffix) removed. Example: if a service
179                 <filename>getty@tty3.service</filename> is requested
180                 and no file by that name is found, systemd will look
181                 for <filename>getty@.service</filename> and
182                 instantiate a service from that configuration file if
183                 it is found. To refer to the instance string from
184                 within the configuration file you may use the special
185                 <literal>%i</literal> specifier in many of the
186                 configuration options. Other specifiers that may be
187                 used are <literal>%n</literal>, <literal>%N</literal>,
188                 <literal>%p</literal>, <literal>%P</literal>,
189                 <literal>%I</literal> and <literal>%f</literal>, for
190                 the full unit name, the unescaped unit name, the
191                 prefix name, the unescaped prefix name, the unescaped
192                 instance name and the unescaped filename,
193                 respectively. The unescaped filename is either the
194                 unescaped instance name (if set) with / prepended (if
195                 necessary), or the prefix name similarly prepended
196                 with /. The prefix name here refers to the string
197                 before the @, i.e. "getty" in the example above, where
198                 "tty3" is the instance name.</para>
199
200                 <para>If a unit file is empty (i.e. has the file size
201                 0) or is symlinked to <filename>/dev/null</filename>
202                 its configuration will not be loaded and it appears
203                 with a load state of <literal>masked</literal>, and
204                 cannot be activated. Use this as an effective way to
205                 fully disable a unit, making it impossible to start it
206                 even manually.</para>
207
208                 <para>The unit file format is covered by the
209                 <ulink
210                 url="http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
211                 Stability Promise</ulink>.</para>
212         </refsect1>
213
214         <refsect1>
215                 <title>Options</title>
216
217                 <para>Unit file may include a [Unit] section, which
218                 carries generic information about the unit that is not
219                 dependent on the type of unit:</para>
220
221                 <variablelist>
222
223                         <varlistentry>
224                                 <term><varname>Description=</varname></term>
225                                 <listitem><para>A free-form string
226                                 describing the unit. This is intended
227                                 for use in UIs to show descriptive
228                                 information along with the unit
229                                 name.</para></listitem>
230                         </varlistentry>
231
232                         <varlistentry>
233                                 <term><varname>Requires=</varname></term>
234
235                                 <listitem><para>Configures requirement
236                                 dependencies on other units. If this
237                                 unit gets activated, the units listed
238                                 here will be activated as well. If one
239                                 of the other units gets deactivated or
240                                 its activation fails, this unit will
241                                 be deactivated. This option may be
242                                 specified more than once, in which
243                                 case requirement dependencies for all
244                                 listed names are created. Note that
245                                 requirement dependencies do not
246                                 influence the order in which services
247                                 are started or stopped. This has to be
248                                 configured independently with the
249                                 <varname>After=</varname> or
250                                 <varname>Before=</varname> options. If
251                                 a unit
252                                 <filename>foo.service</filename>
253                                 requires a unit
254                                 <filename>bar.service</filename> as
255                                 configured with
256                                 <varname>Requires=</varname> and no
257                                 ordering is configured with
258                                 <varname>After=</varname> or
259                                 <varname>Before=</varname>, then both
260                                 units will be started simultaneously
261                                 and without any delay between them if
262                                 <filename>foo.service</filename> is
263                                 activated. Often it is a better choice
264                                 to use <varname>Wants=</varname>
265                                 instead of
266                                 <varname>Requires=</varname> in order
267                                 to achieve a system that is more
268                                 robust when dealing with failing
269                                 services.</para></listitem>
270                         </varlistentry>
271
272                         <varlistentry>
273                                 <term><varname>RequiresOverridable=</varname></term>
274
275                                 <listitem><para>Similar to
276                                 <varname>Requires=</varname>.
