man/systemd.special.xml

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  23
  24 <refentry id="systemd.special">
  25
  26   <refentryinfo>
  27     <title>systemd.special</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.special</refentrytitle>
  42     <manvolnum>7</manvolnum>
  43   </refmeta>
  44
  45   <refnamediv>
  46     <refname>systemd.special</refname>
  47     <refpurpose>Special systemd units</refpurpose>
  48   </refnamediv>
  49
  50   <refsynopsisdiv>
  51     <para><filename>basic.target</filename>,
  52     <filename>bluetooth.target</filename>,
  53     <filename>ctrl-alt-del.target</filename>,
  54     <filename>cryptsetup.target</filename>,
  55     <filename>cryptsetup-pre.target</filename>,
  56     <filename>dbus.service</filename>,
  57     <filename>dbus.socket</filename>,
  58     <filename>default.target</filename>,
  59     <filename>display-manager.service</filename>,
  60     <filename>emergency.target</filename>,
  61     <filename>exit.target</filename>,
  62     <filename>final.target</filename>,
  63     <filename>getty.target</filename>,
  64     <filename>graphical.target</filename>,
  65     <filename>halt.target</filename>,
  66     <filename>hibernate.target</filename>,
  67     <filename>hybrid-sleep.target</filename>,
  68     <filename>initrd-fs.target</filename>,
  69     <filename>kbrequest.target</filename>,
  70     <filename>kexec.target</filename>,
  71     <filename>local-fs.target</filename>,
  72     <filename>local-fs-pre.target</filename>,
  73     <filename>multi-user.target</filename>,
  74     <filename>network.target</filename>,
  75     <filename>network-online.target</filename>,
  76     <filename>network-pre.target</filename>,
  77     <filename>nss-lookup.target</filename>,
  78     <filename>nss-user-lookup.target</filename>,
  79     <filename>paths.target</filename>,
  80     <filename>poweroff.target</filename>,
  81     <filename>printer.target</filename>,
  82     <filename>reboot.target</filename>,
  83     <filename>remote-fs.target</filename>,
  84     <filename>remote-fs-pre.target</filename>,
  85     <filename>rescue.target</filename>,
  86     <filename>initrd-root-device.target</filename>,
  87     <filename>initrd-root-fs.target</filename>,
  88     <filename>rpcbind.target</filename>,
  89     <filename>runlevel2.target</filename>,
  90     <filename>runlevel3.target</filename>,
  91     <filename>runlevel4.target</filename>,
  92     <filename>runlevel5.target</filename>,
  93     <filename>shutdown.target</filename>,
  94     <filename>sigpwr.target</filename>,
  95     <filename>sleep.target</filename>,
  96     <filename>slices.target</filename>,
  97     <filename>smartcard.target</filename>,
  98     <filename>sockets.target</filename>,
  99     <filename>sound.target</filename>,
 100     <filename>suspend.target</filename>,
 101     <filename>swap.target</filename>,
 102     <filename>sysinit.target</filename>,
 103     <filename>syslog.socket</filename>,
 104     <filename>system-update.target</filename>,
 105     <filename>time-sync.target</filename>,
 106     <filename>timers.target</filename>,
 107     <filename>umount.target</filename>,
 108     <filename>-.slice</filename>,
 109     <filename>system.slice</filename>,
 110     <filename>user.slice</filename>,
 111     <filename>machine.slice</filename></para>
 112   </refsynopsisdiv>
 113
 114   <refsect1>
 115     <title>Description</title>
 116
 117     <para>A few units are treated specially by systemd. They have
 118     special internal semantics and cannot be renamed.</para>
 119   </refsect1>
 120
 121   <refsect1>
 122     <title>Special System Units</title>
 123
 124     <variablelist>
 125       <varlistentry>
 126         <term><filename>basic.target</filename></term>
 127         <listitem>
 128           <para>A special target unit covering basic boot-up.</para>
 129
 130           <para>systemd automatically adds dependency of the type
 131           <varname>After=</varname> for this target unit to all
 132           services (except for those with
 133           <varname>DefaultDependencies=no</varname>).</para>
 134
 135           <para>Usually, this should pull-in all local mount points plus
 136           <filename>/var</filename>, <filename>/tmp</filename> and
 137           <filename>/var/tmp</filename>, swap devices, sockets, timers,
 138           path units and other basic initialization necessary for general
 139           purpose daemons. The mentioned mount points are special cased
 140           to allow them to be remote.
