man/systemd.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3         "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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   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">
  25
  26         <refentryinfo>
  27                 <title>systemd</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</refentrytitle>
  42                 <manvolnum>1</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>systemd</refname>
  47                 <refname>init</refname>
  48                 <refpurpose>systemd System and Service Manager</refpurpose>
  49         </refnamediv>
  50
  51         <refsynopsisdiv>
  52                 <cmdsynopsis>
  53                         <command>systemd <arg choice="opt" rep="repeat">OPTIONS</arg></command>
  54                 </cmdsynopsis>
  55                 <cmdsynopsis>
  56                         <command>init <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="req">COMMAND</arg></command>
  57                 </cmdsynopsis>
  58         </refsynopsisdiv>
  59
  60         <refsect1>
  61                 <title>Description</title>
  62
  63                 <para>systemd is a system and service manager for
  64                 Linux operating systems. When run as first process on
  65                 boot (as PID 1), it acts as init system that brings
  66                 up and maintains userspace services.</para>
  67
  68                 <para>For compatibility with SysV, if systemd is called
  69                 as <command>init</command> and a PID that is not
  70                 1, it will execute <command>telinit</command> and pass
  71                 all command line arguments unmodified. That means
  72                 <command>init</command> and <command>telinit</command>
  73                 are mostly equivalent when invoked from normal login sessions. See
  74                 <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  75                 for more information.</para>
  76
  77                 <para>When run as system instance, systemd interprets
  78                 the configuration file
  79                 <filename>system.conf</filename>, otherwise
  80                 <filename>user.conf</filename>. See
  81                 <citerefentry><refentrytitle>systemd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  82                 for more information.</para>
  83         </refsect1>
  84
  85         <refsect1>
  86                 <title>Options</title>
  87
  88                 <para>The following options are understood:</para>
  89
  90                 <variablelist>
  91                         <varlistentry>
  92                                 <term><option>-h</option></term>
  93                                 <term><option>--help</option></term>
  94
  95                                 <listitem><para>Prints a short help
  96                                 text and exits.</para></listitem>
  97                         </varlistentry>
  98                         <varlistentry>
  99                                 <term><option>--test</option></term>
 100
 101                                 <listitem><para>Determine startup
 102                                 sequence, dump it and exit. This is an
 103                                 option useful for debugging
 104                                 only.</para></listitem>
 105                         </varlistentry>
 106                         <varlistentry>
 107                                 <term><option>--dump-configuration-items</option></term>
 108
 109                                 <listitem><para>Dump understood unit
 110                                 configuration items. This outputs a
 111                                 terse but complete list of
 112                                 configuration items understood in unit
 113                                 definition files.</para></listitem>
 114                         </varlistentry>
 115                         <varlistentry>
 116                                 <term><option>--introspect=</option></term>
 117
 118                                 <listitem><para>Extract D-Bus
 119                                 interface introspection data. This is
 120                                 mostly useful at install time
 121                                 to generate data suitable for the
 122                                 D-Bus interfaces
 123                                 repository. Optionally the interface
 124                                 name for the introspection data may be
 125                                 specified. If omitted, the
 126                                 introspection data for all interfaces
 127                                 is dumped.</para></listitem>
 128                         </varlistentry>
 129                         <varlistentry>
 130                                 <term><option>--unit=</option></term>
 131
 132                                 <listitem><para>Set default unit to
 133                                 activate on startup. If not specified
 134                                 defaults to
 135                                 <filename>default.target</filename>.</para></listitem>
 136                         </varlistentry>
 137                         <varlistentry>
 138                                 <term><option>--system</option></term>
 139                                 <term><option>--user</option></term>
 140
 141                                 <listitem><para>Tell systemd to run a
 142                                 system instance (resp. user
 143                                 instance), even if the process ID is
 144                                 not 1 (resp. is 1), i.e. systemd is
 145                                 not (resp. is) run as init process.
