man/systemd.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3         "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   This file is part of systemd.
   7
   8   Copyright 2010 Lennart Poettering
   9
  10   systemd is free software; you can redistribute it and/or modify it
  11   under the terms of the GNU General Public License as published by
  12   the Free Software Foundation; either version 2 of the License, or
  13   (at your option) any later version.
  14
  15   systemd is distributed in the hope that it will be useful, but
  16   WITHOUT ANY WARRANTY; without even the implied warranty of
  17   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  18   General Public License for more details.
  19
  20   You should have received a copy of the GNU General Public License
  21   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  22 -->
  23
  24 <refentry id="systemd">
  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
 172                                 status information while booting. This
 173                                 switch has no effect when run as
 174                                 session instance. Takes a boolean
 175                                 argument which may be omitted
 176                                 which is interpreted as
 177                                 <option>true</option>.</para></listitem>
 178                         </varlistentry>
 179                         <varlistentry>
 180                                 <term><option>--sysv-console=</option></term>
 181
 182                                 <listitem><para>Controls whether
 183                                 output of SysV init scripts will be
 184                                 directed to the console. This switch
 185                                 has no effect when run as session
 186                                 instance. Takes a boolean argument
 187                                 which may be omitted which is
 188                                 interpreted as
 189                                 <option>true</option>.</para></listitem>
 190                         </varlistentry>
 191                         <varlistentry>
 192                                 <term><option>--log-target=</option></term>
 193
 194                                 <listitem><para>Set log
 195                                 target. Argument must be one of
 196                                 <option>console</option>,
 197                                 <option>syslog</option>,
 198                                 <option>kmsg</option>,
 199                                 <option>syslog-or-kmsg</option>,
 200                                 <option>null</option>.</para></listitem>
 201                         </varlistentry>
 202                         <varlistentry>
 203                                 <term><option>--log-level=</option></term>
 204
 205                                 <listitem><para>Set log level. As
 206                                 argument this accepts a numerical log
 207                                 level or the well-known <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 208                                 symbolic names (lowercase):
 209                                 <option>emerg</option>,
 210                                 <option>alert</option>,
 211                                 <option>crit</option>,
 212                                 <option>err</option>,
 213                                 <option>warning</option>,
 214                                 <option>notice</option>,
 215                                 <option>info</option>,
 216                                 <option>debug</option>.</para></listitem>
 217                         </varlistentry>
 218                         <varlistentry>
 219                                 <term><option>--log-color=</option></term>
 220
 221                                 <listitem><para>Highlight important
 222                                 log messages. Argument is a boolean
 223                                 value. If the argument is omitted it
 224                                 defaults to
 225                                 <option>true</option>.</para></listitem>
 226                         </varlistentry>
 227                         <varlistentry>
 228                                 <term><option>--log-location=</option></term>
 229
 230                                 <listitem><para>Include code location
 231                                 in log messages. This is mostly
 232                                 relevant for debugging
 233                                 purposes. Argument is a boolean
 234                                 value. If the argument is omitted
 235                                 it defaults to
 236                                 <option>true</option>.</para></listitem>
 237                         </varlistentry>
 238                 </variablelist>
 239         </refsect1>
 240
 241         <refsect1>
 242                 <title>Concepts</title>
 243
 244                 <para>systemd provides a dependency system between
 245                 various entities called "units". Units encapsulate
 246                 various objects that are relevant for system boot-up
 247                 and maintenance. The majority of units are configured
 248                 in unit configuration files, whose syntax and basic
 249                 set of options is described in
 250                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 251                 however some are created automatically from other
 252                 configuration or dynamically from system state. Units
 253                 may be 'active' (meaning started, bound, plugged in,
 254                 ...  depending on the unit type, see below), or
 255                 'inactive' (meaning stopped, unbound, unplugged, ...),
 256                 as well as in the process of being activated or
 257                 deactivated, i.e. between the two states (these states
 258                 are called 'activating', 'deactivating'). A special
 259                 'failed' state is available as well which is very
 260                 similar to 'inactive' and is entered when the service
 261                 failed in some way (process returned error code on
 262                 exit, or crashed, or an operation timed out). If this
 263                 state is entered the cause will be logged, for later
 264                 reference. Note that the various unit types may have a
 265                 number of additional substates, which are mapped to
 266                 the five generalized unit states described
 267                 here.</para>
 268
