man/systemd.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   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.
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  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/>.
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  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 Session 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 session 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>session.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>--session</option></term>
 140
 141                                 <listitem><para>Tell systemd to run a
 142                                 system instance (resp. session
 143                                 instance), even if the process ID is
 144                                 not 1 (resp. is 1), i.e. systemd is not
 145                                 (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
 151                                 debugging.</para></listitem>
 152                         </varlistentry>
 153                         <varlistentry>
 154                                 <term><option>--dump-core</option></term>
 155
 156                                 <listitem><para>Dump core on crash. This switch has no effect when run as session instance.</para></listitem>
 157                         </varlistentry>
 158                         <varlistentry>
 159                                 <term><option>--crash-shell</option></term>
 160
 161                                 <listitem><para>Run shell on crash. This switch has no effect when run as session instance.</para></listitem>
 162                         </varlistentry>
 163                         <varlistentry>
 164                                 <term><option>--confirm-spawn</option></term>
 165
 166                                 <listitem><para>Ask for confirmation when spawning processes. This switch has no effect when run as session instance.</para></listitem>
 167                         </varlistentry>
 168                         <varlistentry>
 169                                 <term><option>--show-status</option></term>
 170
 171                                 <listitem><para>Show terse service status information while booting. This switch has no effect when run as session instance.</para></listitem>
 172                         </varlistentry>
 173                         <varlistentry>
 174                                 <term><option>--log-target=</option></term>
 175
 176                                 <listitem><para>Set log
 177                                 target. Argument must be one of
 178                                 <option>console</option>,
 179                                 <option>syslog</option>,
 180                                 <option>kmsg</option>,
 181                                 <option>syslog-or-kmsg</option>,
 182                                 <option>null</option>.</para></listitem>
 183                         </varlistentry>
 184                         <varlistentry>
 185                                 <term><option>--log-level=</option></term>
 186
 187                                 <listitem><para>Set log level. As
 188                                 argument this accepts a numerical log
 189                                 level or the well-known <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 190                                 symbolic names (lowercase):
 191                                 <option>emerg</option>,
 192                                 <option>alert</option>,
 193                                 <option>crit</option>,
 194                                 <option>err</option>,
 195                                 <option>warning</option>,
 196                                 <option>notice</option>,
 197                                 <option>info</option>,
 198                                 <option>debug</option>.</para></listitem>
 199                         </varlistentry>
 200                         <varlistentry>
 201                                 <term><option>--log-color=</option></term>
 202
 203                                 <listitem><para>Highlight important
 204                                 log messages. Argument is a boolean
 205                                 value. If the argument is omitted it
 206                                 defaults to
 207                                 <option>true</option>.</para></listitem>
 208                         </varlistentry>
 209                         <varlistentry>
 210                                 <term><option>--log-location=</option></term>
 211
 212                                 <listitem><para>Include code location
 213                                 in log messages. This is mostly
 214                                 relevant for debugging
 215                                 purposes. Argument is a boolean
 216                                 value. If the argument is omitted
 217                                 it defaults to
 218                                 <option>true</option>.</para></listitem>
 219                         </varlistentry>
 220                 </variablelist>
 221         </refsect1>
 222
 223         <refsect1>
 224                 <title>Concepts</title>
 225
 226                 <para>systemd provides a dependency system between
 227                 various entities called "units". Units encapsulate
 228                 various objects that are relevant for system boot-up
 229                 and maintainance. The majority of units are configured
 230                 in unit configuration files, whose syntax and basic
 231                 set of options is described in
 232                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 233                 however some are created automatically from other
 234                 configuration or dynamically from system state. Units
 235                 may be 'active' (meaning started, bound, plugged in,
 236                 ...  depending on the unit type, see below), or
 237                 'inactive' (meaning stopped, unbound, unplugged, ...),
 238                 as well as in the process of being activated or
 239                 deactivated, i.e. between the two states (these states
 240                 are called 'activating', 'deactivating'). A special
 241                 'maintenance' state is available as well which is very
