man/systemd.special.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd.special">
   7
   8   <refentryinfo>
   9     <title>systemd.special</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd.special</refentrytitle>
  15     <manvolnum>7</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd.special</refname>
  20     <refpurpose>Special systemd units</refpurpose>
  21   </refnamediv>
  22
  23   <refsynopsisdiv><para>
  24     <!-- sort alphabetically, targets first --><filename>basic.target</filename>,
  25     <filename>bluetooth.target</filename>,
  26     <filename>cryptsetup-pre.target</filename>,
  27     <filename>cryptsetup.target</filename>,
  28     <filename>veritysetup-pre.target</filename>,
  29     <filename>veritysetup.target</filename>,
  30     <filename>ctrl-alt-del.target</filename>,
  31     <filename>blockdev@.target</filename>,
  32     <filename>boot-complete.target</filename>,
  33     <filename>default.target</filename>,
  34     <filename>emergency.target</filename>,
  35     <filename>exit.target</filename>,
  36     <filename>factory-reset.target</filename>,
  37     <filename>final.target</filename>,
  38     <filename>first-boot-complete.target</filename>,
  39     <filename>getty.target</filename>,
  40     <filename>getty-pre.target</filename>,
  41     <filename>graphical.target</filename>,
  42     <filename>halt.target</filename>,
  43     <filename>hibernate.target</filename>,
  44     <filename>hybrid-sleep.target</filename>,
  45     <filename>suspend-then-hibernate.target</filename>,
  46     <filename>initrd.target</filename>,
  47     <filename>initrd-fs.target</filename>,
  48     <filename>initrd-root-device.target</filename>,
  49     <filename>initrd-root-fs.target</filename>,
  50     <filename>initrd-usr-fs.target</filename>,
  51     <filename>integritysetup-pre.target</filename>,
  52     <filename>integritysetup.target</filename>,
  53     <filename>kbrequest.target</filename>,
  54     <filename>kexec.target</filename>,
  55     <filename>local-fs-pre.target</filename>,
  56     <filename>local-fs.target</filename>,
  57     <filename>machines.target</filename>
  58     <filename>multi-user.target</filename>,
  59     <filename>network-online.target</filename>,
  60     <filename>network-pre.target</filename>,
  61     <filename>network.target</filename>,
  62     <filename>nss-lookup.target</filename>,
  63     <filename>nss-user-lookup.target</filename>,
  64     <filename>paths.target</filename>,
  65     <filename>poweroff.target</filename>,
  66     <filename>printer.target</filename>,
  67     <filename>reboot.target</filename>,
  68     <filename>remote-cryptsetup.target</filename>,
  69     <filename>remote-veritysetup.target</filename>,
  70     <filename>remote-fs-pre.target</filename>,
  71     <filename>remote-fs.target</filename>,
  72     <filename>rescue.target</filename>,
  73     <filename>rpcbind.target</filename>,
  74     <filename>runlevel2.target</filename>,
  75     <filename>runlevel3.target</filename>,
  76     <filename>runlevel4.target</filename>,
  77     <filename>runlevel5.target</filename>,
  78     <filename>shutdown.target</filename>,
  79     <filename>sigpwr.target</filename>,
  80     <filename>sleep.target</filename>,
  81     <filename>slices.target</filename>,
  82     <filename>smartcard.target</filename>,
  83     <filename>sockets.target</filename>,
  84     <filename>soft-reboot.target</filename>,
  85     <filename>sound.target</filename>,
  86     <filename>suspend.target</filename>,
  87     <filename>swap.target</filename>,
  88     <filename>sysinit.target</filename>,
  89     <filename>system-update.target</filename>,
  90     <filename>system-update-pre.target</filename>,
  91     <filename>time-set.target</filename>,
  92     <filename>time-sync.target</filename>,
  93     <filename>timers.target</filename>,
  94     <filename>umount.target</filename>,
  95     <filename>usb-gadget.target</filename>,
  96     <!-- slices --><filename>-.slice</filename>,
  97     <filename>system.slice</filename>,
  98     <filename>user.slice</filename>,
  99     <filename>machine.slice</filename>,
 100     <!-- the rest --><filename>-.mount</filename>,
 101     <filename>dbus.service</filename>,
 102     <filename>dbus.socket</filename>,
 103     <filename>display-manager.service</filename>,
 104     <filename>init.scope</filename>,
 105     <filename>syslog.socket</filename>,
 106     <filename>system-update-cleanup.service</filename>
 107   </para></refsynopsisdiv>
 108
 109   <refsect1>
 110     <title>Description</title>
 111
 112     <para>A few units are treated specially by systemd. Many of them have
 113     special internal semantics and cannot be renamed, while others simply
 114     have a standard meaning and should be present on all systems.</para>
 115   </refsect1>
 116
 117   <refsect1>
 118     <title>Units managed by the system service manager</title>
 119
 120     <refsect2>
 121       <title>Special System Units</title>
 122
 123       <variablelist>
 124         <varlistentry>
 125           <term><filename>-.mount</filename></term>
 126           <listitem>
 127             <para>The root mount point, i.e. the mount unit for the <filename>/</filename>
 128             path. This unit is unconditionally active, during the entire time the system is up, as
 129             this mount point is where the basic userspace is running from.</para>
 130           </listitem>
 131         </varlistentry>
 132
 133         <varlistentry>
 134           <term><filename>basic.target</filename></term>
 135           <listitem>
 136             <para>A special target unit covering basic boot-up.</para>
 137
 138             <para>systemd automatically adds dependency of the type
 139             <varname>After=</varname> for this target unit to all
 140             services (except for those with
 141             <varname>DefaultDependencies=no</varname>).</para>
 142
 143             <para>Usually, this should pull-in all local mount points plus
 144             <filename>/var/</filename>, <filename>/tmp/</filename> and
 145             <filename>/var/tmp/</filename>, swap devices, sockets, timers,
 146             path units and other basic initialization necessary for general
 147             purpose daemons. The mentioned mount points are special cased
 148             to allow them to be remote.