 269                 <para>The following unit types are available:</para>
 270
 271                 <orderedlist>
 272                         <listitem><para>Service units, which control
 273                         daemons and the processes they consist of. For
 274                         details see
 275                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 276
 277                         <listitem><para>Socket units, which
 278                         encapsulate local IPC or network sockets in
 279                         the system, useful for socket-based
 280                         activation. For details about socket units see
 281                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 282                         for details on socket-based activation and
 283                         other forms of activation, see
 284                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 285
 286                         <listitem><para>Target units are useful to
 287                         group units, or provide well-known
 288                         synchronization points during boot-up, see
 289                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 290
 291                         <listitem><para>Device units expose kernel
 292                         devices in systemd and may be used to
 293                         implement device-based activation. For details
 294                         see
 295                         <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 296
 297                         <listitem><para>Mount units control mount
 298                         points in the file system, for details see
 299                         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 300
 301                         <listitem><para>Automount units provide
 302                         automount capabilities, for on-demand mounting
 303                         of file systems as well as parallelized
 304                         boot-up. See
 305                         <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 306
 307                         <listitem><para>Snapshot units can be used to
 308                         temporarily save the state of the set of
 309                         systemd units, which later may be restored by
 310                         activating the saved snapshot unit. For more
 311                         information see
 312                         <citerefentry><refentrytitle>systemd.snapshot</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 313
 314                         <listitem><para>Timer units are useful for
 315                         triggering activation of other units based on
 316                         timers. You may find details in
 317                         <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 318
 319                         <listitem><para>Swap units are very similar to
 320                         mount units and encapsulated memory swap
 321                         partitions or files of the operating
 322                         systemd. They are described in <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 323
 324                         <listitem><para>Path units may be used
 325                         to activate other services when file system
 326                         objects change or are modified. See
 327                         <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 328
 329                 </orderedlist>
 330
 331                 <para>Units are named as their configuration
 332                 files. Some units have special semantics. A detailed
 333                 list you may find in
 334                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 335
 336                 <para>systemd knows various kinds of dependencies,
 337                 including positive and negative requirement
 338                 dependencies (i.e. <varname>Requires=</varname> and
 339                 <varname>Conflicts=</varname>) as well as ordering
 340                 dependencies (<varname>After=</varname> and
 341                 <varname>Before=</varname>). NB: ordering and
 342                 requirement dependencies are orthogonal. If only a
 343                 requirement dependency exists between two units
 344                 (e.g. <filename>foo.service</filename> requires
 345                 <filename>bar.service</filename>), but no ordering
 346                 dependency (e.g. <filename>foo.service</filename>
 347                 after <filename>bar.service</filename>) and both are
 348                 requested to start, they will be started in
 349                 parallel. It is a common pattern that both requirement
 350                 and ordering dependencies are placed between two
 351                 units. Also note that the majority of dependencies are
 352                 implicitly created and maintained by systemd. In most
 353                 cases it should be unnecessary to declare additional
 354                 dependencies manually, however it is possible to do
 355                 this.</para>
 356
 357                 <para>Application programs and units (via
 358                 dependencies) may requests state changes of units. In
 359                 systemd, these requests are encapsulated as 'jobs' and
 360                 maintained in a job queue. Jobs may succeed or can
 361                 fail, their execution is ordered based on the ordering
 362                 dependencies of the units they have been scheduled
 363                 for.</para>
 364
 365                 <para>On boot systemd activates the target unit
 366                 <filename>default.target</filename> whose job is to
 367                 activate on-boot services and other on-boot units by
 368                 pulling them in via dependencies. Usually the unit
 369                 name is just an alias (symlink) for either
 370                 <filename>graphical.target</filename> (for
 371                 fully-featured boots into the UI) or
 372                 <filename>multi-user.target</filename> (for limited
 373                 console-only boots for use in embedded or server
 374                 environments, or similar; a subset of
 375                 graphical.target). However it is at the discretion of
 376                 the administrator to configure it as an alias to any
 377                 other target unit. See
 378                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 379                 for details about these target units.</para>
 380
 381                 <para>Processes systemd spawns are placed in
 382                 individual Linux control groups named after the unit
 383                 which they belong to in the private systemd
