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 maintenance. 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
 452                                 directories only with the
 453                                 <command>enable</command> and
 454                                 <command>disable</command> commands of
 455                                 the
 456                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 457                                 tool.</para></listitem>
 458                         </varlistentry>
 459                 </variablelist>
 460
 461                 <variablelist>
 462                         <varlistentry>
 463                                 <term>Session unit directories</term>
 464
 465                                 <listitem><para>Similar rules apply
 466                                 for the session unit
 467                                 directories. However, here the <ulink
 468                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 469                                 Base Directory specification</ulink>
 470                                 is followed to find
 471                                 units. Applications should place their
 472                                 unit files in the directory returned
 473                                 by <command>pkg-config systemd
 474                                 --variable=systemdsessionunitdir</command>. Global
 475                                 configuration is done in the directory
 476                                 reported by <command>pkg-config
 477                                 systemd
 478                                 --variable=systemdsessionconfdir</command>. The
 479                                 <command>enable</command> and
 480                                 <command>disable</command> commands of
 481                                 the
 482                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 483                                 tool can handle both global (i.e. for
 484                                 all users) and private (for one user)
 485                                 enabling/disabling of
 486                                 units.</para></listitem>
 487                         </varlistentry>
 488                 </variablelist>
 489
 490                 <variablelist>
 491                         <varlistentry>
 492                                 <term>SysV init scripts directory</term>
 493
 494                                 <listitem><para>The location of the
 495                                 SysV init script directory varies
 496                                 between distributions. If systemd
 497                                 cannot find a native unit file for a
 498                                 requested service, it will look for a
 499                                 SysV init script of the same name
 500                                 (with the
 501                                 <filename>.service</filename> suffix
 502                                 removed).</para></listitem>
 503                         </varlistentry>
 504                 </variablelist>
 505
 506                 <variablelist>
 507                         <varlistentry>
 508                                 <term>SysV runlevel link farm directory</term>
 509
 510                                 <listitem><para>The location of the
 511                                 SysV runlevel link farm directory
 512                                 varies between distributions. systemd
 513                                 will take the link farm into account
 514                                 when figuring out whether a service
 515                                 shall be enabled. Note that a service
 516                                 unit with a native unit configuration
 517                                 file cannot be started by activating it
 518                                 in the SysV runlevel link
 519                                 farm.</para></listitem>
 520                         </varlistentry>
 521                 </variablelist>
 522         </refsect1>
 523
 524         <refsect1>
 525                 <title>Signals</title>
 526
 527                 <variablelist>
 528                         <varlistentry>
 529                                 <term>SIGTERM</term>
 530
 531                                 <listitem><para>Upon receiving this
 532                                 signal the systemd system manager
 533                                 serializes its state, reexecutes
 534                                 itself and deserializes the saved
 535                                 state again. This is mostly equivalent
 536                                 to <command>systemctl
 537                                 daemon-reexec</command>.</para>
 538
 539                                 <para>systemd session managers will
 540                                 start the
 541                                 <filename>exit.target</filename> unit
 542                                 when this signal is received. This is
 543                                 mostly equivalent to
 544                                 <command>systemctl --session start
 545                                 exit.target</command>.</para></listitem>
 546                         </varlistentry>
 547
 548                         <varlistentry>
 549                                 <term>SIGINT</term>
 550
 551                                 <listitem><para>Upon receiving this
 552                                 signal the systemd system manager will
 553                                 start the
 554                                 <filename>ctrl-alt-del.target</filename> unit. This
 555                                 is mostly equivalent to
 556                                 <command>systemctl start
 557                                 ctl-alt-del.target</command>.</para>
 558
 559                                 <para>systemd session managers
 560                                 treat this signal the same way as
 561                                 SIGTERM.</para></listitem>
 562                         </varlistentry>
 563
 564                         <varlistentry>
 565                                 <term>SIGWINCH</term>
 566
 567                                 <listitem><para>When this signal is
 568                                 received the systemd system manager
 569                                 will start the
 570                                 <filename>kbrequest.target</filename>