 242                 similar to 'inactive' and is entered when the service
 243                 failed in some way (process returned error code on
 244                 exit, or crashed, or an operation timed out). If this
 245                 state is entered the cause will be logged, for later
 246                 reference. Note that the various unit types may have a
 247                 number of additional substates, which are mapped to
 248                 the five generalized unit states described
 249                 here.</para>
 250
 251                 <para>The following unit types are available:</para>
 252
 253                 <orderedlist>
 254                         <listitem><para>Service units, which control
 255                         daemons and the processes they consist of. For
 256                         details see
 257                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 258
 259                         <listitem><para>Socket units, which
 260                         encapsulate local IPC or network sockets in
 261                         the system, useful for socket-based
 262                         activation. For details about socket units see
 263                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 264                         for details on socket-based activation and
 265                         other forms of activation, see
 266                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 267
 268                         <listitem><para>Target units are useful to
 269                         group units, or provide well-known
 270                         synchronization points during boot-up, see
 271                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 272
 273                         <listitem><para>Device units expose kernel
 274                         devices in systemd and may be used to
 275                         implement device-based activation. For details
 276                         see
 277                         <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 278
 279                         <listitem><para>Mount units control mount
 280                         points in the file system, for details see
 281                         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 282
 283                         <listitem><para>Automount units provide
 284                         automount capabilities, for on-demand mounting
 285                         of file systems as well as parallelized
 286                         boot-up. See
 287                         <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 288
 289                         <listitem><para>Snapshot units can be used to
 290                         temporarily save the state of the set of
 291                         systemd units, which later may be restored by
 292                         activating the saved snapshot unit. For more
 293                         information see
 294                         <citerefentry><refentrytitle>systemd.snapshot</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 295
 296                         <listitem><para>Timer units are useful for
 297                         triggering activation of other units based on
 298                         timers. You may find details in
 299                         <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 300
 301                         <listitem><para>Swap units are very similar to
 302                         mount units and encapsulated memory swap
 303                         partitions or files of the operating
 304                         systemd. They are described in <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 305
 306                         <listitem><para>Path units may be used
 307                         to activate other services when file system
 308                         objects change or are modified. See
 309                         <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 310
 311                 </orderedlist>
 312
 313                 <para>Units are named as their configuration
 314                 files. Some units have special semantics. A detailed
 315                 list you may find in
 316                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 317
 318                 <para>systemd knows various kinds of dependencies,
 319                 including positive and negative requirement
 320                 dependencies (i.e. <varname>Requires=</varname> and
 321                 <varname>Conflicts=</varname>) as well as ordering
 322                 dependencies (<varname>After=</varname> and
 323                 <varname>Before=</varname>). NB: ordering and
 324                 requirement dependencies are orthogonal. If only a
 325                 requirement dependency exists between two units
 326                 (e.g. <filename>foo.service</filename> requires
 327                 <filename>bar.service</filename>), but no ordering
 328                 dependency (e.g. <filename>foo.service</filename>
 329                 after <filename>bar.service</filename>) and both are
 330                 requested to start, they will be started in
 331                 parallel. It is a common pattern that both requirement
 332                 and ordering dependencies are placed between two
 333                 units. Also note that the majority of dependencies are
 334                 implicitly created and maintained by systemd. In most
 335                 cases it should be unnecessary to declare additional
 336                 dependencies manually, however it is possible to do
 337                 this.</para>
 338
 339                 <para>Application programs and units (via
 340                 dependencies) may requests state changes of units. In
 341                 systemd, these requests are encapsulated as 'jobs' and
 342                 maintained in a job queue. Jobs may succeed or can
 343                 fail, their execution is ordered based on the ordering
 344                 dependencies of the units they have been scheduled
 345                 for.</para>
 346
 347                 <para>On boot systemd activates the target unit
 348                 <filename>default.target</filename> whose job is to
 349                 activate on-boot services and other on-boot units by
 350                 pulling them in via dependencies. Usually the unit