 384                 hierarchy. (see <ulink
 385                 url="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
 386                 for more information about control groups, or short
 387                 "cgroups"). systemd uses this to effectively keep
 388                 track of processes. Control group information is
 389                 maintained in the kernel, and is accessible via the
 390                 file system hierarchy (beneath
 391                 <filename>/sys/fs/cgroup/systemd/</filename>), or in tools
 392                 such as
 393                 <citerefentry><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 394                 (<command>ps xawf -eo pid,user,cgroup,args</command>
 395                 is particularly useful to list all processes and the
 396                 systemd units they belong to.).</para>
 397
 398                 <para>systemd is compatible with the SysV init system
 399                 to a large degree: SysV init scripts are supported and
 400                 simply read as an alternative (though limited)
 401                 configuration file format. The SysV
 402                 <filename>/dev/initctl</filename> interface is
 403                 provided, and compatibility implementations of the
 404                 various SysV client tools are available. In addition to
 405                 that, various established Unix functionality such as
 406                 <filename>/etc/fstab</filename> or the
 407                 <filename>utmp</filename> database are
 408                 supported.</para>
 409
 410                 <para>systemd has a minimal transaction system: if a
 411                 unit is requested to start up or shut down it will add
 412                 it and all its dependencies to a temporary
 413                 transaction. Then, it will verify if the transaction
 414                 is consistent (i.e. whether the ordering of all units
 415                 is cycle-free). If it is not, systemd will try to fix
 416                 it up, and removes non-essential jobs from the
 417                 transaction that might remove the loop. Also, systemd
 418                 tries to suppress non-essential jobs in the
 419                 transaction that would stop a running service. Finally
 420                 it is checked whether the jobs of the transaction
 421                 contradict jobs that have already been queued, and
 422                 optionally the transaction is aborted then. If all
 423                 worked out and the transaction is consistent and
 424                 minimized in its impact it is merged with all already
 425                 outstanding jobs and added to the run
 426                 queue. Effectively this means that before executing a
 427                 requested operation, systemd will verify that it makes
 428                 sense, fixing it if possible, and only failing if it
 429                 really cannot work.</para>
 430
 431                 <para>Systemd contains native implementations of
 432                 various tasks that need to be executed as part of the
 433                 boot process. For example, it sets the host name or
 434                 configures the loopback network device. It also sets
 435                 up and mounts various API file systems, such as
 436                 <filename>/sys</filename> or
 437                 <filename>/proc</filename>.</para>
 438
 439                 <para>For more information about the concepts and
 440                 ideas behind systemd please refer to the <ulink
 441                 url="http://0pointer.de/blog/projects/systemd.html">Original
 442                 Design Document</ulink>.</para>
 443
 444                 <para>Note that some but not all interfaces provided
 445                 by systemd are covered by the <ulink
 446                 url="http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
 447                 Stability Promise</ulink>.</para>
 448         </refsect1>
 449
 450         <refsect1>
 451                 <title>Directories</title>
 452
 453                 <variablelist>
 454                         <varlistentry>
 455                                 <term>System unit directories</term>
 456
 457                                 <listitem><para>The systemd system
 458                                 manager reads unit configuration from
 459                                 various directories. Packages that
 460                                 want to install unit files shall place
 461                                 them in the directory returned by
 462                                 <command>pkg-config systemd
 463                                 --variable=systemdsystemunitdir</command>. Other
 464                                 directories checked are
 465                                 <filename>/usr/local/share/systemd/system</filename>
 466                                 and
 467                                 <filename>/usr/share/systemd/system</filename>. User
 468                                 configuration always takes
 469                                 precedence. <command>pkg-config
 470                                 systemd
 471                                 --variable=systemdsystemconfdir</command>
 472                                 returns the path of the system
 473                                 configuration directory. Packages
 474                                 should alter the content of these
 475                                 directories only with the
 476                                 <command>enable</command> and
 477                                 <command>disable</command> commands of
 478                                 the
 479                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 480                                 tool.</para></listitem>
 481                         </varlistentry>
 482                 </variablelist>
 483
 484                 <variablelist>
 485                         <varlistentry>
 486                                 <term>Session unit directories</term>
 487
 488                                 <listitem><para>Similar rules apply
 489                                 for the session unit
 490                                 directories. However, here the <ulink
 491                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 492                                 Base Directory specification</ulink>
 493                                 is followed to find
 494                                 units. Applications should place their
 495                                 unit files in the directory returned
 496                                 by <command>pkg-config systemd