 571                                 unit. This is mostly equivalent to
 572                                 <command>systemctl start
 573                                 kbrequest.target</command>.</para>
 574
 575                                 <para>This signal is ignored by
 576                                 systemd session
 577                                 managers.</para></listitem>
 578                         </varlistentry>
 579
 580                         <varlistentry>
 581                                 <term>SIGPWR</term>
 582
 583                                 <listitem><para>When this signal is
 584                                 received the systemd manager
 585                                 will start the
 586                                 <filename>sigpwr.target</filename>
 587                                 unit. This is mostly equivalent to
 588                                 <command>systemctl start
 589                                 sigpwr.target</command>.</para></listitem>
 590                         </varlistentry>
 591
 592                         <varlistentry>
 593                                 <term>SIGUSR1</term>
 594
 595                                 <listitem><para>When this signal is
 596                                 received the systemd manager will try
 597                                 to reconnect to the D-Bus
 598                                 bus.</para></listitem>
 599                         </varlistentry>
 600
 601                         <varlistentry>
 602                                 <term>SIGUSR2</term>
 603
 604                                 <listitem><para>When this signal is
 605                                 received the systemd manager will log
 606                                 its complete state in human readable
 607                                 form. The data logged is the same as
 608                                 printed by <command>systemctl
 609                                 dump</command>.</para></listitem>
 610                         </varlistentry>
 611
 612                         <varlistentry>
 613                                 <term>SIGHUP</term>
 614
 615                                 <listitem><para>Reloads the complete
 616                                 daemon configuration. This is mostly
 617                                 equivalent to <command>systemctl
 618                                 daemon-reload</command>.</para></listitem>
 619                         </varlistentry>
 620
 621                         <varlistentry>
 622                                 <term>SIGRTMIN+0</term>
 623
 624                                 <listitem><para>Enters default mode, starts the
 625                                 <filename>default.target</filename>
 626                                 unit. This is mostly equivalent to
 627                                 <command>systemctl start
 628                                 default.target</command>.</para></listitem>
 629                         </varlistentry>
 630
 631                         <varlistentry>
 632                                 <term>SIGRTMIN+1</term>
 633
 634                                 <listitem><para>Enters rescue mode,
 635                                 starts the
 636                                 <filename>rescue.target</filename>
 637                                 unit. This is mostly equivalent to
 638                                 <command>systemctl isolate
 639                                 rescue.target</command>.</para></listitem>
 640                         </varlistentry>
 641
 642                         <varlistentry>
 643                                 <term>SIGRTMIN+2</term>
 644
 645                                 <listitem><para>Enters emergency mode,
 646                                 starts the
 647                                 <filename>emergency.service</filename>
 648                                 unit. This is mostly equivalent to
 649                                 <command>systemctl isolate
 650                                 emergency.service</command>.</para></listitem>
 651                         </varlistentry>
 652
 653                         <varlistentry>
 654                                 <term>SIGRTMIN+3</term>
 655
 656                                 <listitem><para>Halts the machine,
 657                                 starts the
 658                                 <filename>halt.target</filename>
 659                                 unit. This is mostly equivalent to
 660                                 <command>systemctl start
 661                                 halt.target</command>.</para></listitem>
 662                         </varlistentry>
 663
 664                         <varlistentry>
 665                                 <term>SIGRTMIN+4</term>
 666
 667                                 <listitem><para>Powers off the machine,
 668                                 starts the
 669                                 <filename>poweroff.target</filename>
 670                                 unit. This is mostly equivalent to
 671                                 <command>systemctl start
 672                                 poweroff.target</command>.</para></listitem>
 673                         </varlistentry>
 674
 675                         <varlistentry>
 676                                 <term>SIGRTMIN+5</term>
 677
 678                                 <listitem><para>Reboots the machine,
 679                                 starts the
 680                                 <filename>reboot.target</filename>
 681                                 unit. This is mostly equivalent to
 682                                 <command>systemctl start
 683                                 reboot.target</command>.</para></listitem>
 684                         </varlistentry>
 685                 </variablelist>
 686         </refsect1>
 687
 688         <refsect1>
 689                 <title>Environment</title>
 690
 691                 <variablelist>
 692                         <varlistentry>
 693                                 <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 694                                 <listitem><para>systemd reads the
 695                                 log level from this environment
 696                                 variable. This can be overridden with
 697                                 <option>--log-level=</option>.</para></listitem>
 698                         </varlistentry>
 699
 700                         <varlistentry>