 141           </para>
 142
 143           <para>This target usually does not pull in any non-target units
 144           directly, but rather does so indirectly via other early boot targets.
 145           It is instead meant as a synchronization point for late boot
 146           services. Refer to
 147           <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 148           for details on the targets involved.
 149           </para>
 150
 151         </listitem>
 152       </varlistentry>
 153       <varlistentry>
 154         <term><filename>ctrl-alt-del.target</filename></term>
 155         <listitem>
 156           <para>systemd starts this target whenever Control+Alt+Del is
 157           pressed on the console. Usually, this should be aliased
 158           (symlinked) to <filename>reboot.target</filename>.</para>
 159         </listitem>
 160       </varlistentry>
 161       <varlistentry>
 162         <term><filename>cryptsetup.target</filename></term>
 163         <listitem>
 164           <para>A target that pulls in setup services for all
 165           encrypted block devices.</para>
 166         </listitem>
 167       </varlistentry>
 168       <varlistentry>
 169         <term><filename>dbus.service</filename></term>
 170         <listitem>
 171           <para>A special unit for the D-Bus bus daemon. As soon as
 172           this service is fully started up systemd will connect to it
 173           and register its service.</para>
 174         </listitem>
 175       </varlistentry>
 176       <varlistentry>
 177         <term><filename>dbus.socket</filename></term>
 178         <listitem>
 179           <para>A special unit for the D-Bus system bus socket. All
 180           units with <varname>Type=dbus</varname> automatically gain a
 181           dependency on this unit.</para>
 182         </listitem>
 183       </varlistentry>
 184       <varlistentry>
 185         <term><filename>default.target</filename></term>
 186         <listitem>
 187           <para>The default unit systemd starts at bootup. Usually,
 188           this should be aliased (symlinked) to
 189           <filename>multi-user.target</filename> or
 190           <filename>graphical.target</filename>.</para>
 191
 192           <para>The default unit systemd starts at bootup can be
 193           overridden with the <varname>systemd.unit=</varname> kernel
 194           command line option.</para>
 195         </listitem>
 196       </varlistentry>
 197       <varlistentry>
 198         <term><filename>display-manager.service</filename></term>
 199         <listitem>
 200           <para>The display manager service. Usually, this should be
 201           aliased (symlinked) to <filename>gdm.service</filename> or a
 202           similar display manager service.</para>
 203         </listitem>
 204       </varlistentry>
 205       <varlistentry>
 206         <term><filename>emergency.target</filename></term>
 207         <listitem>
 208           <para>A special target unit that starts an emergency shell on the main console. This target does not pull in
 209           any services or mounts. It is the most minimal version of starting the system in order to acquire an
 210           interactive shell; the only processes running are usually just the system manager (PID 1) and the shell
 211           process. This unit is supposed to be used with the kernel command line option
 212           <varname>systemd.unit=</varname>; it is also used when a file system check on a required file system fails,
 213           and boot-up cannot continue. Compare with <filename>rescue.target</filename>, which serves a similar purpose,
 214           but also starts the most basic services and mounts all file systems.</para>
 215
 216           <para>Use the <literal>systemd.unit=emergency.target</literal> kernel command line option to boot into this
 217           mode. A short alias for this kernel command line option is <literal>emergency</literal>, for compatibility
 218           with SysV.</para>
 219
 220           <para>In many ways booting into <filename>emergency.target</filename> is similar to the effect of booting
 221           with <literal>init=/bin/sh</literal> on the kernel command line, except that emergency mode provides you with
 222           the full system and service manager, and allows starting individual units in order to continue the boot
 223           process in steps.</para>
 224         </listitem>
 225       </varlistentry>
 226       <varlistentry>
 227         <term><filename>exit.target</filename></term>
 228         <listitem>
 229           <para>A special service unit for shutting down the system or
 230           user service manager. It is equivalent to
 231           <filename>poweroff.target</filename> on non-container
 232           systems, and also works in containers.</para>
 233
 234           <para>systemd will start this unit when it receives a
 235           request to shut down over D-Bus or a
 236           <constant>SIGTERM</constant> or <constant>SIGINT</constant>
 237           signal when running as user service daemon.</para>
 238
 239           <para>Normally, this (indirectly) pulls in
 240           <filename>shutdown.target</filename>, which in turn should be
 241           conflicted by all units that want to be scheduled for
 242           shutdown when the service manager starts to exit.</para>
 243         </listitem>
 244       </varlistentry>
 245       <varlistentry>
 246         <term><filename>final.target</filename></term>
 247         <listitem>
 248           <para>A special target unit that is used during the shutdown
 249           logic and may be used to pull in late services after all
 250           normal services are already terminated and all mounts
 251           unmounted.