277                                 Dependencies listed in
278                                 <varname>RequiresOverridable=</varname>
279                                 which cannot be fulfilled or fail to
280                                 start are ignored if the startup was
281                                 explicitly requested by the user. If
282                                 the start-up was pulled in indirectly
283                                 by some dependency or automatic
284                                 start-up of units that is not
285                                 requested by the user this dependency
286                                 must be fulfilled and otherwise the
287                                 transaction fails. Hence, this option
288                                 may be used to configure dependencies
289                                 that are normally honored unless the
290                                 user explicitly starts up the unit, in
291                                 which case whether they failed or not
292                                 is irrelevant.</para></listitem>
293
294                         </varlistentry>
295                         <varlistentry>
296                                 <term><varname>Requisite=</varname></term>
297                                 <term><varname>RequisiteOverridable=</varname></term>
298
299                                 <listitem><para>Similar to
300                                 <varname>Requires=</varname>
301                                 resp. <varname>RequiresOverridable=</varname>. However,
302                                 if a unit listed here is not started
303                                 already it will not be started and the
304                                 transaction fails
305                                 immediately.</para></listitem>
306                         </varlistentry>
307
308                         <varlistentry>
309                                 <term><varname>Wants=</varname></term>
310
311                                 <listitem><para>A weaker version of
312                                 <varname>Requires=</varname>. A unit
313                                 listed in this option will be started
314                                 if the configuring unit is. However,
315                                 if the listed unit fails to start up
316                                 or cannot be added to the transaction
317                                 this has no impact on the validity of
318                                 the transaction as a whole. This is
319                                 the recommended way to hook start-up
320                                 of one unit to the start-up of another
321                                 unit. Note that dependencies of this
322                                 type may also be configured outside of
323                                 the unit configuration file by
324                                 adding a symlink to a
325                                 <filename>.wants/</filename> directory
326                                 accompanying the unit file. For
327                                 details see above.</para></listitem>
328                         </varlistentry>
329
330                         <varlistentry>
331                                 <term><varname>BindTo=</varname></term>
332
333                                 <listitem><para>Configures requirement
334                                 dependencies, very similar in style to
335                                 <varname>Requires=</varname>, however
336                                 in addition to this behaviour it also
337                                 declares that this unit is stopped
338                                 when any of the units listed suddenly
339                                 disappears. Units can suddenly,
340                                 unexpectedly disappear if a service
341                                 terminates on its own choice, a device
342                                 is unplugged or a mount point
343                                 unmounted without involvement of
344                                 systemd.</para></listitem>
345                         </varlistentry>
346
347                         <varlistentry>
348                                 <term><varname>Conflicts=</varname></term>
349
350                                 <listitem><para>Configures negative
351                                 requirement dependencies. If a unit
352                                 has a
353                                 <varname>Conflicts=</varname> setting
354                                 on another unit, starting the former
355                                 will stop the latter and vice
356                                 versa. Note that this setting is
357                                 independent of and orthogonal to the
358                                 <varname>After=</varname> and
359                                 <varname>Before=</varname> ordering
360                                 dependencies.</para>
361
362                                 <para>If a unit A that conflicts with
363                                 a unit B is scheduled to be started at
364                                 the same time as B, the transaction
365                                 will either fail (in case both are
366                                 required part of the transaction) or
367                                 be modified to be fixed (in case one
368                                 or both jobs are not a required part
369                                 of the transaction). In the latter
370                                 case the job that is not the required
371                                 will be removed, or in case both are
372                                 not required the unit that conflicts
373                                 will be started and the unit that is
374                                 conflicted is
375                                 stopped.</para></listitem>
376                         </varlistentry>
377
378                         <varlistentry>
379                                 <term><varname>Before=</varname></term>
380                                 <term><varname>After=</varname></term>
381
382                                 <listitem><para>Configures ordering
383                                 dependencies between units. If a unit
384                                 <filename>foo.service</filename>
385                                 contains a setting
386                                 <option>Before=bar.service</option>
387                                 and both units are being started,
388                                 <filename>bar.service</filename>'s
389                                 start-up is delayed until
390                                 <filename>foo.service</filename> is
391                                 started up. Note that this setting is
392                                 independent of and orthogonal to the
393                                 requirement dependencies as configured