 701                                 <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 702                                 <listitem><para>systemd reads the
 703                                 log target from this environment
 704                                 variable. This can be overridden with
 705                                 <option>--log-target=</option>.</para></listitem>
 706                         </varlistentry>
 707
 708                         <varlistentry>
 709                                 <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 710                                 <listitem><para>Controls whether
 711                                 systemd highlights important log
 712                                 messages. This can be overridden with
 713                                 <option>--log-color=</option>.</para></listitem>
 714                         </varlistentry>
 715
 716                         <varlistentry>
 717                                 <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 718                                 <listitem><para>Controls whether
 719                                 systemd prints the code location along
 720                                 with log messages. This can be
 721                                 overridden with
 722                                 <option>--log-location=</option>.</para></listitem>
 723                         </varlistentry>
 724
 725                         <varlistentry>
 726                                 <term><varname>$XDG_CONFIG_HOME</varname></term>
 727                                 <term><varname>$XDG_CONFIG_DIRS</varname></term>
 728                                 <term><varname>$XDG_DATA_HOME</varname></term>
 729                                 <term><varname>$XDG_DATA_DIRS</varname></term>
 730
 731                                 <listitem><para>The systemd session
 732                                 manager uses these variables in
 733                                 accordance to the <ulink
 734                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 735                                 Base Directory specification</ulink>
 736                                 to find its configuration.</para></listitem>
 737                         </varlistentry>
 738
 739                         <varlistentry>
 740                                 <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 741
 742                                 <listitem><para>Controls where systemd
 743                                 looks for unit
 744                                 files.</para></listitem>
 745                         </varlistentry>
 746
 747                         <varlistentry>
 748                                 <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 749
 750                                 <listitem><para>Controls where systemd
 751                                 looks for SysV init scripts.</para></listitem>
 752                         </varlistentry>
 753
 754                         <varlistentry>
 755                                 <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 756
 757                                 <listitem><para>Controls where systemd
 758                                 looks for SysV init script runlevel link
 759                                 farms.</para></listitem>
 760                         </varlistentry>
 761
 762                         <varlistentry>
 763                                 <term><varname>$LISTEN_PID</varname></term>
 764                                 <term><varname>$LISTEN_FDS</varname></term>
 765
 766                                 <listitem><para>Set by systemd for
 767                                 supervised processes during
 768                                 socket-based activation. See
 769                                 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 770                                 for more information.
 771                                 </para></listitem>
 772                         </varlistentry>
 773
 774                         <varlistentry>
 775                                 <term><varname>$NOTIFY_SOCKET</varname></term>
 776
 777                                 <listitem><para>Set by systemd for
 778                                 supervised processes for status and
 779                                 start-up completion notification. See
 780                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 781                                 for more information.
 782                                 </para></listitem>
 783                         </varlistentry>
 784                 </variablelist>
 785         </refsect1>
 786
 787         <refsect1>
 788                 <title>Kernel Command Line</title>
 789
 790                 <para>When run as system instance systemd parses a few kernel command line arguments:</para>
 791
 792                 <variablelist>
 793                         <varlistentry>
 794                                 <term><varname>systemd.unit=</varname></term>
 795
 796                                 <listitem><para>Overrides the unit to
 797                                 activate on boot. Defaults to
 798                                 <filename>default.target</filename>. This
 799                                 may be used to temporarily boot into a
 800                                 different boot unit, for example
 801                                 <filename>rescue.target</filename> or
 802                                 <filename>emergency.service</filename>. See
 803                                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 804                                 for details about these
 805                                 units.</para></listitem>
 806                         </varlistentry>
 807
 808                         <varlistentry>
 809                                 <term><varname>systemd.log_target=</varname></term>
 810                                 <term><varname>systemd.log_level=</varname></term>
 811                                 <term><varname>systemd.log_color=</varname></term>
 812                                 <term><varname>systemd.log_location=</varname></term>
 813
 814                                 <listitem><para>Controls log output,
 815                                 with the same effect as the
 816                                 <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
 817                                 environment variables described above.</para></listitem>
 818                         </varlistentry>