 252           </para>
 253         </listitem>
 254       </varlistentry>
 255       <varlistentry>
 256         <term><filename>getty.target</filename></term>
 257         <listitem>
 258           <para>A special target unit that pulls in statically
 259           configured local TTY <filename>getty</filename> instances.
 260           </para>
 261         </listitem>
 262       </varlistentry>
 263       <varlistentry>
 264         <term><filename>graphical.target</filename></term>
 265         <listitem>
 266           <para>A special target unit for setting up a graphical login
 267           screen. This pulls in
 268           <filename>multi-user.target</filename>.</para>
 269
 270           <para>Units that are needed for graphical logins shall add
 271           <varname>Wants=</varname> dependencies for their unit to
 272           this unit (or <filename>multi-user.target</filename>) during
 273           installation. This is best configured via
 274           <varname>WantedBy=graphical.target</varname> in the unit's
 275           <literal>[Install]</literal> section.</para>
 276         </listitem>
 277       </varlistentry>
 278       <varlistentry>
 279         <term><filename>hibernate.target</filename></term>
 280         <listitem>
 281           <para>A special target unit for hibernating the system. This
 282           pulls in <filename>sleep.target</filename>.</para>
 283         </listitem>
 284       </varlistentry>
 285       <varlistentry>
 286         <term><filename>hybrid-sleep.target</filename></term>
 287         <listitem>
 288           <para>A special target unit for hibernating and suspending
 289           the system at the same time. This pulls in
 290           <filename>sleep.target</filename>.</para>
 291         </listitem>
 292       </varlistentry>
 293       <varlistentry>
 294         <term><filename>halt.target</filename></term>
 295         <listitem>
 296           <para>A special target unit for shutting down and halting
 297           the system. Note that this target is distinct from
 298           <filename>poweroff.target</filename> in that it generally
 299           really just halts the system rather than powering it
 300           down.</para>
 301
 302           <para>Applications wanting to halt the system should start
 303           this unit.</para>
 304         </listitem>
 305       </varlistentry>
 306       <varlistentry>
 307         <term><filename>initrd-fs.target</filename></term>
 308         <listitem>
 309           <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 310           automatically adds dependencies of type
 311           <varname>Before=</varname> to
 312           <filename>sysroot-usr.mount</filename> and all mount points
 313           found in <filename>/etc/fstab</filename> that have
 314           <option>x-initrd.mount</option> and not have
 315           <option>noauto</option> mount options set.</para>
 316         </listitem>
 317       </varlistentry>
 318       <varlistentry>
 319         <term><filename>kbrequest.target</filename></term>
 320         <listitem>
 321           <para>systemd starts this target whenever Alt+ArrowUp is
 322           pressed on the console. This is a good candidate to be
 323           aliased (symlinked) to
 324           <filename>rescue.target</filename>.</para>
 325         </listitem>
 326       </varlistentry>
 327       <varlistentry>
 328         <term><filename>kexec.target</filename></term>
 329         <listitem>
 330           <para>A special target unit for shutting down and rebooting
 331           the system via kexec.</para>
 332
 333           <para>Applications wanting to reboot the system with kexec
 334           should start this unit.</para>
 335         </listitem>
 336       </varlistentry>
 337       <varlistentry>
 338         <term><filename>local-fs.target</filename></term>
 339         <listitem>
 340           <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 341           automatically adds dependencies of type
 342           <varname>Before=</varname> to all mount units that refer to
 343           local mount points for this target unit. In addition, it
 344           adds dependencies of type <varname>Wants=</varname> to this
 345           target unit for those mounts listed in
 346           <filename>/etc/fstab</filename> that have the
 347           <option>auto</option> mount option set.</para>
 348         </listitem>
 349       </varlistentry>
 350       <varlistentry>
 351         <term><filename>multi-user.target</filename></term>
 352         <listitem>
 353           <para>A special target unit for setting up a multi-user
 354           system (non-graphical). This is pulled in by
 355           <filename>graphical.target</filename>.</para>
 356
 357           <para>Units that are needed for a multi-user system shall
 358           add <varname>Wants=</varname> dependencies for their unit to
 359           this unit during installation. This is best configured via
 360           <varname>WantedBy=multi-user.target</varname> in the unit's
 361           <literal>[Install]</literal> section.</para>
 362         </listitem>
 363       </varlistentry>
 364       <varlistentry>
 365         <term><filename>network-online.target</filename></term>
 366         <listitem>
 367           <para>Units that strictly require a configured network
 368           connection should pull in
 369           <filename>network-online.target</filename> (via a