 146                                 Normally it should not be necessary to
 147                                 pass these options, as systemd
 148                                 automatically detects the mode it is
 149                                 started in. These options are hence of
 150                                 little use except for debugging. Note
 151                                 that it is not supported booting and
 152                                 maintaining a full system with systemd
 153                                 running in <option>--system</option>
 154                                 mode, but PID not 1. In practice,
 155                                 passing <option>--system</option> explicitly is
 156                                 only useful in conjunction with
 157                                 <option>--test</option>.</para></listitem>
 158                         </varlistentry>
 159                         <varlistentry>
 160                                 <term><option>--dump-core</option></term>
 161
 162                                 <listitem><para>Dump core on
 163                                 crash. This switch has no effect when
 164                                 run as user
 165                                 instance.</para></listitem>
 166                         </varlistentry>
 167                         <varlistentry>
 168                                 <term><option>--crash-shell</option></term>
 169
 170                                 <listitem><para>Run shell on
 171                                 crash. This switch has no effect when
 172                                 run as user
 173                                 instance.</para></listitem>
 174                         </varlistentry>
 175                         <varlistentry>
 176                                 <term><option>--confirm-spawn</option></term>
 177
 178                                 <listitem><para>Ask for confirmation
 179                                 when spawning processes. This switch
 180                                 has no effect when run as user
 181                                 instance.</para></listitem>
 182                         </varlistentry>
 183                         <varlistentry>
 184                                 <term><option>--show-status=</option></term>
 185
 186                                 <listitem><para>Show terse service
 187                                 status information while booting. This
 188                                 switch has no effect when run as user
 189                                 instance. Takes a boolean argument
 190                                 which may be omitted which is
 191                                 interpreted as
 192                                 <option>true</option>.</para></listitem>
 193                         </varlistentry>
 194                         <varlistentry>
 195                                 <term><option>--sysv-console=</option></term>
 196
 197                                 <listitem><para>Controls whether
 198                                 output of SysV init scripts will be
 199                                 directed to the console. This switch
 200                                 has no effect when run as user
 201                                 instance. Takes a boolean argument
 202                                 which may be omitted which is
 203                                 interpreted as
 204                                 <option>true</option>.</para></listitem>
 205                         </varlistentry>
 206                         <varlistentry>
 207                                 <term><option>--log-target=</option></term>
 208
 209                                 <listitem><para>Set log
 210                                 target. Argument must be one of
 211                                 <option>console</option>,
 212                                 <option>syslog</option>,
 213                                 <option>kmsg</option>,
 214                                 <option>syslog-or-kmsg</option>,
 215                                 <option>null</option>.</para></listitem>
 216                         </varlistentry>
 217                         <varlistentry>
 218                                 <term><option>--log-level=</option></term>
 219
 220                                 <listitem><para>Set log level. As
 221                                 argument this accepts a numerical log
 222                                 level or the well-known <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 223                                 symbolic names (lowercase):
 224                                 <option>emerg</option>,
 225                                 <option>alert</option>,
 226                                 <option>crit</option>,
 227                                 <option>err</option>,
 228                                 <option>warning</option>,
 229                                 <option>notice</option>,
 230                                 <option>info</option>,
 231                                 <option>debug</option>.</para></listitem>
 232                         </varlistentry>
 233                         <varlistentry>
 234                                 <term><option>--log-color=</option></term>
 235
 236                                 <listitem><para>Highlight important
 237                                 log messages. Argument is a boolean
 238                                 value. If the argument is omitted it
 239                                 defaults to
 240                                 <option>true</option>.</para></listitem>
 241                         </varlistentry>
 242                         <varlistentry>
 243                                 <term><option>--log-location=</option></term>
 244
 245                                 <listitem><para>Include code location
 246                                 in log messages. This is mostly
 247                                 relevant for debugging
 248                                 purposes. Argument is a boolean
 249                                 value. If the argument is omitted
 250                                 it defaults to
 251                                 <option>true</option>.</para></listitem>
 252                         </varlistentry>
 253                         <varlistentry>
 254                                 <term><option>--default-standard-output=</option></term>
 255                                 <term><option>--default-standard-error=</option></term>
 256
 257                                 <listitem><para>Sets the default
 258                                 output resp. error output for all
 259                                 services and sockets, i.e. controls
 260                                 the default for
 261                                 <option>StandardOutput=</option>
 262                                 resp. <option>StandardExecute=</option>
 263                                 (see
 264                                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 265                                 for details). Takes one of
 266                                 <option>inherit</option>,
 267                                 <option>null</option>,
 268                                 <option>tty</option>,
 269                                 <option>syslog</option>,
 270                                 <option>syslog+console</option>,
 271                                 <option>kmsg</option>,
 272                                 <option>kmsg+console</option>.  If the
 273                                 argument is omitted it defaults to
 274                                 <option>inherit</option>.</para></listitem>
 275                         </varlistentry>
 276                 </variablelist>
 277         </refsect1>
 278
 279         <refsect1>