394                                 by <varname>Requires=</varname>. It is
395                                 a common pattern to include a unit
396                                 name in both the
397                                 <varname>After=</varname> and
398                                 <varname>Requires=</varname> option in
399                                 which case the unit listed will be
400                                 started before the unit that is
401                                 configured with these options. This
402                                 option may be specified more than
403                                 once, in which case ordering
404                                 dependencies for all listed names are
405                                 created. <varname>After=</varname> is
406                                 the inverse of
407                                 <varname>Before=</varname>, i.e. while
408                                 <varname>After=</varname> ensures that
409                                 the configured unit is started after
410                                 the listed unit finished starting up,
411                                 <varname>Before=</varname> ensures the
412                                 opposite, i.e.  that the configured
413                                 unit is fully started up before the
414                                 listed unit is started. Note that when
415                                 two units with an ordering dependency
416                                 between them are shut down, the
417                                 inverse of the start-up order is
418                                 applied. i.e. if a unit is configured
419                                 with <varname>After=</varname> on
420                                 another unit, the former is stopped
421                                 before the latter if both are shut
422                                 down. If one unit with an ordering
423                                 dependency on another unit is shut
424                                 down while the latter is started up,
425                                 the shut down is ordered before the
426                                 start-up regardless whether the
427                                 ordering dependency is actually of
428                                 type <varname>After=</varname> or
429                                 <varname>Before=</varname>. If two
430                                 units have no ordering dependencies
431                                 between them they are shut down
432                                 resp. started up simultaneously, and
433                                 no ordering takes
434                                 place. </para></listitem>
435                         </varlistentry>
436
437                         <varlistentry>
438                                 <term><varname>OnFailure=</varname></term>
439
440                                 <listitem><para>Lists one or more
441                                 units that are activated when this
442                                 unit enters the
443                                 '<literal>failed</literal>'
444                                 state.</para></listitem>
445                         </varlistentry>
446
447                         <varlistentry>
448                                 <term><varname>OnFailureIsolate=</varname></term>
449
450                                 <listitem><para>Takes a boolean
451                                 argument. If <option>true</option> the
452                                 unit listed in
453                                 <varname>OnFailure=</varname> will be
454                                 enqueued in isolation mode, i.e. all
455                                 units that are not its dependency will
456                                 be stopped. If this is set only a
457                                 single unit may be listed in
458                                 <varname>OnFailure=</varname>. Defaults
459                                 to
460                                 <option>false</option>.</para></listitem>
461                         </varlistentry>
462
463                         <varlistentry>
464                                 <term><varname>IgnoreOnIsolate=</varname></term>
465
466                                 <listitem><para>Takes a boolean
467                                 argument. If <option>true</option>
468                                 this unit will not be stopped when
469                                 isolating another unit. Defaults to
470                                 <option>false</option>.</para></listitem>
471                         </varlistentry>
472
473                         <varlistentry>
474                                 <term><varname>StopWhenUnneeded=</varname></term>
475
476                                 <listitem><para>Takes a boolean
477                                 argument. If <option>true</option>
478                                 this unit will be stopped when it is
479                                 no longer used. Note that in order to
480                                 minimize the work to be executed,
481                                 systemd will not stop units by default
482                                 unless they are conflicting with other
483                                 units, or the user explicitly
484                                 requested their shut down. If this
485                                 option is set, a unit will be
486                                 automatically cleaned up if no other
487                                 active unit requires it. Defaults to
488                                 <option>false</option>.</para></listitem>
489                         </varlistentry>
490
491                         <varlistentry>
492                                 <term><varname>RefuseManualStart=</varname></term>
493                                 <term><varname>RefuseManualStop=</varname></term>
494
495                                 <listitem><para>Takes a boolean
496                                 argument. If <option>true</option>
497                                 this unit can only be activated
498                                 (resp. deactivated) indirectly. In
499                                 this case explicit start-up
500                                 (resp. termination) requested by the
501                                 user is denied, however if it is
502                                 started (resp. stopped) as a
503                                 dependency of another unit, start-up
504                                 (resp. termination) will succeed. This
505                                 is mostly a safety feature to ensure
506                                 that the user does not accidentally
507                                 activate units that are not intended
508                                 to be activated explicitly, and not
509                                 accidentally deactivate units that are
510                                 not intended to be deactivated.