 370           <varname>Wants=</varname> type dependency) and order
 371           themselves after it. This target unit is intended to pull in
 372           a service that delays further execution until the network is
 373           sufficiently set up. What precisely this requires is left to
 374           the implementation of the network managing service.</para>
 375
 376           <para>Note the distinction between this unit and
 377           <filename>network.target</filename>. This unit is an active
 378           unit (i.e. pulled in by the consumer rather than the
 379           provider of this functionality) and pulls in a service which
 380           possibly adds substantial delays to further execution. In
 381           contrast, <filename>network.target</filename> is a passive
 382           unit (i.e. pulled in by the provider of the functionality,
 383           rather than the consumer) that usually does not delay
 384           execution much. Usually, <filename>network.target</filename>
 385           is part of the boot of most systems, while
 386           <filename>network-online.target</filename> is not, except
 387           when at least one unit requires it. Also see <ulink
 388           url="http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running
 389           Services After the Network is up</ulink> for more
 390           information.</para>
 391
 392           <para>All mount units for remote network file systems
 393           automatically pull in this unit, and order themselves after
 394           it. Note that networking daemons that simply provide
 395           functionality to other hosts generally do not need to pull
 396           this in.</para>
 397           </listitem>
 398       </varlistentry>
 399       <varlistentry>
 400         <term><filename>paths.target</filename></term>
 401         <listitem>
 402           <para>A special target unit that sets up all path units (see
 403           <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 404           for details) that shall be active after boot.</para>
 405
 406           <para>It is recommended that path units installed by
 407           applications get pulled in via <varname>Wants=</varname>
 408           dependencies from this unit. This is best configured via a
 409           <varname>WantedBy=paths.target</varname> in the path unit's
 410           <literal>[Install]</literal> section.</para>
 411         </listitem>
 412       </varlistentry>
 413       <varlistentry>
 414         <term><filename>poweroff.target</filename></term>
 415         <listitem>
 416           <para>A special target unit for shutting down and powering
 417           off the system.</para>
 418
 419           <para>Applications wanting to power off the system should
 420           start this unit.</para>
 421
 422           <para><filename>runlevel0.target</filename> is an alias for
 423           this target unit, for compatibility with SysV.</para>
 424         </listitem>
 425       </varlistentry>
 426       <varlistentry>
 427         <term><filename>reboot.target</filename></term>
 428         <listitem>
 429           <para>A special target unit for shutting down and rebooting
 430           the system.</para>
 431
 432           <para>Applications wanting to reboot the system should start
 433           this unit.</para>
 434
 435           <para><filename>runlevel6.target</filename> is an alias for
 436           this target unit, for compatibility with SysV.</para>
 437         </listitem>
 438       </varlistentry>
 439       <varlistentry>
 440         <term><filename>remote-fs.target</filename></term>
 441         <listitem>
 442           <para>Similar to <filename>local-fs.target</filename>, but
 443           for remote mount points.</para>
 444
 445           <para>systemd automatically adds dependencies of type
 446           <varname>After=</varname> for this target unit to all SysV
 447           init script service units with an LSB header referring to
 448           the <literal>$remote_fs</literal> facility.</para>
 449         </listitem>
 450       </varlistentry>
 451       <varlistentry>
 452         <term><filename>rescue.target</filename></term>
 453         <listitem>
 454           <para>A special target unit that pulls in the base system (including system mounts) and spawns a rescue
 455           shell. Isolate to this target in order to administer the system in single-user mode with all file systems
 456           mounted but with no services running, except for the most basic. Compare with
 457           <filename>emergency.target</filename>, which is much more reduced and does not provide the file systems or
 458           most basic services.</para>
 459
 460           <para><filename>runlevel1.target</filename> is an alias for this target unit, for compatibility with
 461           SysV.</para>
 462
 463           <para>Use the <literal>systemd.unit=rescue.target</literal> kernel command line option to boot into this
 464           mode. A short alias for this kernel command line option is <literal>1</literal>, for compatibility with
 465           SysV.</para>
 466         </listitem>
 467       </varlistentry>
 468       <varlistentry>
 469         <term><filename>initrd-root-device.target</filename></term>
 470         <listitem>
 471           <para>A special initrd target unit that is reached when the root filesystem device is available, but before
 472           it has been mounted.