 497                                 --variable=systemdsessionunitdir</command>. Global
 498                                 configuration is done in the directory
 499                                 reported by <command>pkg-config
 500                                 systemd
 501                                 --variable=systemdsessionconfdir</command>. The
 502                                 <command>enable</command> and
 503                                 <command>disable</command> commands of
 504                                 the
 505                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 506                                 tool can handle both global (i.e. for
 507                                 all users) and private (for one user)
 508                                 enabling/disabling of
 509                                 units.</para></listitem>
 510                         </varlistentry>
 511                 </variablelist>
 512
 513                 <variablelist>
 514                         <varlistentry>
 515                                 <term>SysV init scripts directory</term>
 516
 517                                 <listitem><para>The location of the
 518                                 SysV init script directory varies
 519                                 between distributions. If systemd
 520                                 cannot find a native unit file for a
 521                                 requested service, it will look for a
 522                                 SysV init script of the same name
 523                                 (with the
 524                                 <filename>.service</filename> suffix
 525                                 removed).</para></listitem>
 526                         </varlistentry>
 527                 </variablelist>
 528
 529                 <variablelist>
 530                         <varlistentry>
 531                                 <term>SysV runlevel link farm directory</term>
 532
 533                                 <listitem><para>The location of the
 534                                 SysV runlevel link farm directory
 535                                 varies between distributions. systemd
 536                                 will take the link farm into account
 537                                 when figuring out whether a service
 538                                 shall be enabled. Note that a service
 539                                 unit with a native unit configuration
 540                                 file cannot be started by activating it
 541                                 in the SysV runlevel link
 542                                 farm.</para></listitem>
 543                         </varlistentry>
 544                 </variablelist>
 545         </refsect1>
 546
 547         <refsect1>
 548                 <title>Signals</title>
 549
 550                 <variablelist>
 551                         <varlistentry>
 552                                 <term>SIGTERM</term>
 553
 554                                 <listitem><para>Upon receiving this
 555                                 signal the systemd system manager
 556                                 serializes its state, reexecutes
 557                                 itself and deserializes the saved
 558                                 state again. This is mostly equivalent
 559                                 to <command>systemctl
 560                                 daemon-reexec</command>.</para>
 561
 562                                 <para>systemd session managers will
 563                                 start the
 564                                 <filename>exit.target</filename> unit
 565                                 when this signal is received. This is
 566                                 mostly equivalent to
 567                                 <command>systemctl --session start
 568                                 exit.target</command>.</para></listitem>
 569                         </varlistentry>
 570
 571                         <varlistentry>
 572                                 <term>SIGINT</term>
 573
 574                                 <listitem><para>Upon receiving this
 575                                 signal the systemd system manager will
 576                                 start the
 577                                 <filename>ctrl-alt-del.target</filename> unit. This
 578                                 is mostly equivalent to
 579                                 <command>systemctl start
 580                                 ctl-alt-del.target</command>.</para>
 581
 582                                 <para>systemd session managers
 583                                 treat this signal the same way as
 584                                 SIGTERM.</para></listitem>
 585                         </varlistentry>
 586
 587                         <varlistentry>
 588                                 <term>SIGWINCH</term>
 589
 590                                 <listitem><para>When this signal is
 591                                 received the systemd system manager
 592                                 will start the
 593                                 <filename>kbrequest.target</filename>
 594                                 unit. This is mostly equivalent to
 595                                 <command>systemctl start
 596                                 kbrequest.target</command>.</para>
 597
 598                                 <para>This signal is ignored by
 599                                 systemd session
 600                                 managers.</para></listitem>
 601                         </varlistentry>
 602
 603                         <varlistentry>
 604                                 <term>SIGPWR</term>
 605
 606                                 <listitem><para>When this signal is
 607                                 received the systemd manager
 608                                 will start the
 609                                 <filename>sigpwr.target</filename>
 610                                 unit. This is mostly equivalent to
 611                                 <command>systemctl start
 612                                 sigpwr.target</command>.</para></listitem>
 613                         </varlistentry>
 614
 615                         <varlistentry>
 616                                 <term>SIGUSR1</term>
 617
 618                                 <listitem><para>When this signal is
 619                                 received the systemd manager will try
 620                                 to reconnect to the D-Bus
 621                                 bus.</para></listitem>
 622                         </varlistentry>
 623