 351                 name is just an alias (symlink) for either
 352                 <filename>graphical.target</filename> (for
 353                 fully-featured boots into the UI) or
 354                 <filename>multi-user.target</filename> (for limited
 355                 console-only boots for use in embedded or server
 356                 environments, or similar; a subset of
 357                 graphical.target). However it is at the discretion of
 358                 the administrator to configure it as an alias to any
 359                 other target unit. See
 360                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 361                 for details about these target units.</para>
 362
 363                 <para>Processes systemd spawns are placed in
 364                 individual Linux control groups named after the unit
 365                 which they belong to in the private systemd
 366                 hierarchy. (see <ulink
 367                 url="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
 368                 for more information about control groups, or short
 369                 "cgroups"). systemd uses this to effectively keep
 370                 track of processes. Control group information is
 371                 maintained in the kernel, and is accessible via the
 372                 file system hierarchy (beneath
 373                 <filename>/cgroup/systemd/</filename>), or in tools
 374                 such as
 375                 <citerefentry><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 376                 (<command>ps xawf -eo pid,user,cgroup,args</command>
 377                 is particularly useful to list all processes and the
 378                 systemd units they belong to.).</para>
 379
 380                 <para>systemd is compatible with the SysV init system
 381                 to a large degree: SysV init scripts are supported and
 382                 simply read as an alternative (though limited)
 383                 configuration file format. The SysV
 384                 <filename>/dev/initctl</filename> interface is
 385                 provided, and compatibility implementations of the
 386                 various SysV client tools are available. In addition to
 387                 that, various established Unix functionality such as
 388                 <filename>/etc/fstab</filename> or the
 389                 <filename>utmp</filename> database are
 390                 supported.</para>
 391
 392                 <para>systemd has a minimal transaction system: if a
 393                 unit is requested to start up or shut down it will add
 394                 it and all its dependencies to a temporary
 395                 transaction. Then, it will verify if the transaction
 396                 is consistent (i.e. whether the ordering of all units
 397                 is cycle-free). If it is not, systemd will try to fix
 398                 it up, and removes non-essential jobs from the
 399                 transaction that might remove the loop. Also, systemd
 400                 tries to suppress non-essential jobs in the
 401                 transaction that would stop a running service. Finally
 402                 it is checked whether the jobs of the transaction
 403                 contradict jobs that have already been queued, and
 404                 optionally the transaction is aborted then. If all
 405                 worked out and the transaction is consistent and
 406                 minimized in its impact it is merged with all already
 407                 outstanding jobs and added to the run
 408                 queue. Effectively this means that before executing a
 409                 requested operation, systemd will verify that it makes
 410                 sense, fixing it if possible, and only failing if it
 411                 really cannot work.</para>
 412
 413                 <para>Systemd contains native implementations of
 414                 various tasks that need to be executed as part of the
 415                 boot process. For example, it sets the host name or
 416                 configures the loopback network device. It also sets
 417                 up and mounts various API file systems, such as
 418                 <filename>/sys</filename> or
 419                 <filename>/proc</filename>.</para>
 420
 421                 <para>For more information about the concepts and
 422                 ideas behind systemd please refer to the <ulink
 423                 url="http://0pointer.de/blog/projects/systemd.html">Original
 424                 Design Document</ulink>.</para>
 425         </refsect1>
 426
 427         <refsect1>
 428                 <title>Directories</title>
 429
 430                 <variablelist>
 431                         <varlistentry>
 432                                 <term>System unit directories</term>
 433
 434                                 <listitem><para>The systemd system
 435                                 manager reads unit configuration from
 436                                 various directories. Packages that
 437                                 want to install unit files shall place
 438                                 them in the directory returned by
 439                                 <command>pkg-config systemd
 440                                 --variable=systemdsystemunitdir</command>. Other
 441                                 directories checked are
 442                                 <filename>/usr/local/share/systemd/system</filename>
 443                                 and
 444                                 <filename>/usr/share/systemd/system</filename>. User
 445                                 configuration always takes
 446                                 precedence. <command>pkg-config
 447                                 systemd
 448                                 --variable=systemdsystemconfdir</command>
 449                                 returns the path of the system
 450                                 configuration directory. Packages
 451                                 should alter the content of these directories
 452                                 only with the
 453                                 <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 454                                 tool.</para></listitem>
 455                         </varlistentry>
 456                 </variablelist>