 149             </para>
 150
 151             <para>This target usually does not pull in any non-target units
 152             directly, but rather does so indirectly via other early boot targets.
 153             It is instead meant as a synchronization point for late boot
 154             services. Refer to
 155             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 156             for details on the targets involved.
 157             </para>
 158           </listitem>
 159         </varlistentry>
 160         <varlistentry>
 161           <term><filename>boot-complete.target</filename></term>
 162           <listitem>
 163             <para>This target is intended as generic synchronization point for services that shall determine or act on
 164             whether the boot process completed successfully. Order units that are required to succeed for a boot process
 165             to be considered successful before this unit, and add a <varname>Requires=</varname> dependency from the
 166             target unit to them. Order units that shall only run when the boot process is considered successful after the
 167             target unit and pull in the target from it, also with <varname>Requires=</varname>. Note that by default this
 168             target unit is not part of the initial boot transaction, but is supposed to be pulled in only if required by
 169             units that want to run only on successful boots.</para>
 170
 171             <para>See
 172             <citerefentry><refentrytitle>systemd-boot-check-no-failures.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 173             for a service that implements a generic system health check and orders itself before
 174             <filename>boot-complete.target</filename>.</para>
 175
 176             <para>See
 177             <citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 178             for a service that propagates boot success information to the boot loader, and orders itself after
 179             <filename>boot-complete.target</filename>.</para>
 180           </listitem>
 181         </varlistentry>
 182         <varlistentry>
 183           <term><filename>ctrl-alt-del.target</filename></term>
 184           <listitem>
 185             <para>systemd starts this target whenever Control+Alt+Del is
 186             pressed on the console. Usually, this should be aliased
 187             (symlinked) to <filename>reboot.target</filename>.</para>
 188           </listitem>
 189         </varlistentry>
 190         <varlistentry>
 191           <term><filename>cryptsetup.target</filename></term>
 192           <listitem>
 193             <para>A target that pulls in setup services for all
 194             encrypted block devices.</para>
 195           </listitem>
 196         </varlistentry>
 197         <varlistentry>
 198           <term><filename>veritysetup.target</filename></term>
 199           <listitem>
 200             <para>A target that pulls in setup services for all
 201             verity integrity protected block devices.</para>
 202           </listitem>
 203         </varlistentry>
 204         <varlistentry>
 205           <term><filename>dbus.service</filename></term>
 206           <listitem>
 207             <para>A special unit for the D-Bus bus daemon. As soon as
 208             this service is fully started up systemd will connect to it
 209             and register its service.</para>
 210           </listitem>
 211         </varlistentry>
 212         <varlistentry>
 213           <term><filename>dbus.socket</filename></term>
 214           <listitem>
 215             <para>A special unit for the D-Bus system bus socket. All
 216             units with <varname>Type=dbus</varname> automatically gain a
 217             dependency on this unit.</para>
 218           </listitem>
 219         </varlistentry>
 220         <varlistentry>
 221           <term><filename>default.target</filename></term>
 222           <listitem>
 223             <para>The default unit systemd starts at bootup. Usually, this should be aliased (symlinked) to
 224             <filename>multi-user.target</filename> or <filename>graphical.target</filename>. See
 225             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
 226             more discussion.</para>
 227
 228             <para>The default unit systemd starts at bootup can be overridden with the
 229             <varname>systemd.unit=</varname> kernel command line option, or more conveniently, with the short
 230             names like <varname>single</varname>, <varname>rescue</varname>, <varname>1</varname>,
 231             <varname>3</varname>, <varname>5</varname>, …; see
 232             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 233           </listitem>
 234         </varlistentry>
 235         <varlistentry>
 236           <term><filename>display-manager.service</filename></term>
 237           <listitem>
 238             <para>The display manager service. Usually, this should be
 239             aliased (symlinked) to <filename>gdm.service</filename> or a
 240             similar display manager service.</para>
 241           </listitem>
 242         </varlistentry>
 243         <varlistentry>
 244           <term><filename>emergency.target</filename></term>
 245           <listitem>
 246             <para>A special target unit that starts an emergency shell on the main console. This
 247             target does not pull in other services or mounts. It is the most minimal version of
 248             starting the system in order to acquire an interactive shell; the only processes running
 249             are usually just the system manager (PID 1) and the shell process. This unit may be used
 250             by specifying <varname>emergency</varname> on the kernel command line; it is
 251             also used when a file system check on a required file system fails and boot-up cannot
 252             continue. Compare with <filename>rescue.target</filename>, which serves a similar
 253             purpose, but also starts the most basic services and mounts all file systems.</para>
 254
 255             <para>In many ways booting into <filename>emergency.target</filename> is similar to the
 256             effect of booting with <literal>init=/bin/sh</literal> on the kernel command line,
 257             except that emergency mode provides you with the full system and service manager, and
 258             allows starting individual units in order to continue the boot process in steps.</para>
 259
 260             <para>Note that depending on how <filename>emergency.target</filename> is reached, the root file
 261             system might be mounted read-only or read-write (no remounting is done specially for this
 262             target). For example, the system may boot with root mounted read-only when <varname>ro</varname>
 263             is used on the kernel command line and remain this way for <filename>emergency.target</filename>,