 819
 820                         <varlistentry>
 821                                 <term><varname>systemd.dump_core=</varname></term>
 822
 823                                 <listitem><para>Takes a boolean
 824                                 argument. If <option>true</option>
 825                                 systemd dumps core when it
 826                                 crashes. Otherwise no core dump is
 827                                 created. Defaults to
 828                                 <option>true</option>.</para></listitem>
 829                         </varlistentry>
 830
 831                         <varlistentry>
 832                                 <term><varname>systemd.crash_shell=</varname></term>
 833
 834                                 <listitem><para>Takes a boolean
 835                                 argument. If <option>true</option>
 836                                 systemd spawns a shell when it
 837                                 crashes. Otherwise no core dump is
 838                                 created. Defaults to
 839                                 <option>false</option>, for security
 840                                 reasons, as the shell is not protected
 841                                 by any password
 842                                 authentication.</para></listitem>
 843                         </varlistentry>
 844
 845                         <varlistentry>
 846                                 <term><varname>systemd.crash_chvt=</varname></term>
 847
 848                                 <listitem><para>Takes an integer
 849                                 argument. If positive systemd
 850                                 activates the specified virtual
 851                                 terminal when it crashes. Defaults to
 852                                 <literal>-1</literal>.</para></listitem>
 853                         </varlistentry>
 854
 855                         <varlistentry>
 856                                 <term><varname>systemd.show_status=</varname></term>
 857
 858                                 <listitem><para>Takes a boolean
 859                                 argument. If <option>true</option>
 860                                 shows terse service status updates on
 861                                 the console during bootup. Defaults to
 862                                 <option>true</option>.</para></listitem>
 863                         </varlistentry>
 864
 865                 </variablelist>
 866         </refsect1>
 867
 868         <refsect1>
 869                 <title>Sockets and FIFOs</title>
 870
 871                 <variablelist>
 872                         <varlistentry>
 873                                 <term><filename>@/org/freedesktop/systemd1/notify</filename></term>
 874
 875                                 <listitem><para>Daemon status
 876                                 notification socket. This is an AF_UNIX
 877                                 datagram socket in the Linux abstract
 878                                 namespace, and is used to implement
 879                                 the daemon notification logic as
 880                                 implemented by
 881                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
 882
 883                         </varlistentry>
 884
 885                         <varlistentry>
 886                                 <term><filename>@/org/freedesktop/systemd1/logger</filename></term>
 887
 888                                 <listitem><para>Used internally by the
 889                                 <filename>systemd-logger.service</filename>
 890                                 unit to connect STDOUT and/or STDERR
 891                                 of spawned processes to
 892                                 <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 893                                 or the kernel log buffer. This is an
 894                                 AF_UNIX stream socket in the Linux
 895                                 abstract namespace.</para></listitem>
 896                         </varlistentry>
 897
 898                         <varlistentry>
 899                                 <term><filename>@/org/freedesktop/systemd1/private</filename></term>
 900
 901                                 <listitem><para>Used internally as
 902                                 communication channel between
 903                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 904                                 and the systemd process. This is an
 905                                 AF_UNIX stream socket in the Linux
 906                                 abstract namespace. This interface is
 907                                 private to systemd and should not be
 908                                 used in external
 909                                 projects.</para></listitem>
 910                         </varlistentry>
 911
 912                         <varlistentry>
 913                                 <term><filename>/dev/initctl</filename></term>
 914
 915                                 <listitem><para>Limited compatibility
 916                                 support for the SysV client interface,
 917                                 as implemented by the
 918                                 <filename>systemd-initctl.service</filename>
 919                                 unit. This is a named pipe in the file
 920                                 system. This interface is obsolete and
 921                                 should not be used in new
 922                                 applications.</para></listitem>
 923                         </varlistentry>
 924                 </variablelist>
 925         </refsect1>
 926
 927         <refsect1>
 928                 <title>See Also</title>
 929                 <para>
 930                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 931                         <citerefentry><refentrytitle>systemadm</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 932                         <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 933                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 934                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 935                         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 936                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 937                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 938                 </para>
 939         </refsect1>
 940
 941 </refentry>