 457
 458                 <variablelist>
 459                         <varlistentry>
 460                                 <term>Session unit directories</term>
 461
 462                                 <listitem><para>Similar rules apply
 463                                 for the session unit
 464                                 directories. However, here the <ulink
 465                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 466                                 Base Directory specification</ulink>
 467                                 is followed to find
 468                                 units. Applications should place their
 469                                 unit files in the directory returned
 470                                 by <command>pkg-config systemd
 471                                 --variable=systemdsessionunitdir</command>. Global
 472                                 configuration is done in the
 473                                 directory reported by
 474                                 <command>pkg-config systemd
 475                                 --variable=systemdsessionconfdir</command>. The
 476                                 <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 477                                 tool can handle both global (i.e. for
 478                                 all users) and private (for one user)
 479                                 enabling/disabling of
 480                                 units.</para></listitem>
 481                         </varlistentry>
 482                 </variablelist>
 483
 484                 <variablelist>
 485                         <varlistentry>
 486                                 <term>SysV init scripts directory</term>
 487
 488                                 <listitem><para>The location of the
 489                                 SysV init script directory varies
 490                                 between distributions. If systemd
 491                                 cannot find a native unit file for a
 492                                 requested service, it will look for a
 493                                 SysV init script of the same name
 494                                 (with the
 495                                 <filename>.service</filename> suffix
 496                                 removed).</para></listitem>
 497                         </varlistentry>
 498                 </variablelist>
 499
 500                 <variablelist>
 501                         <varlistentry>
 502                                 <term>SysV runlevel link farm directory</term>
 503
 504                                 <listitem><para>The location of the
 505                                 SysV runlevel link farm directory
 506                                 varies between distributions. systemd
 507                                 will take the link farm into account
 508                                 when figuring out whether a service
 509                                 shall be enabled. Note that a service
 510                                 unit with a native unit configuration
 511                                 file cannot be started by activating it
 512                                 in the SysV runlevel link
 513                                 farm.</para></listitem>
 514                         </varlistentry>
 515                 </variablelist>
 516         </refsect1>
 517
 518         <refsect1>
 519                 <title>Signals</title>
 520
 521                 <variablelist>
 522                         <varlistentry>
 523                                 <term>SIGTERM</term>
 524
 525                                 <listitem><para>Upon receiving this
 526                                 signal the systemd system manager
 527                                 serializes its state, reexecutes
 528                                 itself and deserializes the saved
 529                                 state again. This is mostly equivalent
 530                                 to <command>systemctl
 531                                 daemon-reexec</command>.</para>
 532
 533                                 <para>systemd session managers will
 534                                 start the
 535                                 <filename>exit.target</filename> unit
 536                                 when this signal is received. This is
 537                                 mostly equivalent to
 538                                 <command>systemctl --session start
 539                                 exit.target</command>.</para></listitem>
 540                         </varlistentry>
 541
 542                         <varlistentry>
 543                                 <term>SIGINT</term>
 544
 545                                 <listitem><para>Upon receiving this
 546                                 signal the systemd system manager will
 547                                 start the
 548                                 <filename>ctrl-alt-del.target</filename> unit. This
 549                                 is mostly equivalent to
 550                                 <command>systemctl start
 551                                 ctl-alt-del.target</command>.</para>
 552
 553                                 <para>systemd session managers
 554                                 treat this signal the same way as
 555                                 SIGTERM.</para></listitem>
 556                         </varlistentry>
 557
 558                         <varlistentry>
 559                                 <term>SIGWINCH</term>
 560
 561                                 <listitem><para>When this signal is
 562                                 received the systemd system manager
 563                                 will start the
 564                                 <filename>kbrequest.target</filename>
 565                                 unit. This is mostly equivalent to
 566                                 <command>systemctl start
 567                                 kbrequest.target</command>.</para>
 568
 569                                 <para>This signal is ignored by
 570                                 systemd session
 571                                 managers.</para></listitem>
 572                         </varlistentry>
 573
 574                         <varlistentry>
 575                                 <term>SIGPWR</term>
 576