 473           <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 474           and
 475           <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 476           automatically setup the appropriate dependencies to make this happen.
 477           </para>
 478         </listitem>
 479       </varlistentry>
 480       <varlistentry>
 481         <term><filename>initrd-root-fs.target</filename></term>
 482         <listitem>
 483           <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 484           automatically adds dependencies of type
 485           <varname>Before=</varname> to the
 486           <filename>sysroot.mount</filename> unit, which is generated
 487           from the kernel command line.
 488           </para>
 489         </listitem>
 490       </varlistentry>
 491       <varlistentry>
 492         <term><filename>runlevel2.target</filename></term>
 493         <term><filename>runlevel3.target</filename></term>
 494         <term><filename>runlevel4.target</filename></term>
 495         <term><filename>runlevel5.target</filename></term>
 496         <listitem>
 497           <para>These are targets that are called whenever the SysV
 498           compatibility code asks for runlevel 2, 3, 4, 5,
 499           respectively. It is a good idea to make this an alias for
 500           (i.e. symlink to) <filename>graphical.target</filename>
 501           (for runlevel 5) or <filename>multi-user.target</filename>
 502           (the others).</para>
 503         </listitem>
 504       </varlistentry>
 505       <varlistentry>
 506         <term><filename>shutdown.target</filename></term>
 507         <listitem>
 508           <para>A special target unit that terminates the services on
 509           system shutdown.</para>
 510
 511           <para>Services that shall be terminated on system shutdown
 512           shall add <varname>Conflicts=</varname> and
 513           <varname>Before=</varname> dependencies to this unit for
 514           their service unit, which is implicitly done when
 515           <varname>DefaultDependencies=yes</varname> is set (the
 516           default).</para>
 517         </listitem>
 518       </varlistentry>
 519       <varlistentry>
 520         <term><filename>sigpwr.target</filename></term>
 521         <listitem>
 522           <para>A special target that is started when systemd receives
 523           the SIGPWR process signal, which is normally sent by the
 524           kernel or UPS daemons when power fails.</para>
 525         </listitem>
 526       </varlistentry>
 527       <varlistentry>
 528         <term><filename>sleep.target</filename></term>
 529         <listitem>
 530           <para>A special target unit that is pulled in by
 531           <filename>suspend.target</filename>,
 532           <filename>hibernate.target</filename> and
 533           <filename>hybrid-sleep.target</filename> and may be used to
 534           hook units into the sleep state logic.</para>
 535         </listitem>
 536       </varlistentry>
 537       <varlistentry>
 538         <term><filename>slices.target</filename></term>
 539         <listitem>
 540           <para>A special target unit that sets up all slice units (see
 541           <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 542           details) that shall be active after boot. By default the generic <filename>user.slice</filename>,
 543           <filename>system.slice</filename>, <filename>machines.slice</filename> slice units, as well as the root
 544           slice unit <filename>-.slice</filename> are pulled in and ordered before this unit (see below).</para>
 545
 546           <para>It's a good idea to add <varname>WantedBy=slices.target</varname> lines to the <literal>[Install]</literal>
 547           section of all slices units that may be installed dynamically.</para>
 548         </listitem>
 549       </varlistentry>
 550       <varlistentry>
 551         <term><filename>sockets.target</filename></term>
 552         <listitem>
 553           <para>A special target unit that sets up all socket
 554           units (see
 555           <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 556           for details) that shall be active after boot.</para>
 557
 558           <para>Services that can be socket-activated shall add
 559           <varname>Wants=</varname> dependencies to this unit for
 560           their socket unit during installation. This is best
 561           configured via a <varname>WantedBy=sockets.target</varname>
 562           in the socket unit's <literal>[Install]</literal>
 563           section.</para>
 564         </listitem>
 565       </varlistentry>
 566       <varlistentry>
 567         <term><filename>suspend.target</filename></term>
 568         <listitem>
 569           <para>A special target unit for suspending the system. This
 570           pulls in <filename>sleep.target</filename>.</para>
 571         </listitem>
 572       </varlistentry>
 573       <varlistentry>
 574         <term><filename>swap.target</filename></term>
 575         <listitem>
 576           <para>Similar to <filename>local-fs.target</filename>, but
 577           for swap partitions and swap files.</para>
 578         </listitem>
 579       </varlistentry>
 580       <varlistentry>
 581         <term><filename>sysinit.target</filename></term>
 582         <listitem>
 583           <para>systemd automatically adds dependencies of the types
 584           <varname>Requires=</varname> and <varname>After=</varname>
 585           for this target unit to all services (except for those with
 586           <varname>DefaultDependencies=no</varname>).</para>
 587
 588           <para>This target pulls in the services required for system
 589           initialization. System services pulled in by this target should
 590           declare <varname>DefaultDependencies=no</varname> and specify
 591           all their dependencies manually, including access to anything
 592           more than a read only root filesystem. For details on the
 593           dependencies of this target, refer to
 594           <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 595           </para>
 596         </listitem>
 597       </varlistentry>
 598       <varlistentry>
 599         <term><filename>syslog.socket</filename></term>
 600         <listitem>
 601           <para>The socket unit syslog implementations should listen
 602           on. All userspace log messages will be made available on
 603           this socket. For more information about syslog integration,
 604           please consult the <ulink
 605           url="http://www.freedesktop.org/wiki/Software/systemd/syslog">Syslog
 606           Interface</ulink> document.</para>
 607         </listitem>
 608       </varlistentry>
 609       <varlistentry>
 610         <term><filename>system-update.target</filename></term>
 611         <listitem>
 612           <para>A special target unit that is used for off-line system
 613           updates.