 264             or the system may transition to <filename>emergency.target</filename> after the system has been
 265             partially booted and disks have already been remounted read-write.</para>
 266           </listitem>
 267         </varlistentry>
 268         <varlistentry>
 269           <term><filename>exit.target</filename></term>
 270           <listitem>
 271             <para>A special service unit for shutting down the system or
 272             user service manager. It is equivalent to
 273             <filename>poweroff.target</filename> on non-container
 274             systems, and also works in containers.</para>
 275
 276             <para>systemd will start this unit when it receives the
 277             <constant>SIGTERM</constant> or <constant>SIGINT</constant>
 278             signal when running as user service daemon.</para>
 279
 280             <para>Normally, this (indirectly) pulls in
 281             <filename>shutdown.target</filename>, which in turn should be
 282             conflicted by all units that want to be scheduled for
 283             shutdown when the service manager starts to exit.</para>
 284           </listitem>
 285         </varlistentry>
 286         <varlistentry>
 287           <term><filename>factory-reset.target</filename></term>
 288           <listitem>
 289             <para>A special target to trigger a factory reset.</para>
 290           </listitem>
 291         </varlistentry>
 292         <varlistentry>
 293           <term><filename>final.target</filename></term>
 294           <listitem>
 295             <para>A special target unit that is used during the shutdown
 296             logic and may be used to pull in late services after all
 297             normal services are already terminated and all mounts
 298             unmounted.
 299             </para>
 300           </listitem>
 301         </varlistentry>
 302         <varlistentry>
 303           <term><filename>getty.target</filename></term>
 304           <listitem>
 305             <para>A special target unit that pulls in statically
 306             configured local TTY <filename>getty</filename> instances.
 307             </para>
 308           </listitem>
 309         </varlistentry>
 310         <varlistentry>
 311           <term><filename>graphical.target</filename></term>
 312           <listitem>
 313             <para>A special target unit for setting up a graphical login
 314             screen. This pulls in
 315             <filename>multi-user.target</filename>.</para>
 316
 317             <para>Units that are needed for graphical logins shall add
 318             <varname>Wants=</varname> dependencies for their unit to
 319             this unit (or <filename>multi-user.target</filename>) during
 320             installation. This is best configured via
 321             <varname>WantedBy=graphical.target</varname> in the unit's
 322             [Install] section.</para>
 323           </listitem>
 324         </varlistentry>
 325         <varlistentry>
 326           <term><filename>hibernate.target</filename></term>
 327           <listitem>
 328             <para>A special target unit for hibernating the system. This
 329             pulls in <filename>sleep.target</filename>.</para>
 330           </listitem>
 331         </varlistentry>
 332         <varlistentry>
 333           <term><filename>hybrid-sleep.target</filename></term>
 334           <listitem>
 335             <para>A special target unit for hibernating and suspending
 336             the system at the same time. This pulls in
 337             <filename>sleep.target</filename>.</para>
 338           </listitem>
 339         </varlistentry>
 340         <varlistentry>
 341           <term><filename>suspend-then-hibernate.target</filename></term>
 342           <listitem>
 343             <para>A special target unit for suspending the system for a period
 344             of time, waking it and putting it into hibernate. This pulls in
 345             <filename>sleep.target</filename>.</para>
 346           </listitem>
 347         </varlistentry>
 348
 349         <varlistentry>
 350           <term><filename>halt.target</filename></term>
 351           <listitem>
 352             <para>A special target unit for shutting down and halting
 353             the system. Note that this target is distinct from
 354             <filename>poweroff.target</filename> in that it generally
 355             really just halts the system rather than powering it
 356             down.</para>
 357
 358             <para>Applications wanting to halt the system should not start this unit
 359             directly, but should instead execute <command>systemctl halt</command>
 360             (possibly with the <option>--no-block</option> option) or call
 361             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 362             <command>org.freedesktop.systemd1.Manager.Halt</command> D-Bus method
 363             directly.</para>
 364           </listitem>
 365         </varlistentry>
 366         <varlistentry>
 367           <term><filename>init.scope</filename></term>
 368           <listitem>
 369             <para>This scope unit is where the system and service manager (PID 1) itself resides. It
 370             is active as long as the system is running.</para>
 371           </listitem>
 372         </varlistentry>
 373         <varlistentry>
 374           <term><filename>initrd.target</filename></term>
 375           <listitem>
 376             <para>This is the default target in the initrd, similar to <filename>default.target</filename> in
 377             the main system. It is used to mount the real root and transition to it. See
 378             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
 379             more discussion.</para>
 380           </listitem>
 381         </varlistentry>
 382         <varlistentry>
 383           <term><filename>initrd-fs.target</filename></term>
 384           <listitem>
 385             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 386             automatically adds dependencies of type <varname>Before=</varname> to
 387             <filename>sysroot-usr.mount</filename> and all mount points found in
 388             <filename>/etc/fstab</filename> that have the <option>x-initrd.mount</option> mount option set
 389             and do not have the <option>noauto</option> mount option set. It is also indirectly ordered after
 390             <filename>sysroot.mount</filename>. Thus, once this target is reached the
 391             <filename>/sysroot/</filename> hierarchy is fully set up, in preparation for the transition to
 392             the host OS.</para>
 393           </listitem>
 394         </varlistentry>
 395         <varlistentry>
 396           <term><filename>initrd-root-device.target</filename></term>
 397           <listitem>
 398             <para>A special initrd target unit that is reached when the root filesystem device is available, but before
 399             it has been mounted.