 577                                 <listitem><para>When this signal is
 578                                 received the systemd manager
 579                                 will start the
 580                                 <filename>sigpwr.target</filename>
 581                                 unit. This is mostly equivalent to
 582                                 <command>systemctl start
 583                                 sigpwr.target</command>.</para></listitem>
 584                         </varlistentry>
 585
 586                         <varlistentry>
 587                                 <term>SIGUSR1</term>
 588
 589                                 <listitem><para>When this signal is
 590                                 received the systemd manager will try
 591                                 to reconnect to the D-Bus
 592                                 bus.</para></listitem>
 593                         </varlistentry>
 594
 595                         <varlistentry>
 596                                 <term>SIGUSR2</term>
 597
 598                                 <listitem><para>When this signal is
 599                                 received the systemd manager will log
 600                                 its complete state in human readable
 601                                 form. The data logged is the same as
 602                                 printed by <command>systemctl
 603                                 dump</command>.</para></listitem>
 604                         </varlistentry>
 605
 606                         <varlistentry>
 607                                 <term>SIGHUP</term>
 608
 609                                 <listitem><para>Reloads the complete
 610                                 daemon configuration. This is mostly
 611                                 equivalent to <command>systemctl
 612                                 daemon-reload</command>.</para></listitem>
 613                         </varlistentry>
 614
 615                         <varlistentry>
 616                                 <term>SIGRTMIN+0</term>
 617
 618                                 <listitem><para>Enters default mode, starts the
 619                                 <filename>default.target</filename>
 620                                 unit. This is mostly equivalent to
 621                                 <command>systemctl start
 622                                 default.target</command>.</para></listitem>
 623                         </varlistentry>
 624
 625                         <varlistentry>
 626                                 <term>SIGRTMIN+1</term>
 627
 628                                 <listitem><para>Enters rescue mode,
 629                                 starts the
 630                                 <filename>rescue.target</filename>
 631                                 unit. This is mostly equivalent to
 632                                 <command>systemctl isolate
 633                                 rescue.target</command>.</para></listitem>
 634                         </varlistentry>
 635
 636                         <varlistentry>
 637                                 <term>SIGRTMIN+2</term>
 638
 639                                 <listitem><para>Enters emergency mode,
 640                                 starts the
 641                                 <filename>emergency.service</filename>
 642                                 unit. This is mostly equivalent to
 643                                 <command>systemctl isolate
 644                                 emergency.service</command>.</para></listitem>
 645                         </varlistentry>
 646
 647                         <varlistentry>
 648                                 <term>SIGRTMIN+3</term>
 649
 650                                 <listitem><para>Halts the machine,
 651                                 starts the
 652                                 <filename>halt.target</filename>
 653                                 unit. This is mostly equivalent to
 654                                 <command>systemctl start
 655                                 halt.target</command>.</para></listitem>
 656                         </varlistentry>
 657
 658                         <varlistentry>
 659                                 <term>SIGRTMIN+4</term>
 660
 661                                 <listitem><para>Powers off the machine,
 662                                 starts the
 663                                 <filename>poweroff.target</filename>
 664                                 unit. This is mostly equivalent to
 665                                 <command>systemctl start
 666                                 poweroff.target</command>.</para></listitem>
 667                         </varlistentry>
 668
 669                         <varlistentry>
 670                                 <term>SIGRTMIN+5</term>
 671
 672                                 <listitem><para>Reboots the machine,
 673                                 starts the
 674                                 <filename>reboot.target</filename>
 675                                 unit. This is mostly equivalent to
 676                                 <command>systemctl start
 677                                 reboot.target</command>.</para></listitem>
 678                         </varlistentry>
 679                 </variablelist>
 680         </refsect1>
 681
 682         <refsect1>
 683                 <title>Environment</title>
 684
 685                 <variablelist>
 686                         <varlistentry>
 687                                 <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 688                                 <listitem><para>systemd reads the
 689                                 log level from this environment
 690                                 variable. This can be overridden with
 691                                 <option>--log-level=</option>.</para></listitem>
 692                         </varlistentry>
 693
 694                         <varlistentry>
 695                                 <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 696                                 <listitem><para>systemd reads the
 697                                 log target from this environment
 698                                 variable. This can be overridden with
 699                                 <option>--log-target=</option>.</para></listitem>
 700                         </varlistentry>
 701