 280                 <title>Concepts</title>
 281
 282                 <para>systemd provides a dependency system between
 283                 various entities called "units". Units encapsulate
 284                 various objects that are relevant for system boot-up
 285                 and maintenance. The majority of units are configured
 286                 in unit configuration files, whose syntax and basic
 287                 set of options is described in
 288                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 289                 however some are created automatically from other
 290                 configuration or dynamically from system state. Units
 291                 may be 'active' (meaning started, bound, plugged in,
 292                 ...  depending on the unit type, see below), or
 293                 'inactive' (meaning stopped, unbound, unplugged, ...),
 294                 as well as in the process of being activated or
 295                 deactivated, i.e. between the two states (these states
 296                 are called 'activating', 'deactivating'). A special
 297                 'failed' state is available as well which is very
 298                 similar to 'inactive' and is entered when the service
 299                 failed in some way (process returned error code on
 300                 exit, or crashed, or an operation timed out). If this
 301                 state is entered the cause will be logged, for later
 302                 reference. Note that the various unit types may have a
 303                 number of additional substates, which are mapped to
 304                 the five generalized unit states described
 305                 here.</para>
 306
 307                 <para>The following unit types are available:</para>
 308
 309                 <orderedlist>
 310                         <listitem><para>Service units, which control
 311                         daemons and the processes they consist of. For
 312                         details see
 313                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 314
 315                         <listitem><para>Socket units, which
 316                         encapsulate local IPC or network sockets in
 317                         the system, useful for socket-based
 318                         activation. For details about socket units see
 319                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 320                         for details on socket-based activation and
 321                         other forms of activation, see
 322                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 323
 324                         <listitem><para>Target units are useful to
 325                         group units, or provide well-known
 326                         synchronization points during boot-up, see
 327                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 328
 329                         <listitem><para>Device units expose kernel
 330                         devices in systemd and may be used to
 331                         implement device-based activation. For details
 332                         see
 333                         <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 334
 335                         <listitem><para>Mount units control mount
 336                         points in the file system, for details see
 337                         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 338
 339                         <listitem><para>Automount units provide
 340                         automount capabilities, for on-demand mounting
 341                         of file systems as well as parallelized
 342                         boot-up. See
 343                         <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 344
 345                         <listitem><para>Snapshot units can be used to
 346                         temporarily save the state of the set of
 347                         systemd units, which later may be restored by
 348                         activating the saved snapshot unit. For more
 349                         information see
 350                         <citerefentry><refentrytitle>systemd.snapshot</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 351
 352                         <listitem><para>Timer units are useful for
 353                         triggering activation of other units based on
 354                         timers. You may find details in
 355                         <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 356
 357                         <listitem><para>Swap units are very similar to
 358                         mount units and encapsulate memory swap
 359                         partitions or files of the operating
 360                         system. They are described in <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 361
 362                         <listitem><para>Path units may be used
 363                         to activate other services when file system
 364                         objects change or are modified. See
 365                         <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 366
 367                 </orderedlist>
 368
 369                 <para>Units are named as their configuration
 370                 files. Some units have special semantics. A detailed
 371                 list is available in
 372                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 373
 374                 <para>systemd knows various kinds of dependencies,
 375                 including positive and negative requirement
 376                 dependencies (i.e. <varname>Requires=</varname> and
 377                 <varname>Conflicts=</varname>) as well as ordering
 378                 dependencies (<varname>After=</varname> and
 379                 <varname>Before=</varname>). NB: ordering and
 380                 requirement dependencies are orthogonal. If only a
 381                 requirement dependency exists between two units
 382                 (e.g. <filename>foo.service</filename> requires
 383                 <filename>bar.service</filename>), but no ordering
 384                 dependency (e.g. <filename>foo.service</filename>
 385                 after <filename>bar.service</filename>) and both are
 386                 requested to start, they will be started in
 387                 parallel. It is a common pattern that both requirement
 388                 and ordering dependencies are placed between two
 389                 units. Also note that the majority of dependencies are
 390                 implicitly created and maintained by systemd. In most
 391                 cases it should be unnecessary to declare additional
 392                 dependencies manually, however it is possible to do
 393                 this.</para>
 394
 395                 <para>Application programs and units (via
 396                 dependencies) may request state changes of units. In
 397                 systemd, these requests are encapsulated as 'jobs' and
 398                 maintained in a job queue. Jobs may succeed or can
 399                 fail, their execution is ordered based on the ordering
 400                 dependencies of the units they have been scheduled
 401                 for.</para>
 402
 403                 <para>On boot systemd activates the target unit
 404                 <filename>default.target</filename> whose job is to
 405                 activate on-boot services and other on-boot units by
 406                 pulling them in via dependencies. Usually the unit
 407                 name is just an alias (symlink) for either
 408                 <filename>graphical.target</filename> (for
 409                 fully-featured boots into the UI) or
 410                 <filename>multi-user.target</filename> (for limited
 411                 console-only boots for use in embedded or server