511                                 These options default to
512                                 <option>false</option>.</para></listitem>
513                         </varlistentry>
514
515                         <varlistentry>
516                                 <term><varname>AllowIsolate=</varname></term>
517
518                                 <listitem><para>Takes a boolean
519                                 argument. If <option>true</option>
520                                 this unit may be used with the
521                                 <command>systemctl isolate</command>
522                                 command. Otherwise this will be
523                                 refused. It probably is a good idea to
524                                 leave this disabled except for target
525                                 units that shall be used similar to
526                                 runlevels in SysV init systems, just
527                                 as a precaution to avoid unusable
528                                 system states. This option defaults to
529                                 <option>false</option>.</para></listitem>
530                         </varlistentry>
531
532                         <varlistentry>
533                                 <term><varname>DefaultDependencies=</varname></term>
534
535                                 <listitem><para>Takes a boolean
536                                 argument. If <option>true</option>
537                                 (the default), a few default
538                                 dependencies will implicitly be
539                                 created for the unit. The actual
540                                 dependencies created depend on the
541                                 unit type. For example, for service
542                                 units, these dependencies ensure that
543                                 the service is started only after
544                                 basic system initialization is
545                                 completed and is properly terminated on
546                                 system shutdown. See the respective
547                                 man pages for details. Generally, only
548                                 services involved with early boot or
549                                 late shutdown should set this option
550                                 to <option>false</option>. It is
551                                 highly recommended to leave this
552                                 option enabled for the majority of
553                                 common units. If set to
554                                 <option>false</option> this option
555                                 does not disable all implicit
556                                 dependencies, just non-essential
557                                 ones.</para></listitem>
558                         </varlistentry>
559
560                         <varlistentry>
561                                 <term><varname>JobTimeoutSec=</varname></term>
562
563                                 <listitem><para>When clients are
564                                 waiting for a job of this unit to
565                                 complete, time out after the specified
566                                 time. If this time limit is reached
567                                 the job will be cancelled, the unit
568                                 however will not change state or even
569                                 enter the '<literal>failed</literal>'
570                                 mode. This value defaults to 0 (job
571                                 timeouts disabled), except for device
572                                 units. NB: this timeout is independent
573                                 from any unit-specific timeout (for
574                                 example, the timeout set with
575                                 <varname>Timeout=</varname> in service
576                                 units) as the job timeout has no
577                                 effect on the unit itself, only on the
578                                 job that might be pending for it. Or
579                                 in other words: unit-specific timeouts
580                                 are useful to abort unit state
581                                 changes, and revert them. The job
582                                 timeout set with this option however
583                                 is useful to abort only the job
584                                 waiting for the unit state to
585                                 change.</para></listitem>
586                         </varlistentry>
587
588                         <varlistentry>
589                                 <term><varname>ConditionPathExists=</varname></term>
590                                 <term><varname>ConditionPathIsDirectory=</varname></term>
591                                 <term><varname>ConditionDirectoryNotEmpty=</varname></term>
592                                 <term><varname>ConditionKernelCommandLine=</varname></term>
593                                 <term><varname>ConditionVirtualization=</varname></term>
594                                 <term><varname>ConditionSecurity=</varname></term>
595                                 <term><varname>ConditionNull=</varname></term>
596
597                                 <listitem><para>Before starting a unit
598                                 verify that the specified condition is
599                                 true. With
600                                 <varname>ConditionPathExists=</varname>
601                                 a file existance condition can be
602                                 checked before a unit is started. If
603                                 the specified absolute path name does
604                                 not exist startup of a unit will not
605                                 actually happen, however the unit is
606                                 still useful for ordering purposes in
607                                 this case. The condition is checked at
608                                 the time the queued start job is to be
609                                 executed. If the absolute path name
610                                 passed to
611                                 <varname>ConditionPathExists=</varname>
612                                 is prefixed with an exclamation mark
613                                 (!), the test is negated, and the unit
614                                 only started if the path does not
615                                 exist. <varname>ConditionPathIsDirectory=</varname>
616                                 is similar to
617                                 <varname>ConditionPathExists=</varname>
618                                 but verifies whether a certain path
619                                 exists and is a directory.