 614           <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 615           will redirect the boot process to this target if
 616           <filename>/system-update</filename> exists. For more
 617           information see the <ulink
 618           url="http://freedesktop.org/wiki/Software/systemd/SystemUpdates">System
 619           Updates Specification</ulink>.</para>
 620         </listitem>
 621       </varlistentry>
 622       <varlistentry>
 623         <term><filename>timers.target</filename></term>
 624         <listitem>
 625           <para>A special target unit that sets up all timer units
 626           (see
 627           <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 628           for details) that shall be active after boot.</para>
 629
 630           <para>It is recommended that timer units installed by
 631           applications get pulled in via <varname>Wants=</varname>
 632           dependencies from this unit. This is best configured via
 633           <varname>WantedBy=timers.target</varname> in the timer
 634           unit's <literal>[Install]</literal> section.</para>
 635         </listitem>
 636       </varlistentry>
 637       <varlistentry>
 638         <term><filename>umount.target</filename></term>
 639         <listitem>
 640           <para>A special target unit that unmounts all mount and
 641           automount points on system shutdown.</para>
 642
 643           <para>Mounts that shall be unmounted on system shutdown
 644           shall add Conflicts dependencies to this unit for their
 645           mount unit, which is implicitly done when
 646           <varname>DefaultDependencies=yes</varname> is set (the
 647           default).</para>
 648         </listitem>
 649       </varlistentry>
 650
 651     </variablelist>
 652   </refsect1>
 653
 654   <refsect1>
 655     <title>Special System Units for Devices</title>
 656
 657     <para>Some target units are automatically pulled in as devices of
 658     certain kinds show up in the system. These may be used to
 659     automatically activate various services based on the specific type
 660     of the available hardware.</para>
 661
 662     <variablelist>
 663       <varlistentry>
 664         <term><filename>bluetooth.target</filename></term>
 665         <listitem>
 666           <para>This target is started automatically as soon as a
 667           Bluetooth controller is plugged in or becomes available at
 668           boot.</para>
 669
 670           <para>This may be used to pull in Bluetooth management
 671           daemons dynamically when Bluetooth hardware is found.</para>
 672         </listitem>
 673       </varlistentry>
 674       <varlistentry>
 675         <term><filename>printer.target</filename></term>
 676         <listitem>
 677           <para>This target is started automatically as soon as a
 678           printer is plugged in or becomes available at boot.</para>
 679
 680           <para>This may be used to pull in printer management daemons
 681           dynamically when printer hardware is found.</para>
 682         </listitem>
 683       </varlistentry>
 684       <varlistentry>
 685         <term><filename>smartcard.target</filename></term>
 686         <listitem>
 687           <para>This target is started automatically as soon as a
 688           smartcard controller is plugged in or becomes available at
 689           boot.</para>
 690
 691           <para>This may be used to pull in smartcard management
 692           daemons dynamically when smartcard hardware is found.</para>
 693         </listitem>
 694       </varlistentry>
 695       <varlistentry>
 696         <term><filename>sound.target</filename></term>
 697         <listitem>
 698           <para>This target is started automatically as soon as a
 699           sound card is plugged in or becomes available at
 700           boot.</para>
 701
 702           <para>This may be used to pull in audio management daemons
 703           dynamically when audio hardware is found.</para>
 704         </listitem>
 705       </varlistentry>
 706     </variablelist>
 707   </refsect1>
 708
 709   <refsect1>
 710     <title>Special Passive System Units </title>
 711
 712     <para>A number of special system targets are defined that can be
 713     used to properly order boot-up of optional services. These targets
 714     are generally not part of the initial boot transaction, unless
 715     they are explicitly pulled in by one of the implementing services.