 412                 environments, or similar; a subset of
 413                 graphical.target). However it is at the discretion of
 414                 the administrator to configure it as an alias to any
 415                 other target unit. See
 416                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 417                 for details about these target units.</para>
 418
 419                 <para>Processes systemd spawns are placed in
 420                 individual Linux control groups named after the unit
 421                 which they belong to in the private systemd
 422                 hierarchy. (see <ulink
 423                 url="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
 424                 for more information about control groups, or short
 425                 "cgroups"). systemd uses this to effectively keep
 426                 track of processes. Control group information is
 427                 maintained in the kernel, and is accessible via the
 428                 file system hierarchy (beneath
 429                 <filename>/sys/fs/cgroup/systemd/</filename>), or in tools
 430                 such as
 431                 <citerefentry><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 432                 (<command>ps xawf -eo pid,user,cgroup,args</command>
 433                 is particularly useful to list all processes and the
 434                 systemd units they belong to.).</para>
 435
 436                 <para>systemd is compatible with the SysV init system
 437                 to a large degree: SysV init scripts are supported and
 438                 simply read as an alternative (though limited)
 439                 configuration file format. The SysV
 440                 <filename>/dev/initctl</filename> interface is
 441                 provided, and compatibility implementations of the
 442                 various SysV client tools are available. In addition to
 443                 that, various established Unix functionality such as
 444                 <filename>/etc/fstab</filename> or the
 445                 <filename>utmp</filename> database are
 446                 supported.</para>
 447
 448                 <para>systemd has a minimal transaction system: if a
 449                 unit is requested to start up or shut down it will add
 450                 it and all its dependencies to a temporary
 451                 transaction. Then, it will verify if the transaction
 452                 is consistent (i.e. whether the ordering of all units
 453                 is cycle-free). If it is not, systemd will try to fix
 454                 it up, and removes non-essential jobs from the
 455                 transaction that might remove the loop. Also, systemd
 456                 tries to suppress non-essential jobs in the
 457                 transaction that would stop a running service. Finally
 458                 it is checked whether the jobs of the transaction
 459                 contradict jobs that have already been queued, and
 460                 optionally the transaction is aborted then. If all
 461                 worked out and the transaction is consistent and
 462                 minimized in its impact it is merged with all already
 463                 outstanding jobs and added to the run
 464                 queue. Effectively this means that before executing a
 465                 requested operation, systemd will verify that it makes
 466                 sense, fixing it if possible, and only failing if it
 467                 really cannot work.</para>
 468
 469                 <para>Systemd contains native implementations of
 470                 various tasks that need to be executed as part of the
 471                 boot process. For example, it sets the host name or
 472                 configures the loopback network device. It also sets
 473                 up and mounts various API file systems, such as
 474                 <filename>/sys</filename> or
 475                 <filename>/proc</filename>.</para>
 476
 477                 <para>For more information about the concepts and
 478                 ideas behind systemd please refer to the <ulink
 479                 url="http://0pointer.de/blog/projects/systemd.html">Original
 480                 Design Document</ulink>.</para>
 481
 482                 <para>Note that some but not all interfaces provided
 483                 by systemd are covered by the <ulink
 484                 url="http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
 485                 Stability Promise</ulink>.</para>
 486         </refsect1>
 487
 488         <refsect1>
 489                 <title>Directories</title>
 490
 491                 <variablelist>
 492                         <varlistentry>
 493                                 <term>System unit directories</term>
 494
 495                                 <listitem><para>The systemd system
 496                                 manager reads unit configuration from
 497                                 various directories. Packages that
 498                                 want to install unit files shall place
 499                                 them in the directory returned by
 500                                 <command>pkg-config systemd
 501                                 --variable=systemdsystemunitdir</command>. Other
 502                                 directories checked are
 503                                 <filename>/usr/local/lib/systemd/system</filename>
 504                                 and
 505                                 <filename>/usr/lib/systemd/system</filename>. User
 506                                 configuration always takes
 507                                 precedence. <command>pkg-config
 508                                 systemd
 509                                 --variable=systemdsystemconfdir</command>
 510                                 returns the path of the system
 511                                 configuration directory. Packages
 512                                 should alter the content of these
 513                                 directories only with the
 514                                 <command>enable</command> and
 515                                 <command>disable</command> commands of
 516                                 the
 517                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 518                                 tool.</para></listitem>
 519                         </varlistentry>
 520                 </variablelist>
 521
 522                 <variablelist>
 523                         <varlistentry>
 524                                 <term>User unit directories</term>
 525
 526                                 <listitem><para>Similar rules apply
 527                                 for the user unit
 528                                 directories. However, here the <ulink
 529                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 530                                 Base Directory specification</ulink>
 531                                 is followed to find
 532                                 units. Applications should place their
 533                                 unit files in the directory returned
 534                                 by <command>pkg-config systemd
 535                                 --variable=systemduserunitdir</command>. Global
 536                                 configuration is done in the directory
 537                                 reported by <command>pkg-config
 538                                 systemd
 539                                 --variable=systemduserconfdir</command>. The
 540                                 <command>enable</command> and
 541                                 <command>disable</command> commands of
 542                                 the
 543                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 544                                 tool can handle both global (i.e. for