620                                 <varname>ConditionDirectoryNotEmpty=</varname>
621                                 is similar to
622                                 <varname>ConditionPathExists=</varname>
623                                 but verifies whether a certain path
624                                 exists and is a non-empty
625                                 directory. Similarly
626                                 <varname>ConditionKernelCommandLine=</varname>
627                                 may be used to check whether a
628                                 specific kernel command line option is
629                                 set (or if prefixed with the
630                                 exclamation mark unset). The argument
631                                 must either be a single word, or an
632                                 assignment (i.e. two words, separated
633                                 by the equality sign). In the former
634                                 case the kernel command line is
635                                 searched for the word appearing as is,
636                                 or as left hand side of an
637                                 assignment. In the latter case the
638                                 exact assignment is looked for with
639                                 right and left hand side
640                                 matching. <varname>ConditionVirtualization=</varname>
641                                 may be used to check whether the
642                                 system is executed in a virtualized
643                                 environment and optionally test
644                                 whether it is a specific
645                                 implementation. Takes either boolean
646                                 value to check if being executed in
647                                 any virtual environment or one of the
648                                 <varname>qemu</varname>,
649                                 <varname>kvm</varname>,
650                                 <varname>vmware</varname>,
651                                 <varname>microsoft</varname>,
652                                 <varname>oracle</varname>,
653                                 <varname>xen</varname>,
654                                 <varname>pidns</varname>,
655                                 <varname>openvz</varname> to test
656                                 against a specific implementation. The
657                                 test may be negated by prepending an
658                                 exclamation mark.
659                                 <varname>ConditionSecurity=</varname>
660                                 may be used to check whether the given security
661                                 module is enabled on the system.
662                                 Currently the only recognized value is
663                                 <varname>selinux</varname>.
664                                 The test may be negated by prepending an
665                                 exclamation mark. Finally,
666                                 <varname>ConditionNull=</varname> may
667                                 be used to add a constant condition
668                                 check value to the unit. It takes a
669                                 boolean argument. If set to
670                                 <varname>false</varname> the condition
671                                 will always fail, otherwise
672                                 succeed. If multiple conditions are
673                                 specified the unit will be executed if
674                                 all of them apply (i.e. a logical AND
675                                 is applied). Condition checks can be
676                                 prefixed with a pipe symbol (|) in
677                                 which case a condition becomes a
678                                 triggering condition. If at least one
679                                 triggering condition is defined for a
680                                 unit then the unit will be executed if
681                                 at least one of the triggering
682                                 conditions apply and all of the
683                                 non-triggering conditions. If you
684                                 prefix an argument with the pipe
685                                 symbol and an exclamation mark the
686                                 pipe symbol must be passed first, the
687                                 exclamation second.</para></listitem>
688                         </varlistentry>
689
690                         <varlistentry>
691                                 <term><varname>Names=</varname></term>
692
693                                 <listitem><para>Additional names for
694                                 this unit. The names listed here must
695                                 have the same suffix (i.e. type) as
696                                 the unit file name. This option may be
697                                 specified more than once, in which
698                                 case all listed names are used. Note
699                                 that this option is different from the
700                                 <varname>Alias=</varname> option from
701                                 the [Install] section mentioned
702                                 below. See below for details. Note
703                                 that in almost all cases this option
704                                 is not what you want. A symlink alias
705                                 in the file system is generally
706                                 preferable since it can be used as
707                                 lookup key. If a unit with a symlinked
708                                 alias name is not loaded and needs to
709                                 be it is easily found via the
710                                 symlink. However, if a unit with an
711                                 alias name configured with this
712                                 setting is not loaded it will not be
713                                 discovered. This settings' only use is