 400             <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 401             and
 402             <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 403             automatically setup the appropriate dependencies to make this happen.
 404             </para>
 405           </listitem>
 406         </varlistentry>
 407         <varlistentry>
 408           <term><filename>initrd-root-fs.target</filename></term>
 409           <listitem>
 410             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 411             automatically adds dependencies of type <varname>Before=</varname> to the
 412             <filename>sysroot.mount</filename> unit, which is generated from the kernel command line's
 413             <varname>root=</varname> setting (or equivalent).</para>
 414           </listitem>
 415         </varlistentry>
 416         <varlistentry>
 417           <term><filename>initrd-usr-fs.target</filename></term>
 418           <listitem>
 419             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 420             automatically adds dependencies of type <varname>Before=</varname> to the
 421             <filename>sysusr-usr.mount</filename> unit, which is generated from the kernel command line's
 422             <varname>usr=</varname> switch. Services may order themselves after this target unit in order to
 423             run once the <filename>/sysusr/</filename> hierarchy becomes available, on systems that come up
 424             initially without a root file system, but with an initialized <filename>/usr/</filename> and need
 425             to access that before setting up the root file system to ultimately switch to. On systems where
 426             <varname>usr=</varname> is not used this target is ordered after
 427             <filename>sysroot.mount</filename> and thus mostly equivalent to
 428             <filename>initrd-root-fs.target</filename>. In effect on any system once this target is reached
 429             the file system backing <filename>/usr/</filename> is mounted, though possibly at two different
 430             locations, either below the <filename>/sysusr/</filename> or the <filename>/sysroot/</filename>
 431             hierarchies.</para>
 432           </listitem>
 433         </varlistentry>
 434         <varlistentry>
 435           <term><filename>kbrequest.target</filename></term>
 436           <listitem>
 437             <para>systemd starts this target whenever Alt+ArrowUp is
 438             pressed on the console. Note that any user with physical access
 439             to the machine will be able to do this, without authentication,
 440             so this should be used carefully.</para>
 441           </listitem>
 442         </varlistentry>
 443         <varlistentry>
 444           <term><filename>kexec.target</filename></term>
 445           <listitem>
 446             <para>A special target unit for shutting down and rebooting the system via kexec.</para>
 447
 448             <para>Applications wanting to reboot the system should not start this unit directly, but should
 449             instead execute <command>systemctl kexec</command> (possibly with the
 450             <option>--no-block</option> option) or call
 451             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 452             <function>org.freedesktop.login1.Manager.RebootWithFlags()</function> D-Bus method
 453             directly.</para>
 454
 455             <para>See
 456             <citerefentry><refentrytitle>systemd-kexec.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 457             for further details of the operation this target pulls in.</para>
 458           </listitem>
 459         </varlistentry>
 460         <varlistentry>
 461           <term><filename>local-fs.target</filename></term>
 462           <listitem>
 463             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 464             automatically adds dependencies of type
 465             <varname>Before=</varname> to all mount units that refer to
 466             local mount points for this target unit. In addition, it
 467             adds dependencies of type <varname>Wants=</varname> to this
 468             target unit for those mounts listed in
 469             <filename>/etc/fstab</filename> that have the
 470             <option>auto</option> mount option set.</para>
 471           </listitem>
 472         </varlistentry>
 473         <varlistentry>
 474           <term><filename>machines.target</filename></term>
 475           <listitem>
 476             <para>A standard target unit for starting all the containers
 477             and other virtual machines. See <filename>systemd-nspawn@.service</filename>
 478             for an example.</para>
 479           </listitem>
 480         </varlistentry>
 481         <varlistentry>
 482           <term><filename>multi-user.target</filename></term>
 483           <listitem>
 484             <para>A special target unit for setting up a multi-user
 485             system (non-graphical). This is pulled in by
 486             <filename>graphical.target</filename>.</para>
 487
 488             <para>Units that are needed for a multi-user system shall
 489             add <varname>Wants=</varname> dependencies for their unit to
 490             this unit during installation. This is best configured via
 491             <varname>WantedBy=multi-user.target</varname> in the unit's
 492             [Install] section.</para>
 493           </listitem>
 494         </varlistentry>
 495         <varlistentry>
 496           <term><filename>network-online.target</filename></term>
 497           <listitem>
 498             <para>Units that strictly require a configured network
 499             connection should pull in
 500             <filename>network-online.target</filename> (via a
 501             <varname>Wants=</varname> type dependency) and order
 502             themselves after it. This target unit is intended to pull in
 503             a service that delays further execution until the network is
 504             sufficiently set up. What precisely this requires is left to
 505             the implementation of the network managing service.</para>
 506
 507             <para>Note the distinction between this unit and <filename>network.target</filename>. This unit
 508             is an active unit (i.e. pulled in by the consumer rather than the provider of this functionality)