 545                                 all users) and private (for one user)
 546                                 enabling/disabling of
 547                                 units.</para></listitem>
 548                         </varlistentry>
 549                 </variablelist>
 550
 551                 <variablelist>
 552                         <varlistentry>
 553                                 <term>SysV init scripts directory</term>
 554
 555                                 <listitem><para>The location of the
 556                                 SysV init script directory varies
 557                                 between distributions. If systemd
 558                                 cannot find a native unit file for a
 559                                 requested service, it will look for a
 560                                 SysV init script of the same name
 561                                 (with the
 562                                 <filename>.service</filename> suffix
 563                                 removed).</para></listitem>
 564                         </varlistentry>
 565                 </variablelist>
 566
 567                 <variablelist>
 568                         <varlistentry>
 569                                 <term>SysV runlevel link farm directory</term>
 570
 571                                 <listitem><para>The location of the
 572                                 SysV runlevel link farm directory
 573                                 varies between distributions. systemd
 574                                 will take the link farm into account
 575                                 when figuring out whether a service
 576                                 shall be enabled. Note that a service
 577                                 unit with a native unit configuration
 578                                 file cannot be started by activating it
 579                                 in the SysV runlevel link
 580                                 farm.</para></listitem>
 581                         </varlistentry>
 582                 </variablelist>
 583         </refsect1>
 584
 585         <refsect1>
 586                 <title>Signals</title>
 587
 588                 <variablelist>
 589                         <varlistentry>
 590                                 <term>SIGTERM</term>
 591
 592                                 <listitem><para>Upon receiving this
 593                                 signal the systemd system manager
 594                                 serializes its state, reexecutes
 595                                 itself and deserializes the saved
 596                                 state again. This is mostly equivalent
 597                                 to <command>systemctl
 598                                 daemon-reexec</command>.</para>
 599
 600                                 <para>systemd user managers will
 601                                 start the
 602                                 <filename>exit.target</filename> unit
 603                                 when this signal is received. This is
 604                                 mostly equivalent to
 605                                 <command>systemctl --user start
 606                                 exit.target</command>.</para></listitem>
 607                         </varlistentry>
 608
 609                         <varlistentry>
 610                                 <term>SIGINT</term>
 611
 612                                 <listitem><para>Upon receiving this
 613                                 signal the systemd system manager will
 614                                 start the
 615                                 <filename>ctrl-alt-del.target</filename> unit. This
 616                                 is mostly equivalent to
 617                                 <command>systemctl start
 618                                 ctl-alt-del.target</command>.</para>
 619
 620                                 <para>systemd user managers
 621                                 treat this signal the same way as
 622                                 SIGTERM.</para></listitem>
 623                         </varlistentry>
 624
 625                         <varlistentry>
 626                                 <term>SIGWINCH</term>
 627
 628                                 <listitem><para>When this signal is
 629                                 received the systemd system manager
 630                                 will start the
 631                                 <filename>kbrequest.target</filename>
 632                                 unit. This is mostly equivalent to
 633                                 <command>systemctl start
 634                                 kbrequest.target</command>.</para>
 635
 636                                 <para>This signal is ignored by
 637                                 systemd user
 638                                 managers.</para></listitem>
 639                         </varlistentry>
 640
 641                         <varlistentry>
 642                                 <term>SIGPWR</term>
 643
 644                                 <listitem><para>When this signal is
 645                                 received the systemd manager
 646                                 will start the
 647                                 <filename>sigpwr.target</filename>
 648                                 unit. This is mostly equivalent to
 649                                 <command>systemctl start
 650                                 sigpwr.target</command>.</para></listitem>
 651                         </varlistentry>
 652
 653                         <varlistentry>
 654                                 <term>SIGUSR1</term>
 655
 656                                 <listitem><para>When this signal is
 657                                 received the systemd manager will try
 658                                 to reconnect to the D-Bus
 659                                 bus.</para></listitem>
 660                         </varlistentry>
 661
 662                         <varlistentry>
 663                                 <term>SIGUSR2</term>
 664
 665                                 <listitem><para>When this signal is
 666                                 received the systemd manager will log
 667                                 its complete state in human readable
 668                                 form. The data logged is the same as
 669                                 printed by <command>systemctl
 670                                 dump</command>.</para></listitem>
 671                         </varlistentry>
 672
 673                         <varlistentry>
 674                                 <term>SIGHUP</term>
 675
 676                                 <listitem><para>Reloads the complete
 677                                 daemon configuration. This is mostly
 678                                 equivalent to <command>systemctl
 679                                 daemon-reload</command>.</para></listitem>
 680                         </varlistentry>
 681
 682                         <varlistentry>
 683                                 <term>SIGRTMIN+0</term>
 684
 685                                 <listitem><para>Enters default mode, starts the
 686                                 <filename>default.target</filename>
 687                                 unit. This is mostly equivalent to
 688                                 <command>systemctl start
 689                                 default.target</command>.</para></listitem>
 690                         </varlistentry>
 691
 692                         <varlistentry>
 693                                 <term>SIGRTMIN+1</term>
 694
 695                                 <listitem><para>Enters rescue mode,
 696                                 starts the
 697                                 <filename>rescue.target</filename>