 702                         <varlistentry>
 703                                 <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 704                                 <listitem><para>Controls whether
 705                                 systemd highlights important log
 706                                 messages. This can be overridden with
 707                                 <option>--log-color=</option>.</para></listitem>
 708                         </varlistentry>
 709
 710                         <varlistentry>
 711                                 <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 712                                 <listitem><para>Controls whether
 713                                 systemd prints the code location along
 714                                 with log messages. This can be
 715                                 overridden with
 716                                 <option>--log-location=</option>.</para></listitem>
 717                         </varlistentry>
 718
 719                         <varlistentry>
 720                                 <term><varname>$XDG_CONFIG_HOME</varname></term>
 721                                 <term><varname>$XDG_CONFIG_DIRS</varname></term>
 722                                 <term><varname>$XDG_DATA_HOME</varname></term>
 723                                 <term><varname>$XDG_DATA_DIRS</varname></term>
 724
 725                                 <listitem><para>The systemd session
 726                                 manager uses these variables in
 727                                 accordance to the <ulink
 728                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 729                                 Base Directory specification</ulink>
 730                                 to find its configuration.</para></listitem>
 731                         </varlistentry>
 732
 733                         <varlistentry>
 734                                 <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 735
 736                                 <listitem><para>Controls where systemd
 737                                 looks for unit
 738                                 files.</para></listitem>
 739                         </varlistentry>
 740
 741                         <varlistentry>
 742                                 <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 743
 744                                 <listitem><para>Controls where systemd
 745                                 looks for SysV init scripts.</para></listitem>
 746                         </varlistentry>
 747
 748                         <varlistentry>
 749                                 <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 750
 751                                 <listitem><para>Controls where systemd
 752                                 looks for SysV init script runlevel link
 753                                 farms.</para></listitem>
 754                         </varlistentry>
 755
 756                         <varlistentry>
 757                                 <term><varname>$LISTEN_PID</varname></term>
 758                                 <term><varname>$LISTEN_FDS</varname></term>
 759
 760                                 <listitem><para>Set by systemd for
 761                                 supervised processes during
 762                                 socket-based activation. See
 763                                 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 764                                 for more information.
 765                                 </para></listitem>
 766                         </varlistentry>
 767
 768                         <varlistentry>
 769                                 <term><varname>$NOTIFY_SOCKET</varname></term>
 770
 771                                 <listitem><para>Set by systemd for
 772                                 supervised processes for status and
 773                                 start-up completion notification. See
 774                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 775                                 for more information.
 776                                 </para></listitem>
 777                         </varlistentry>
 778                 </variablelist>
 779         </refsect1>
 780
 781         <refsect1>
 782                 <title>Kernel Command Line</title>
 783
 784                 <para>When run as system instance systemd parses a few kernel command line arguments:</para>
 785
 786                 <variablelist>
 787                         <varlistentry>
 788                                 <term><varname>systemd.unit=</varname></term>
 789
 790                                 <listitem><para>Overrides the unit to
 791                                 activate on boot. Defaults to
 792                                 <filename>default.target</filename>. This
 793                                 may be used to temporarily boot into a
 794                                 different boot unit, for example
 795                                 <filename>rescue.target</filename> or
 796                                 <filename>emergency.service</filename>. See
 797                                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 798                                 for details about these
 799                                 units.</para></listitem>
 800                         </varlistentry>
 801
 802                         <varlistentry>
 803                                 <term><varname>systemd.log_target=</varname></term>
 804                                 <term><varname>systemd.log_level=</varname></term>
 805                                 <term><varname>systemd.log_color=</varname></term>
 806                                 <term><varname>systemd.log_location=</varname></term>
 807
 808                                 <listitem><para>Controls log output,
 809                                 with the same effect as the
 810                                 <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
 811                                 environment variables described above.</para></listitem>
 812                         </varlistentry>
 813
 814                         <varlistentry>
 815                                 <term><varname>systemd.dump_core=</varname></term>
 816
 817                                 <listitem><para>Takes a boolean