 509             and pulls in a service which possibly adds substantial delays to further execution. In contrast,
 510             <filename>network.target</filename> is a passive unit (i.e. pulled in by the provider of the
 511             functionality, rather than the consumer) that usually does not delay execution much. Usually,
 512             <filename>network.target</filename> is part of the boot of most systems, while
 513             <filename>network-online.target</filename> is not, except when at least one unit requires
 514             it. Also see <ulink url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is
 515             Up</ulink> for more information.</para>
 516
 517             <para>All mount units for remote network file systems automatically pull in this unit, and order
 518             themselves after it. Note that networking daemons that simply <emphasis>provide</emphasis>
 519             functionality to other hosts (as opposed to <emphasis>consume</emphasis> functionality of other
 520             hosts) generally do not need to pull this in.</para>
 521
 522             <para>systemd automatically adds dependencies of type <varname>Wants=</varname> and
 523             <varname>After=</varname> for this target unit to all SysV init script service units
 524             with an LSB header referring to the <literal>$network</literal> facility.</para>
 525
 526             <para>Note that this unit is only useful during the original system start-up
 527             logic. After the system has completed booting up, it will not track the online state of
 528             the system anymore. Due to this it cannot be used as a network connection monitor
 529             concept, it is purely a one-time system start-up concept.</para>
 530             </listitem>
 531         </varlistentry>
 532         <varlistentry>
 533           <term><filename>paths.target</filename></term>
 534           <listitem>
 535             <para>A special target unit that sets up all path units (see
 536             <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 537             for details) that shall be active after boot.</para>
 538
 539             <para>It is recommended that path units installed by
 540             applications get pulled in via <varname>Wants=</varname>
 541             dependencies from this unit. This is best configured via a
 542             <varname>WantedBy=paths.target</varname> in the path unit's
 543             [Install] section.</para>
 544           </listitem>
 545         </varlistentry>
 546         <varlistentry>
 547           <term><filename>poweroff.target</filename></term>
 548           <listitem>
 549             <para>A special target unit for shutting down and powering
 550             off the system.</para>
 551
 552             <para>Applications wanting to power off the system should not start this unit
 553             directly, but should instead execute <command>systemctl poweroff</command>
 554             (possibly with the <option>--no-block</option> option) or call
 555             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 556             <command>org.freedesktop.login1.Manager.PowerOff</command> D-Bus method
 557             directly.</para>
 558
 559             <para><filename>runlevel0.target</filename> is an alias for
 560             this target unit, for compatibility with SysV.</para>
 561           </listitem>
 562         </varlistentry>
 563         <varlistentry>
 564           <term><filename>reboot.target</filename></term>
 565           <listitem>
 566             <para>A special target unit for shutting down and rebooting the system.</para>
 567
 568             <para>Applications wanting to reboot the system should not start this unit directly, but should
 569             instead execute <command>systemctl reboot</command> (possibly with the
 570             <option>--no-block</option> option) or call
 571             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 572             <function>org.freedesktop.login1.Manager.Reboot()</function> D-Bus method directly.</para>
 573
 574             <para>See
 575             <citerefentry><refentrytitle>systemd-reboot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 576             for further details of the operation this target pulls in.</para>
 577
 578             <para><filename>runlevel6.target</filename> is an alias for this target unit, for compatibility
 579             with SysV.</para>
 580           </listitem>
 581         </varlistentry>
 582         <varlistentry>
 583           <term><filename>remote-cryptsetup.target</filename></term>
 584           <listitem>
 585             <para>Similar to <filename>cryptsetup.target</filename>, but for encrypted
 586             devices which are accessed over the network. It is used for
 587             <citerefentry><refentrytitle>crypttab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 588             entries marked with <option>_netdev</option>.</para>
 589           </listitem>
 590         </varlistentry>
 591         <varlistentry>
 592           <term><filename>remote-veritysetup.target</filename></term>
 593           <listitem>
 594             <para>Similar to <filename>veritysetup.target</filename>, but for verity
 595             integrity protected devices which are accessed over the network. It is used for
 596             <citerefentry><refentrytitle>veritytab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 597             entries marked with <option>_netdev</option>.</para>
 598           </listitem>
 599         </varlistentry>
 600         <varlistentry>
 601           <term><filename>remote-fs.target</filename></term>
 602           <listitem>
 603             <para>Similar to <filename>local-fs.target</filename>, but
 604             for remote mount points.</para>
 605
 606             <para>systemd automatically adds dependencies of type
 607             <varname>After=</varname> for this target unit to all SysV
 608             init script service units with an LSB header referring to
 609             the <literal>$remote_fs</literal> facility.</para>
 610           </listitem>
 611         </varlistentry>
 612         <varlistentry>
 613           <term><filename>rescue.target</filename></term>
 614           <listitem>
 615             <para>A special target unit that pulls in the base system (including system mounts) and
 616             spawns a rescue shell. Isolate to this target in order to administer the system in