 698                                 unit. This is mostly equivalent to
 699                                 <command>systemctl isolate
 700                                 rescue.target</command>.</para></listitem>
 701                         </varlistentry>
 702
 703                         <varlistentry>
 704                                 <term>SIGRTMIN+2</term>
 705
 706                                 <listitem><para>Enters emergency mode,
 707                                 starts the
 708                                 <filename>emergency.service</filename>
 709                                 unit. This is mostly equivalent to
 710                                 <command>systemctl isolate
 711                                 emergency.service</command>.</para></listitem>
 712                         </varlistentry>
 713
 714                         <varlistentry>
 715                                 <term>SIGRTMIN+3</term>
 716
 717                                 <listitem><para>Halts the machine,
 718                                 starts the
 719                                 <filename>halt.target</filename>
 720                                 unit. This is mostly equivalent to
 721                                 <command>systemctl start
 722                                 halt.target</command>.</para></listitem>
 723                         </varlistentry>
 724
 725                         <varlistentry>
 726                                 <term>SIGRTMIN+4</term>
 727
 728                                 <listitem><para>Powers off the machine,
 729                                 starts the
 730                                 <filename>poweroff.target</filename>
 731                                 unit. This is mostly equivalent to
 732                                 <command>systemctl start
 733                                 poweroff.target</command>.</para></listitem>
 734                         </varlistentry>
 735
 736                         <varlistentry>
 737                                 <term>SIGRTMIN+5</term>
 738
 739                                 <listitem><para>Reboots the machine,
 740                                 starts the
 741                                 <filename>reboot.target</filename>
 742                                 unit. This is mostly equivalent to
 743                                 <command>systemctl start
 744                                 reboot.target</command>.</para></listitem>
 745                         </varlistentry>
 746
 747                         <varlistentry>
 748                                 <term>SIGRTMIN+6</term>
 749
 750                                 <listitem><para>Reboots the machine via kexec,
 751                                 starts the
 752                                 <filename>kexec.target</filename>
 753                                 unit. This is mostly equivalent to
 754                                 <command>systemctl start
 755                                 kexec.target</command>.</para></listitem>
 756                         </varlistentry>
 757
 758                         <varlistentry>
 759                                 <term>SIGRTMIN+13</term>
 760
 761                                 <listitem><para>Immediately halts the machine.</para></listitem>
 762                         </varlistentry>
 763
 764                         <varlistentry>
 765                                 <term>SIGRTMIN+14</term>
 766
 767                                 <listitem><para>Immediately powers off the machine.</para></listitem>
 768                         </varlistentry>
 769
 770                         <varlistentry>
 771                                 <term>SIGRTMIN+15</term>
 772
 773                                 <listitem><para>Immediately reboots the machine.</para></listitem>
 774                         </varlistentry>
 775
 776                         <varlistentry>
 777                                 <term>SIGRTMIN+16</term>
 778
 779                                 <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 780                         </varlistentry>
 781
 782                         <varlistentry>
 783                                 <term>SIGRTMIN+20</term>
 784
 785                                 <listitem><para>Enables display of
 786                                 status messages on the console, as
 787                                 controlled via
 788                                 <varname>systemd.show_status=1</varname>
 789                                 on the kernel command
 790                                 line.</para></listitem>
 791                         </varlistentry>
 792
 793                         <varlistentry>
 794                                 <term>SIGRTMIN+21</term>
 795
 796                                 <listitem><para>Disables display of
 797                                 status messages on the console, as
 798                                 controlled via
 799                                 <varname>systemd.show_status=0</varname>
 800                                 on the kernel command
 801                                 line.</para></listitem>
 802                         </varlistentry>
 803
 804                         <varlistentry>
 805                                 <term>SIGRTMIN+22</term>
 806                                 <term>SIGRTMIN+23</term>
 807
 808                                 <listitem><para>Sets the log level to
 809                                 <literal>debug</literal>
 810                                 (resp. <literal>info</literal> on
 811                                 <literal>SIGRTMIN+32</literal>), as
 812                                 controlled via
 813                                 <varname>systemd.log_level=debug</varname>
 814                                 (resp. <varname>systemd.log_level=info</varname>
 815                                 on <literal>SIGRTMIN+23</literal>) on
 816                                 the kernel command
 817                                 line.</para></listitem>
 818                         </varlistentry>
 819
 820                         <varlistentry>
 821                                 <term>SIGRTMIN+27</term>
 822                                 <term>SIGRTMIN+28</term>
 823                                 <term>SIGRTMIN+29</term>
 824
 825                                 <listitem><para>Sets the log level to
 826                                 <literal>console</literal>
 827                                 (resp. <literal>kmsg</literal> on
 828                                 <literal>SIGRTMIN+28</literal>;
 829                                 resp. <literal>syslog-or-kmsg</literal>
 830                                 on <literal>SIGRTMIN+29</literal>), as
 831                                 controlled via
 832                                 <varname>systemd.log_target=console</varname>
 833                                 (resp. <varname>systemd.log_target=kmsg</varname>
 834                                 on <literal>SIGRTMIN+28</literal>;
 835                                 resp
 836                                 <varname>systemd.log_target=syslog-or-kmsg</varname>
 837                                 on <literal>SIGRTMIN+29</literal>) on
 838                                 the kernel command
 839                                 line.</para></listitem>
 840                         </varlistentry>
 841                 </variablelist>
 842         </refsect1>
 843
 844         <refsect1>
 845                 <title>Environment</title>
 846
 847                 <variablelist>
 848                         <varlistentry>
 849                                 <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 850                                 <listitem><para>systemd reads the
 851                                 log level from this environment
 852                                 variable. This can be overridden with