 818                                 argument. If <option>true</option>
 819                                 systemd dumps core when it
 820                                 crashes. Otherwise no core dump is
 821                                 created. Defaults to
 822                                 <option>true</option>.</para></listitem>
 823                         </varlistentry>
 824
 825                         <varlistentry>
 826                                 <term><varname>systemd.crash_shell=</varname></term>
 827
 828                                 <listitem><para>Takes a boolean
 829                                 argument. If <option>true</option>
 830                                 systemd spawns a shell when it
 831                                 crashes. Otherwise no core dump is
 832                                 created. Defaults to
 833                                 <option>false</option>, for security
 834                                 reasons, as the shell is not protected
 835                                 by any password
 836                                 authentication.</para></listitem>
 837                         </varlistentry>
 838
 839                         <varlistentry>
 840                                 <term><varname>systemd.crash_chvt=</varname></term>
 841
 842                                 <listitem><para>Takes an integer
 843                                 argument. If positive systemd
 844                                 activates the specified virtual
 845                                 terminal when it crashes. Defaults to
 846                                 <literal>-1</literal>.</para></listitem>
 847                         </varlistentry>
 848
 849                         <varlistentry>
 850                                 <term><varname>systemd.show_status=</varname></term>
 851
 852                                 <listitem><para>Takes a boolean
 853                                 argument. If <option>true</option>
 854                                 shows terse service status updates on
 855                                 the console during bootup. Defaults to
 856                                 <option>true</option>.</para></listitem>
 857                         </varlistentry>
 858
 859                 </variablelist>
 860         </refsect1>
 861
 862         <refsect1>
 863                 <title>Sockets and FIFOs</title>
 864
 865                 <variablelist>
 866                         <varlistentry>
 867                                 <term><filename>@/org/freedesktop/systemd1/notify</filename></term>
 868
 869                                 <listitem><para>Daemon status
 870                                 notification socket. This is an AF_UNIX
 871                                 datagram socket in the Linux abstract
 872                                 namespace, and is used to implement
 873                                 the daemon notification logic as
 874                                 implemented by
 875                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
 876
 877                         </varlistentry>
 878
 879                         <varlistentry>
 880                                 <term><filename>@/org/freedesktop/systemd1/logger</filename></term>
 881
 882                                 <listitem><para>Used internally by the
 883                                 <filename>systemd-logger.service</filename>
 884                                 unit to connect STDOUT and/or STDERR
 885                                 of spawned processes to
 886                                 <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 887                                 or the kernel log buffer. This is an
 888                                 AF_UNIX stream socket in the Linux
 889                                 abstract namespace.</para></listitem>
 890                         </varlistentry>
 891
 892                         <varlistentry>
 893                                 <term><filename>@/org/freedesktop/systemd1/private</filename></term>
 894
 895                                 <listitem><para>Used internally as
 896                                 communication channel between
 897                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 898                                 and the systemd process. This is an
 899                                 AF_UNIX stream socket in the Linux
 900                                 abstract namespace. This interface is
 901                                 private to systemd and should not be
 902                                 used in external
 903                                 projects.</para></listitem>
 904                         </varlistentry>
 905
 906                         <varlistentry>
 907                                 <term><filename>/dev/initctl</filename></term>
 908
 909                                 <listitem><para>Limited compatibility
 910                                 support for the SysV client interface,
 911                                 as implemented by the
 912                                 <filename>systemd-initctl.service</filename>
 913                                 unit. This is a named pipe in the file
 914                                 system. This interface is obsolete and
 915                                 should not be used in new
 916                                 applications.</para></listitem>
 917                         </varlistentry>
 918                 </variablelist>
 919         </refsect1>
 920
 921         <refsect1>
 922                 <title>See Also</title>
 923                 <para>
 924                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 925                         <citerefentry><refentrytitle>systemadm</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 926                         <citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 927                         <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 928                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 929                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 930                         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 931                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 932                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 933                 </para>
 934         </refsect1>
 935
 936 </refentry>