 617             single-user mode with all file systems mounted but with no services running, except for
 618             the most basic. Compare with <filename>emergency.target</filename>, which is much more
 619             reduced and does not provide the file systems or most basic services. Compare with
 620             <filename>multi-user.target</filename>, this target could be seen as
 621             <filename>single-user.target</filename>.</para>
 622
 623             <para><filename>runlevel1.target</filename> is an alias for this target unit, for
 624             compatibility with SysV.</para>
 625
 626             <para>Use the <literal>systemd.unit=rescue.target</literal> kernel command line option
 627             to boot into this mode. A short alias for this kernel command line option is
 628             <literal>1</literal>, for compatibility with SysV.</para>
 629           </listitem>
 630         </varlistentry>
 631         <varlistentry>
 632           <term><filename>runlevel2.target</filename></term>
 633           <term><filename>runlevel3.target</filename></term>
 634           <term><filename>runlevel4.target</filename></term>
 635           <term><filename>runlevel5.target</filename></term>
 636           <listitem>
 637             <para>These are targets that are called whenever the SysV
 638             compatibility code asks for runlevel 2, 3, 4, 5,
 639             respectively. It is a good idea to make this an alias for
 640             (i.e. symlink to) <filename>graphical.target</filename>
 641             (for runlevel 5) or <filename>multi-user.target</filename>
 642             (the others).</para>
 643           </listitem>
 644         </varlistentry>
 645         <varlistentry>
 646           <term><filename>shutdown.target</filename></term>
 647           <listitem>
 648             <para>A special target unit that terminates the services on
 649             system shutdown.</para>
 650
 651             <para>Services that shall be terminated on system shutdown
 652             shall add <varname>Conflicts=</varname> and
 653             <varname>Before=</varname> dependencies to this unit for
 654             their service unit, which is implicitly done when
 655             <varname>DefaultDependencies=yes</varname> is set (the
 656             default).</para>
 657           </listitem>
 658         </varlistentry>
 659         <varlistentry>
 660           <term><filename>sigpwr.target</filename></term>
 661           <listitem>
 662             <para>A special target that is started when systemd receives
 663             the SIGPWR process signal, which is normally sent by the
 664             kernel or UPS daemons when power fails.</para>
 665           </listitem>
 666         </varlistentry>
 667         <varlistentry>
 668           <term><filename>sleep.target</filename></term>
 669           <listitem>
 670             <para>A special target unit that is pulled in by
 671             <filename>suspend.target</filename>,
 672             <filename>hibernate.target</filename> and
 673             <filename>hybrid-sleep.target</filename> and may be used to
 674             hook units into the sleep state logic.</para>
 675           </listitem>
 676         </varlistentry>
 677         <varlistentry>
 678           <term><filename>slices.target</filename></term>
 679           <listitem>
 680             <para>A special target unit that sets up all slice units (see
 681             <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 682             for details) that shall always be active after boot. By default the generic
 683             <filename>system.slice</filename> slice unit as well as the root slice unit
 684             <filename>-.slice</filename> are pulled in and ordered before this unit (see
 685             below).</para>
 686
 687             <para>Adding slice units to <filename>slices.target</filename> is generally not
 688             necessary. Instead, when some unit that uses <varname>Slice=</varname> is started, the
 689             specified slice will be started automatically. Adding
 690             <varname>WantedBy=slices.target</varname> lines to the [Install]
 691             section should only be done for units that need to be always active. In that case care
 692             needs to be taken to avoid creating a loop through the automatic dependencies on
 693             "parent" slices.</para>
 694           </listitem>
 695         </varlistentry>
 696         <varlistentry>
 697           <term><filename>sockets.target</filename></term>
 698           <listitem>
 699             <para>A special target unit that sets up all socket
 700             units (see
 701             <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 702             for details) that shall be active after boot.</para>
 703
 704             <para>Services that can be socket-activated shall add
 705             <varname>Wants=</varname> dependencies to this unit for
 706             their socket unit during installation. This is best
 707             configured via a <varname>WantedBy=sockets.target</varname>
 708             in the socket unit's [Install]
 709             section.</para>
 710           </listitem>
 711         </varlistentry>
 712         <varlistentry>
 713           <term><filename>soft-reboot.target</filename></term>
 714           <listitem>
 715             <para>A special target unit for shutting down and rebooting the userspace of the system (leaving
 716             the kernel running).</para>
 717
 718             <para>Applications wanting to reboot the system should not start this unit directly, but should
 719             instead execute <command>systemctl soft-reboot</command> (possibly with the
 720             <option>--no-block</option> option) or call
 721             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 722             <function>org.freedesktop.login1.Manager.RebootWithFlags()</function> D-Bus method
 723             directly.</para>
 724
 725             <para>See
 726             <citerefentry><refentrytitle>systemd-soft-reboot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 727             for further details of the operation this target pulls in.</para>
 728           </listitem>
 729         </varlistentry>
 730         <varlistentry>
 731           <term><filename>suspend.target</filename></term>
 732           <listitem>
 733             <para>A special target unit for suspending the system. This