 624                         <varlistentry>
 625                                 <term>SIGUSR2</term>
 626
 627                                 <listitem><para>When this signal is
 628                                 received the systemd manager will log
 629                                 its complete state in human readable
 630                                 form. The data logged is the same as
 631                                 printed by <command>systemctl
 632                                 dump</command>.</para></listitem>
 633                         </varlistentry>
 634
 635                         <varlistentry>
 636                                 <term>SIGHUP</term>
 637
 638                                 <listitem><para>Reloads the complete
 639                                 daemon configuration. This is mostly
 640                                 equivalent to <command>systemctl
 641                                 daemon-reload</command>.</para></listitem>
 642                         </varlistentry>
 643
 644                         <varlistentry>
 645                                 <term>SIGRTMIN+0</term>
 646
 647                                 <listitem><para>Enters default mode, starts the
 648                                 <filename>default.target</filename>
 649                                 unit. This is mostly equivalent to
 650                                 <command>systemctl start
 651                                 default.target</command>.</para></listitem>
 652                         </varlistentry>
 653
 654                         <varlistentry>
 655                                 <term>SIGRTMIN+1</term>
 656
 657                                 <listitem><para>Enters rescue mode,
 658                                 starts the
 659                                 <filename>rescue.target</filename>
 660                                 unit. This is mostly equivalent to
 661                                 <command>systemctl isolate
 662                                 rescue.target</command>.</para></listitem>
 663                         </varlistentry>
 664
 665                         <varlistentry>
 666                                 <term>SIGRTMIN+2</term>
 667
 668                                 <listitem><para>Enters emergency mode,
 669                                 starts the
 670                                 <filename>emergency.service</filename>
 671                                 unit. This is mostly equivalent to
 672                                 <command>systemctl isolate
 673                                 emergency.service</command>.</para></listitem>
 674                         </varlistentry>
 675
 676                         <varlistentry>
 677                                 <term>SIGRTMIN+3</term>
 678
 679                                 <listitem><para>Halts the machine,
 680                                 starts the
 681                                 <filename>halt.target</filename>
 682                                 unit. This is mostly equivalent to
 683                                 <command>systemctl start
 684                                 halt.target</command>.</para></listitem>
 685                         </varlistentry>
 686
 687                         <varlistentry>
 688                                 <term>SIGRTMIN+4</term>
 689
 690                                 <listitem><para>Powers off the machine,
 691                                 starts the
 692                                 <filename>poweroff.target</filename>
 693                                 unit. This is mostly equivalent to
 694                                 <command>systemctl start
 695                                 poweroff.target</command>.</para></listitem>
 696                         </varlistentry>
 697
 698                         <varlistentry>
 699                                 <term>SIGRTMIN+5</term>
 700
 701                                 <listitem><para>Reboots the machine,
 702                                 starts the
 703                                 <filename>reboot.target</filename>
 704                                 unit. This is mostly equivalent to
 705                                 <command>systemctl start
 706                                 reboot.target</command>.</para></listitem>
 707                         </varlistentry>
 708                 </variablelist>
 709         </refsect1>
 710
 711         <refsect1>
 712                 <title>Environment</title>
 713
 714                 <variablelist>
 715                         <varlistentry>
 716                                 <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 717                                 <listitem><para>systemd reads the
 718                                 log level from this environment
 719                                 variable. This can be overridden with
 720                                 <option>--log-level=</option>.</para></listitem>
 721                         </varlistentry>
 722
 723                         <varlistentry>
 724                                 <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 725                                 <listitem><para>systemd reads the
 726                                 log target from this environment
 727                                 variable. This can be overridden with
 728                                 <option>--log-target=</option>.</para></listitem>
 729                         </varlistentry>
 730
 731                         <varlistentry>
 732                                 <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 733                                 <listitem><para>Controls whether
 734                                 systemd highlights important log
 735                                 messages. This can be overridden with
 736                                 <option>--log-color=</option>.</para></listitem>
 737                         </varlistentry>
 738
 739                         <varlistentry>
 740                                 <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 741                                 <listitem><para>Controls whether
 742                                 systemd prints the code location along
 743                                 with log messages. This can be
 744                                 overridden with
 745                                 <option>--log-location=</option>.</para></listitem>
 746                         </varlistentry>
 747
 748                         <varlistentry>
 749                                 <term><varname>$XDG_CONFIG_HOME</varname></term>
 750                                 <term><varname>$XDG_CONFIG_DIRS</varname></term>