 853                                 <option>--log-level=</option>.</para></listitem>
 854                         </varlistentry>
 855
 856                         <varlistentry>
 857                                 <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 858                                 <listitem><para>systemd reads the
 859                                 log target from this environment
 860                                 variable. This can be overridden with
 861                                 <option>--log-target=</option>.</para></listitem>
 862                         </varlistentry>
 863
 864                         <varlistentry>
 865                                 <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 866                                 <listitem><para>Controls whether
 867                                 systemd highlights important log
 868                                 messages. This can be overridden with
 869                                 <option>--log-color=</option>.</para></listitem>
 870                         </varlistentry>
 871
 872                         <varlistentry>
 873                                 <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 874                                 <listitem><para>Controls whether
 875                                 systemd prints the code location along
 876                                 with log messages. This can be
 877                                 overridden with
 878                                 <option>--log-location=</option>.</para></listitem>
 879                         </varlistentry>
 880
 881                         <varlistentry>
 882                                 <term><varname>$XDG_CONFIG_HOME</varname></term>
 883                                 <term><varname>$XDG_CONFIG_DIRS</varname></term>
 884                                 <term><varname>$XDG_DATA_HOME</varname></term>
 885                                 <term><varname>$XDG_DATA_DIRS</varname></term>
 886
 887                                 <listitem><para>The systemd user
 888                                 manager uses these variables in
 889                                 accordance to the <ulink
 890                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 891                                 Base Directory specification</ulink>
 892                                 to find its configuration.</para></listitem>
 893                         </varlistentry>
 894
 895                         <varlistentry>
 896                                 <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 897
 898                                 <listitem><para>Controls where systemd
 899                                 looks for unit
 900                                 files.</para></listitem>
 901                         </varlistentry>
 902
 903                         <varlistentry>
 904                                 <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 905
 906                                 <listitem><para>Controls where systemd
 907                                 looks for SysV init scripts.</para></listitem>
 908                         </varlistentry>
 909
 910                         <varlistentry>
 911                                 <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 912
 913                                 <listitem><para>Controls where systemd
 914                                 looks for SysV init script runlevel link
 915                                 farms.</para></listitem>
 916                         </varlistentry>
 917
 918                         <varlistentry>
 919                                 <term><varname>$LISTEN_PID</varname></term>
 920                                 <term><varname>$LISTEN_FDS</varname></term>
 921
 922                                 <listitem><para>Set by systemd for
 923                                 supervised processes during
 924                                 socket-based activation. See
 925                                 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 926                                 for more information.
 927                                 </para></listitem>
 928                         </varlistentry>
 929
 930                         <varlistentry>
 931                                 <term><varname>$NOTIFY_SOCKET</varname></term>
 932
 933                                 <listitem><para>Set by systemd for
 934                                 supervised processes for status and
 935                                 start-up completion notification. See
 936                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 937                                 for more information.
 938                                 </para></listitem>
 939                         </varlistentry>
 940                 </variablelist>
 941         </refsect1>
 942
 943         <refsect1>
 944                 <title>Kernel Command Line</title>
 945
 946                 <para>When run as system instance systemd parses a few kernel command line arguments:</para>
 947
 948                 <variablelist>
 949                         <varlistentry>
 950                                 <term><varname>systemd.unit=</varname></term>
 951
 952                                 <listitem><para>Overrides the unit to
 953                                 activate on boot. Defaults to
 954                                 <filename>default.target</filename>. This
 955                                 may be used to temporarily boot into a
 956                                 different boot unit, for example
 957                                 <filename>rescue.target</filename> or
 958                                 <filename>emergency.service</filename>. See
 959                                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 960                                 for details about these
 961                                 units.</para></listitem>
 962                         </varlistentry>
 963
 964                         <varlistentry>
 965                                 <term><varname>systemd.dump_core=</varname></term>
 966
 967                                 <listitem><para>Takes a boolean
 968                                 argument. If <option>true</option>
 969                                 systemd dumps core when it
 970                                 crashes. Otherwise no core dump is
 971                                 created. Defaults to
 972                                 <option>true</option>.</para></listitem>
 973                         </varlistentry>
 974
 975                         <varlistentry>
 976                                 <term><varname>systemd.crash_shell=</varname></term>
 977
 978                                 <listitem><para>Takes a boolean
 979                                 argument. If <option>true</option>
 980                                 systemd spawns a shell when it
 981                                 crashes. Otherwise no shell is
 982                                 spawned. Defaults to
 983                                 <option>false</option>, for security
 984                                 reasons, as the shell is not protected
 985                                 by any password
 986                                 authentication.</para></listitem>
 987                         </varlistentry>
 988
 989                         <varlistentry>
 990                                 <term><varname>systemd.crash_chvt=</varname></term>
 991
 992                                 <listitem><para>Takes an integer
 993                                 argument. If positive systemd
 994                                 activates the specified virtual
 995                                 terminal when it crashes. Defaults to
 996                                 <literal>-1</literal>.</para></listitem>
 997                         </varlistentry>