 734             pulls in <filename>sleep.target</filename>.</para>
 735           </listitem>
 736         </varlistentry>
 737         <varlistentry>
 738           <term><filename>swap.target</filename></term>
 739           <listitem>
 740             <para>Similar to <filename>local-fs.target</filename>, but
 741             for swap partitions and swap files.</para>
 742           </listitem>
 743         </varlistentry>
 744         <varlistentry>
 745           <term><filename>sysinit.target</filename></term>
 746           <listitem>
 747             <para>systemd automatically adds dependencies of the types
 748             <varname>Requires=</varname> and <varname>After=</varname>
 749             for this target unit to all services (except for those with
 750             <varname>DefaultDependencies=no</varname>).</para>
 751
 752             <para>This target pulls in the services required for system
 753             initialization. System services pulled in by this target should
 754             declare <varname>DefaultDependencies=no</varname> and specify
 755             all their dependencies manually, including access to anything
 756             more than a read only root filesystem. For details on the
 757             dependencies of this target, refer to
 758             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 759             </para>
 760           </listitem>
 761         </varlistentry>
 762         <varlistentry>
 763           <term><filename>syslog.socket</filename></term>
 764           <listitem>
 765             <para>The socket unit syslog implementations should listen
 766             on. All userspace log messages will be made available on
 767             this socket. For more information about syslog integration,
 768             please consult the <ulink
 769             url="https://www.freedesktop.org/wiki/Software/systemd/syslog">Syslog
 770             Interface</ulink> document.</para>
 771           </listitem>
 772         </varlistentry>
 773         <varlistentry>
 774           <term><filename>system-update.target</filename></term>
 775           <term><filename>system-update-pre.target</filename></term>
 776           <term><filename>system-update-cleanup.service</filename></term>
 777           <listitem>
 778             <para>A special target unit that is used for offline system updates.
 779             <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 780             will redirect the boot process to this target if <filename>/system-update</filename> or
 781             <filename>/etc/system-update</filename> exists. For more information see
 782             <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 783             </para>
 784
 785             <para>Updates should happen before the <filename>system-update.target</filename> is
 786             reached, and the services which implement them should cause the machine to reboot. The
 787             main units executing the update should order themselves after
 788             <filename>system-update-pre.target</filename> but not pull it in. Services which want to
 789             run during system updates only, but before the actual system update is executed should
 790             order themselves before this unit and pull it in. As a safety measure, if this does not
 791             happen, and <filename>/system-update</filename> or
 792             <filename>/etc/system-update</filename> still exists after
 793             <filename>system-update.target</filename> is reached,
 794             <filename>system-update-cleanup.service</filename> will remove the symlinks and reboot
 795             the machine.</para>
 796           </listitem>
 797         </varlistentry>
 798         <varlistentry>
 799           <term><filename>timers.target</filename></term>
 800           <listitem>
 801             <para>A special target unit that sets up all timer units
 802             (see
 803             <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 804             for details) that shall be active after boot.</para>
 805
 806             <para>It is recommended that timer units installed by
 807             applications get pulled in via <varname>Wants=</varname>
 808             dependencies from this unit. This is best configured via
 809             <varname>WantedBy=timers.target</varname> in the timer
 810             unit's [Install] section.</para>
 811           </listitem>
 812         </varlistentry>
 813         <varlistentry>
 814           <term><filename>umount.target</filename></term>
 815           <listitem>
 816             <para>A special target unit that unmounts all mount and
 817             automount points on system shutdown.</para>
 818
 819             <para>Mounts that shall be unmounted on system shutdown
 820             shall add Conflicts dependencies to this unit for their
 821             mount unit, which is implicitly done when
 822             <varname>DefaultDependencies=yes</varname> is set (the
 823             default).</para>
 824           </listitem>
 825         </varlistentry>
 826
 827       </variablelist>
 828     </refsect2>
 829
 830     <refsect2>
 831       <title>Special System Units for Devices</title>
 832
 833       <para>Some target units are automatically pulled in as devices of
 834       certain kinds show up in the system. These may be used to
 835       automatically activate various services based on the specific type
 836       of the available hardware.</para>
 837
 838       <variablelist>
 839         <varlistentry>
 840           <term><filename>bluetooth.target</filename></term>
 841           <listitem>
 842             <para>This target is started automatically as soon as a
 843             Bluetooth controller is plugged in or becomes available at
 844             boot.</para>
 845
 846             <para>This may be used to pull in Bluetooth management
 847             daemons dynamically when Bluetooth hardware is found.</para>
 848           </listitem>
 849         </varlistentry>
 850         <varlistentry>
 851           <term><filename>printer.target</filename></term>
 852           <listitem>
 853             <para>This target is started automatically as soon as a
 854             printer is plugged in or becomes available at boot.</para>
 855
 856             <para>This may be used to pull in printer management daemons
 857             dynamically when printer hardware is found.</para>
 858           </listitem>
 859         </varlistentry>