 716     Note specifically that these <emphasis>passive</emphasis> target
 717     units are generally not pulled in by the consumer of a service,
 718     but by the provider of the service. This means: a consuming
 719     service should order itself after these targets (as appropriate),
 720     but not pull it in. A providing service should order itself before
 721     these targets (as appropriate) and pull it in (via a
 722     <varname>Wants=</varname> type dependency).</para>
 723
 724     <para>Note that these passive units cannot be started manually,
 725     i.e. <literal>systemctl start time-sync.target</literal> will fail
 726     with an error. They can only be pulled in by dependency. This is
 727     enforced since they exist for ordering purposes only and thus are
 728     not useful as only unit within a transaction.</para>
 729
 730     <variablelist>
 731       <varlistentry>
 732         <term><filename>cryptsetup-pre.target</filename></term>
 733         <listitem>
 734           <para>This passive target unit may be pulled in by services
 735           that want to run before any encrypted block device is set
 736           up. All encrypted block devices are set up after this target
 737           has been reached. Since the shutdown order is implicitly the
 738           reverse start-up order between units, this target is
 739           particularly useful to ensure that a service is shut down
 740           only after all encrypted block devices are fully
 741           stopped.</para>
 742         </listitem>
 743       </varlistentry>
 744       <varlistentry>
 745         <term><filename>local-fs-pre.target</filename></term>
 746         <listitem>
 747           <para>This target unit is
 748           automatically ordered before
 749           all local mount points marked
 750           with <option>auto</option>
 751           (see above). It can be used to
 752           execute certain units before
 753           all local mounts.</para>
 754         </listitem>
 755       </varlistentry>
 756       <varlistentry>
 757         <term><filename>network.target</filename></term>
 758         <listitem>
 759           <para>This unit is supposed to indicate when network
 760           functionality is available, but it is only very weakly
 761           defined what that is supposed to mean, with one exception:
 762           at shutdown, a unit that is ordered after
 763           <filename>network.target</filename> will be stopped before
 764           the network — to whatever level it might be set up then —
 765           is shut down. It is hence useful when writing service files
 766           that require network access on shutdown, which should order
 767           themselves after this target, but not pull it in. Also see
 768           <ulink url="http://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running
 769           Services After the Network is up</ulink> for more
 770           information. Also see
 771           <filename>network-online.target</filename> described
 772           above.</para>
 773
 774           <para>systemd automatically adds dependencies of type
 775           <varname>After=</varname> for this target unit to all SysV
 776           init script service units with an LSB header referring to
 777           the <literal>$network</literal> facility.</para>
 778         </listitem>
 779       </varlistentry>
 780       <varlistentry>
 781         <term><filename>network-pre.target</filename></term>
 782         <listitem>
 783           <para>This passive target unit may be pulled in by services
 784           that want to run before any network is set up, for example
 785           for the purpose of setting up a firewall. All network
 786           management software orders itself after this target, but
 787           does not pull it in.</para>
 788         </listitem>
 789       </varlistentry>
 790       <varlistentry>
 791         <term><filename>nss-lookup.target</filename></term>
 792         <listitem>
 793           <para>A target that should be used as synchronization point
 794           for all host/network name service lookups. Note that this is
 795           independent of user/group name lookups for which
 796           <filename>nss-user-lookup.target</filename> should be used.