 998
 999                         <varlistentry>
1000                                 <term><varname>systemd.confirm_spawn=</varname></term>
1001
1002                                 <listitem><para>Takes a boolean
1003                                 argument. If <option>true</option>
1004                                 asks for confirmation when spawning
1005                                 processes. Defaults to
1006                                 <option>false</option>.</para></listitem>
1007                         </varlistentry>
1008
1009                         <varlistentry>
1010                                 <term><varname>systemd.show_status=</varname></term>
1011
1012                                 <listitem><para>Takes a boolean
1013                                 argument. If <option>true</option>
1014                                 shows terse service status updates on
1015                                 the console during bootup. Defaults to
1016                                 <option>true</option>.</para></listitem>
1017                         </varlistentry>
1018
1019                         <varlistentry>
1020                                 <term><varname>systemd.sysv_console=</varname></term>
1021
1022                                 <listitem><para>Takes a boolean
1023                                 argument. If <option>true</option>
1024                                 output of SysV init scripts will be
1025                                 directed to the console. Defaults to
1026                                 <option>true</option>, unless
1027                                 <option>quiet</option> is passed as
1028                                 kernel command line option in which
1029                                 case it defaults to
1030                                 <option>false</option>.</para></listitem>
1031                         </varlistentry>
1032
1033                         <varlistentry>
1034                                 <term><varname>systemd.log_target=</varname></term>
1035                                 <term><varname>systemd.log_level=</varname></term>
1036                                 <term><varname>systemd.log_color=</varname></term>
1037                                 <term><varname>systemd.log_location=</varname></term>
1038
1039                                 <listitem><para>Controls log output,
1040                                 with the same effect as the
1041                                 <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
1042                                 environment variables described above.</para></listitem>
1043                         </varlistentry>
1044
1045                         <varlistentry>
1046                                 <term><varname>systemd.default_standard_output=</varname></term>
1047                                 <term><varname>systemd.default_standard_error=</varname></term>
1048                                 <listitem><para>Controls default
1049                                 standard output/error output for
1050                                 services, with the same effect as the
1051                                 <option>--default-standard-output=</option>
1052                                 resp. <option>--default-standard-error=</option>
1053                                 command line arguments described
1054                                 above.</para></listitem>
1055                         </varlistentry>
1056
1057                 </variablelist>
1058         </refsect1>
1059
1060         <refsect1>
1061                 <title>Sockets and FIFOs</title>
1062
1063                 <variablelist>
1064                         <varlistentry>
1065                                 <term><filename>/run/systemd/notify</filename></term>
1066
1067                                 <listitem><para>Daemon status
1068                                 notification socket. This is an
1069                                 AF_UNIX datagram socket and is used to
1070                                 implement the daemon notification
1071                                 logic as implemented by
1072                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1073
1074                         </varlistentry>
1075
1076                         <varlistentry>
1077                                 <term><filename>/run/systemd/logger</filename></term>
1078
1079                                 <listitem><para>Used internally by the
1080                                 <filename>systemd-logger.service</filename>
1081                                 unit to connect STDOUT and/or STDERR
1082                                 of spawned processes to
1083                                 <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
1084                                 or the kernel log buffer. This is an
1085                                 AF_UNIX stream
1086                                 socket.</para></listitem>
1087                         </varlistentry>
1088
1089                         <varlistentry>
1090                                 <term><filename>/run/systemd/shutdownd</filename></term>
1091
1092                                 <listitem><para>Used internally by the
1093                                 <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1094                                 tool to implement delayed
1095                                 shutdowns. This is an AF_UNIX datagram
1096                                 socket.</para></listitem>
1097                         </varlistentry>
1098
1099                         <varlistentry>
1100                                 <term><filename>/run/systemd/private</filename></term>
1101
1102                                 <listitem><para>Used internally as
1103                                 communication channel between
1104                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1105                                 and the systemd process. This is an
1106                                 AF_UNIX stream socket. This interface
1107                                 is private to systemd and should not
1108                                 be used in external
1109                                 projects.</para></listitem>
1110                         </varlistentry>
1111
1112                         <varlistentry>
1113                                 <term><filename>/dev/initctl</filename></term>
1114
1115                                 <listitem><para>Limited compatibility
1116                                 support for the SysV client interface,
1117                                 as implemented by the
1118                                 <filename>systemd-initctl.service</filename>
1119                                 unit. This is a named pipe in the file
1120                                 system. This interface is obsolete and
1121                                 should not be used in new
1122                                 applications.</para></listitem>
1123                         </varlistentry>
1124                 </variablelist>
1125         </refsect1>
1126
1127         <refsect1>
1128                 <title>See Also</title>
1129                 <para>
1130                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1131                         <citerefentry><refentrytitle>systemadm</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1132                         <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1133                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1134                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1135                         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1136                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1137                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1138                 </para>
1139         </refsect1>
1140
1141 </refentry>