714                                 in conjunction with service
715                                 instances.</para>
716                                 </listitem>
717                         </varlistentry>
718                 </variablelist>
719
720                 <para>Unit file may include a [Install] section, which
721                 carries installation information for the unit. This
722                 section is not interpreted by
723                 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
724                 during runtime. It is used exclusively by the
725                 <command>enable</command> and
726                 <command>disable</command> commands of the
727                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
728                 tool during installation of a unit:</para>
729
730                 <variablelist>
731                         <varlistentry>
732                                 <term><varname>Alias=</varname></term>
733
734                                 <listitem><para>Additional names this
735                                 unit shall be installed under. The
736                                 names listed here must have the same
737                                 suffix (i.e. type) as the unit file
738                                 name. This option may be specified
739                                 more than once, in which case all
740                                 listed names are used. At installation
741                                 time,
742                                 <command>systemctl enable</command>
743                                 will create symlinks from these names
744                                 to the unit file name. Note that this
745                                 is different from the
746                                 <varname>Names=</varname> option from
747                                 the [Unit] section mentioned above:
748                                 The names from
749                                 <varname>Names=</varname> apply
750                                 unconditionally if the unit is
751                                 loaded. The names from
752                                 <varname>Alias=</varname> apply only
753                                 if the unit has actually been
754                                 installed with the
755                                 <command>systemctl enable</command>
756                                 command.  Also, if systemd searches for a
757                                 unit, it will discover symlinked alias
758                                 names as configured with
759                                 <varname>Alias=</varname>, but not
760                                 names configured with
761                                 <varname>Names=</varname> only. It is
762                                 a common pattern to list a name in
763                                 both options. In this case, a unit
764                                 will be active under all names if
765                                 installed, but also if not installed
766                                 but requested explicitly under its
767                                 main name.</para></listitem>
768                         </varlistentry>
769
770                         <varlistentry>
771                                 <term><varname>WantedBy=</varname></term>
772
773                                 <listitem><para>Installs a symlink in
774                                 the <filename>.wants/</filename>
775                                 subdirectory for a unit. This has the
776                                 effect that when the listed unit name
777                                 is activated the unit listing it is
778                                 activated
779                                 too. <command>WantedBy=foo.service</command>
780                                 in a service
781                                 <filename>bar.service</filename> is
782                                 mostly equivalent to
783                                 <command>Alias=foo.service.wants/bar.service</command>
784                                 in the same file.</para></listitem>
785                         </varlistentry>
786
787                         <varlistentry>
788                                 <term><varname>Also=</varname></term>
789
790                                 <listitem><para>Additional units to
791                                 install when this unit is
792                                 installed. If the user requests
793                                 installation of a unit with this
794                                 option configured,
795                                 <command>systemctl enable</command>
796                                 will automatically install units
797                                 listed in this option as
798                                 well.</para></listitem>
799                         </varlistentry>
800                 </variablelist>
801
802         </refsect1>
803
804         <refsect1>
805                 <title>See Also</title>
806                 <para>
807                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
808                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
809                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
810                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
811                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
812                         <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
813                         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
814                         <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
815                         <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
816                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
817                         <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
818                         <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
819                         <citerefentry><refentrytitle>systemd.snapshot</refentrytitle><manvolnum>5</manvolnum></citerefentry>
820                 </para>
821         </refsect1>
822
823 </refentry>