 797           All services for which the availability of full host/network
 798           name resolution is essential should be ordered after this
 799           target, but not pull it in. systemd automatically adds
 800           dependencies of type <varname>After=</varname> for this
 801           target unit to all SysV init script service units with an
 802           LSB header referring to the <literal>$named</literal>
 803           facility.</para>
 804         </listitem>
 805       </varlistentry>
 806       <varlistentry>
 807         <term><filename>nss-user-lookup.target</filename></term>
 808         <listitem>
 809           <para>A target that should be used as synchronization point
 810           for all user/group name service lookups. Note that this is
 811           independent of host/network name lookups for which
 812           <filename>nss-lookup.target</filename> should be used. All
 813           services for which the availability of the full user/group
 814           database is essential should be ordered after this target,
 815           but not pull it in. Note that system users are always
 816           resolvable, and hence do not require any special ordering
 817           against this target.</para>
 818         </listitem>
 819       </varlistentry>
 820       <varlistentry>
 821         <term><filename>remote-fs-pre.target</filename></term>
 822         <listitem>
 823           <para>This target unit is automatically ordered before all
 824           remote mount point units (see above). It can be used to run
 825           certain units before the remote mounts are established. Note
 826           that this unit is generally not part of the initial
 827           transaction, unless the unit that wants to be ordered before
 828           all remote mounts pulls it in via a
 829           <varname>Wants=</varname> type dependency. If the unit wants
 830           to be pulled in by the first remote mount showing up, it
 831           should use <filename>network-online.target</filename> (see
 832           above).</para>
 833         </listitem>
 834       </varlistentry>
 835       <varlistentry>
 836         <term><filename>rpcbind.target</filename></term>
 837         <listitem>
 838           <para>The portmapper/rpcbind pulls in this target and orders
 839           itself before it, to indicate its availability. systemd
 840           automatically adds dependencies of type
 841           <varname>After=</varname> for this target unit to all SysV
 842           init script service units with an LSB header referring to
 843           the <literal>$portmap</literal> facility.</para>
 844         </listitem>
 845       </varlistentry>
 846       <varlistentry>
 847         <term><filename>time-sync.target</filename></term>
 848         <listitem>
 849           <para>Services responsible for synchronizing the system
 850           clock from a remote source (such as NTP client
 851           implementations) should pull in this target and order
 852           themselves before it. All services where correct time is
 853           essential should be ordered after this unit, but not pull it
 854           in. systemd automatically adds dependencies of type
 855           <varname>After=</varname> for this target unit to all SysV
 856           init script service units with an LSB header referring to
 857           the <literal>$time</literal> facility. </para>
 858         </listitem>
 859       </varlistentry>
 860     </variablelist>
 861   </refsect1>
 862
 863   <refsect1>
 864     <title>Special User Units</title>
 865
 866     <para>When systemd runs as a user instance, the following special
 867     units are available, which have similar definitions as their
 868     system counterparts:
 869     <filename>exit.target</filename>,
 870     <filename>default.target</filename>,
 871     <filename>shutdown.target</filename>,
 872     <filename>sockets.target</filename>,
 873     <filename>timers.target</filename>,
 874     <filename>paths.target</filename>,
 875     <filename>bluetooth.target</filename>,
 876     <filename>printer.target</filename>,
 877     <filename>smartcard.target</filename>,
 878     <filename>sound.target</filename>.</para>
 879   </refsect1>
 880
 881   <refsect1>
 882     <title>Special Slice Units</title>
 883
 884     <para>There are four <literal>.slice</literal> units which form
 885     the basis of the hierarchy for assignment of resources for
 886     services, users, and virtual machines or containers.</para>
 887
 888     <variablelist>
 889       <varlistentry>
 890         <term><filename>-.slice</filename></term>
 891         <listitem>
 892           <para>The root slice is the root of the hierarchy. It
 893           usually does not contain units directly, but may be used to
 894           set defaults for the whole tree.</para>
 895         </listitem>
 896       </varlistentry>
 897
 898       <varlistentry>
 899         <term><filename>system.slice</filename></term>
 900         <listitem>
 901           <para>By default, all system services started by
 902           <command>systemd</command> are found in this slice.</para>
 903         </listitem>
 904       </varlistentry>
 905
 906       <varlistentry>
 907         <term><filename>user.slice</filename></term>
 908         <listitem>
 909           <para>By default, all user processes and services started on
 910           behalf of the user, including the per-user systemd instance
 911           are found in this slice.</para>
 912         </listitem>
 913       </varlistentry>
 914
 915       <varlistentry>
 916         <term><filename>machine.slice</filename></term>
 917         <listitem>
 918           <para>By default, all virtual machines and containers
 919           registered with <command>systemd-machined</command> are
 920           found in this slice.
 921           </para>
 922         </listitem>
 923       </varlistentry>
 924     </variablelist>
 925   </refsect1>
 926
 927   <refsect1>
 928       <title>See Also</title>
 929       <para>
 930         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 931         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 932         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 933         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 934         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 935         <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 936         <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 937         <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 938       </para>
 939   </refsect1>
 940
 941 </refentry>