 751                                 <term><varname>$XDG_DATA_HOME</varname></term>
 752                                 <term><varname>$XDG_DATA_DIRS</varname></term>
 753
 754                                 <listitem><para>The systemd session
 755                                 manager uses these variables in
 756                                 accordance to the <ulink
 757                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 758                                 Base Directory specification</ulink>
 759                                 to find its configuration.</para></listitem>
 760                         </varlistentry>
 761
 762                         <varlistentry>
 763                                 <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 764
 765                                 <listitem><para>Controls where systemd
 766                                 looks for unit
 767                                 files.</para></listitem>
 768                         </varlistentry>
 769
 770                         <varlistentry>
 771                                 <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 772
 773                                 <listitem><para>Controls where systemd
 774                                 looks for SysV init scripts.</para></listitem>
 775                         </varlistentry>
 776
 777                         <varlistentry>
 778                                 <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 779
 780                                 <listitem><para>Controls where systemd
 781                                 looks for SysV init script runlevel link
 782                                 farms.</para></listitem>
 783                         </varlistentry>
 784
 785                         <varlistentry>
 786                                 <term><varname>$LISTEN_PID</varname></term>
 787                                 <term><varname>$LISTEN_FDS</varname></term>
 788
 789                                 <listitem><para>Set by systemd for
 790                                 supervised processes during
 791                                 socket-based activation. See
 792                                 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 793                                 for more information.
 794                                 </para></listitem>
 795                         </varlistentry>
 796
 797                         <varlistentry>
 798                                 <term><varname>$NOTIFY_SOCKET</varname></term>
 799
 800                                 <listitem><para>Set by systemd for
 801                                 supervised processes for status and
 802                                 start-up completion notification. See
 803                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 804                                 for more information.
 805                                 </para></listitem>
 806                         </varlistentry>
 807                 </variablelist>
 808         </refsect1>
 809
 810         <refsect1>
 811                 <title>Kernel Command Line</title>
 812
 813                 <para>When run as system instance systemd parses a few kernel command line arguments:</para>
 814
 815                 <variablelist>
 816                         <varlistentry>
 817                                 <term><varname>systemd.unit=</varname></term>
 818
 819                                 <listitem><para>Overrides the unit to
 820                                 activate on boot. Defaults to
 821                                 <filename>default.target</filename>. This
 822                                 may be used to temporarily boot into a
 823                                 different boot unit, for example
 824                                 <filename>rescue.target</filename> or
 825                                 <filename>emergency.service</filename>. See
 826                                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 827                                 for details about these
 828                                 units.</para></listitem>
 829                         </varlistentry>
 830
 831                         <varlistentry>
 832                                 <term><varname>systemd.dump_core=</varname></term>
 833
 834                                 <listitem><para>Takes a boolean
 835                                 argument. If <option>true</option>
 836                                 systemd dumps core when it
 837                                 crashes. Otherwise no core dump is
 838                                 created. Defaults to
 839                                 <option>true</option>.</para></listitem>
 840                         </varlistentry>
 841
 842                         <varlistentry>
 843                                 <term><varname>systemd.crash_shell=</varname></term>
 844
 845                                 <listitem><para>Takes a boolean
 846                                 argument. If <option>true</option>
 847                                 systemd spawns a shell when it
 848                                 crashes. Otherwise no core dump is
 849                                 created. Defaults to
 850                                 <option>false</option>, for security
 851                                 reasons, as the shell is not protected
 852                                 by any password
 853                                 authentication.</para></listitem>
 854                         </varlistentry>
 855
 856                         <varlistentry>
 857                                 <term><varname>systemd.crash_chvt=</varname></term>
 858
 859                                 <listitem><para>Takes an integer
 860                                 argument. If positive systemd
 861                                 activates the specified virtual
 862                                 terminal when it crashes. Defaults to
 863                                 <literal>-1</literal>.</para></listitem>
 864                         </varlistentry>
 865
 866                         <varlistentry>
 867                                 <term><varname>systemd.confirm_spawn=</varname></term>
 868
 869                                 <listitem><para>Takes a boolean
 870                                 argument. If <option>true</option>
 871                                 asks for confirmation when spawning
 872                                 processes. Defaults to
 873                                 <option>false</option>.</para></listitem>
 874                         </varlistentry>
 875
 876                         <varlistentry>
 877                                 <term><varname>systemd.show_status=</varname></term>
 878
 879                                 <listitem><para>Takes a boolean
 880                                 argument. If <option>true</option>
 881                                 shows terse service status updates on
 882                                 the console during bootup. Defaults to
 883                                 <option>true</option>.</para></listitem>
 884                         </varlistentry>
 885
 886                         <varlistentry>
 887                                 <term><varname>systemd.sysv_console=</varname></term>
 888
 889                                 <listitem><para>Takes a boolean
 890                                 argument. If <option>true</option>
 891                                 output of SysV init scripts will be
 892                                 directed to the console. Defaults to
 893                                 <option>true</option>, unless
 894                                 <option>quiet</option> is passed as
 895                                 kernel command line option in which
 896                                 case it defaults to
 897                                 <option>false</option>.</para></listitem>
 898                         </varlistentry>
 899
 900                         <varlistentry>
 901                                 <term><varname>systemd.log_target=</varname></term>
 902                                 <term><varname>systemd.log_level=</varname></term>
 903                                 <term><varname>systemd.log_color=</varname></term>
 904                                 <term><varname>systemd.log_location=</varname></term>
 905
 906                                 <listitem><para>Controls log output,
 907                                 with the same effect as the
 908                                 <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
 909                                 environment variables described above.</para></listitem>
 910                         </varlistentry>
 911
 912                 </variablelist>
 913         </refsect1>
 914
 915         <refsect1>
 916                 <title>Sockets and FIFOs</title>
 917
 918                 <variablelist>
 919                         <varlistentry>
 920                                 <term><filename>@/org/freedesktop/systemd1/notify</filename></term>
 921
 922                                 <listitem><para>Daemon status
 923                                 notification socket. This is an AF_UNIX
 924                                 datagram socket in the Linux abstract
 925                                 namespace, and is used to implement
 926                                 the daemon notification logic as
 927                                 implemented by
 928                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
 929
 930                         </varlistentry>
 931
 932                         <varlistentry>
 933                                 <term><filename>@/org/freedesktop/systemd1/logger</filename></term>
 934
 935                                 <listitem><para>Used internally by the
 936                                 <filename>systemd-logger.service</filename>
 937                                 unit to connect STDOUT and/or STDERR
 938                                 of spawned processes to
 939                                 <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 940                                 or the kernel log buffer. This is an
 941                                 AF_UNIX stream socket in the Linux
 942                                 abstract namespace.</para></listitem>
 943                         </varlistentry>
 944
 945                         <varlistentry>
 946                                 <term><filename>@/org/freedesktop/systemd1/shutdown</filename></term>
 947
 948                                 <listitem><para>Used internally by the
 949                                 <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 950                                 tool to implement delayed
 951                                 shutdowns. This is an AF_UNIX datagram
 952                                 socket in the Linux abstract
 953                                 namespace.</para></listitem>
 954                         </varlistentry>
 955
 956                         <varlistentry>
 957                                 <term><filename>@/org/freedesktop/systemd1/private</filename></term>
 958
 959                                 <listitem><para>Used internally as
 960                                 communication channel between
 961                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 962                                 and the systemd process. This is an
 963                                 AF_UNIX stream socket in the Linux
 964                                 abstract namespace. This interface is
 965                                 private to systemd and should not be
 966                                 used in external
 967                                 projects.</para></listitem>
 968                         </varlistentry>
 969
 970                         <varlistentry>
 971                                 <term><filename>/dev/initctl</filename></term>
 972
 973                                 <listitem><para>Limited compatibility
 974                                 support for the SysV client interface,
 975                                 as implemented by the
 976                                 <filename>systemd-initctl.service</filename>
 977                                 unit. This is a named pipe in the file
 978                                 system. This interface is obsolete and
 979                                 should not be used in new
 980                                 applications.</para></listitem>
 981                         </varlistentry>
 982                 </variablelist>
 983         </refsect1>
 984
 985         <refsect1>
 986                 <title>See Also</title>
 987                 <para>
 988                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 989                         <citerefentry><refentrytitle>systemadm</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 990                         <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 991                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 992                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 993                         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 994                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 995                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 996                 </para>
 997         </refsect1>
 998
 999 </refentry>