 860         <varlistentry>
 861           <term><filename>smartcard.target</filename></term>
 862           <listitem>
 863             <para>This target is started automatically as soon as a
 864             smartcard controller is plugged in or becomes available at
 865             boot.</para>
 866
 867             <para>This may be used to pull in smartcard management
 868             daemons dynamically when smartcard hardware is found.</para>
 869           </listitem>
 870         </varlistentry>
 871         <varlistentry>
 872           <term><filename>sound.target</filename></term>
 873           <listitem>
 874             <para>This target is started automatically as soon as a
 875             sound card is plugged in or becomes available at
 876             boot.</para>
 877
 878             <para>This may be used to pull in audio management daemons
 879             dynamically when audio hardware is found.</para>
 880           </listitem>
 881         </varlistentry>
 882         <varlistentry>
 883           <term><filename>usb-gadget.target</filename></term>
 884           <listitem>
 885             <para>This target is started automatically as soon as a
 886             USB Device Controller becomes available at boot.</para>
 887
 888             <para>This may be used to pull in usb gadget
 889             dynamically when UDC hardware is found.</para>
 890           </listitem>
 891         </varlistentry>
 892       </variablelist>
 893     </refsect2>
 894
 895     <refsect2>
 896       <title>Special Passive System Units </title>
 897
 898       <para>A number of special system targets are defined that can be
 899       used to properly order boot-up of optional services. These targets
 900       are generally not part of the initial boot transaction, unless
 901       they are explicitly pulled in by one of the implementing services.
 902       Note specifically that these <emphasis>passive</emphasis> target
 903       units are generally not pulled in by the consumer of a service,
 904       but by the provider of the service. This means: a consuming
 905       service should order itself after these targets (as appropriate),
 906       but not pull it in. A providing service should order itself before
 907       these targets (as appropriate) and pull it in (via a
 908       <varname>Wants=</varname> type dependency).</para>
 909
 910       <para>Note that these passive units cannot be started manually,
 911       i.e. <literal>systemctl start time-sync.target</literal> will fail
 912       with an error. They can only be pulled in by dependency. This is
 913       enforced since they exist for ordering purposes only and thus are
 914       not useful as only unit within a transaction.</para>
 915
 916       <variablelist>
 917         <varlistentry>
 918           <term><filename>blockdev@.target</filename></term>
 919           <listitem><para>This template unit is used to order mount units and other consumers of block
 920           devices after services that synthesize these block devices. In particular, this is intended to be
 921           used with storage services (such as
 922           <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>/
 923           <citerefentry><refentrytitle>systemd-veritysetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
 924           that allocate and manage a virtual block device. Storage services are ordered before an instance of
 925           <filename>blockdev@.target</filename>, and the consumer units after it. The ordering is
 926           particularly relevant during shutdown, as it ensures that the mount is deactivated first and the
 927           service backing the mount later. The <filename>blockdev@.target</filename> instance should be
 928           pulled in via a <option>Wants=</option> dependency of the storage daemon and thus generally not be
 929           part of any transaction unless a storage daemon is used. The instance name for instances of this
 930           template unit must be a properly escaped block device node path, e.g.
 931           <filename index="false">blockdev@dev-mapper-foobar.target</filename> for the storage device
 932           <filename index="false">/dev/mapper/foobar</filename>.</para></listitem>
 933         </varlistentry>
 934         <varlistentry>
 935           <term><filename>cryptsetup-pre.target</filename></term>
 936           <listitem>
 937             <para>This passive target unit may be pulled in by services
 938             that want to run before any encrypted block device is set
 939             up. All encrypted block devices are set up after this target
 940             has been reached. Since the shutdown order is implicitly the
 941             reverse start-up order between units, this target is
 942             particularly useful to ensure that a service is shut down
 943             only after all encrypted block devices are fully
 944             stopped.</para>
 945           </listitem>
 946         </varlistentry>
 947         <varlistentry>
 948           <term><filename>veritysetup-pre.target</filename></term>
 949           <listitem>
 950             <para>This passive target unit may be pulled in by services
 951             that want to run before any verity integrity protected block
 952             device is set up. All verity integrity protected block
 953             devices are set up after this target has been reached. Since
 954             the shutdown order is implicitly the reverse start-up order
 955             between units, this target is particularly useful to ensure
 956             that a service is shut down only after all verity integrity
 957             protected block devices are fully stopped.</para>
 958           </listitem>
 959         </varlistentry>
 960         <varlistentry>
 961           <term><filename>first-boot-complete.target</filename></term>
 962           <listitem>
 963             <para>This passive target is intended as a synchronization point for units that need to run once
 964             during the first boot.  Only after all units ordered before this target have finished, will the
 965             <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 966             be committed to disk, marking the first boot as completed.  If the boot is aborted at any time
 967             before that, the next boot will re-run any units with <varname>ConditionFirstBoot=yes</varname>.
 968             </para>
 969           </listitem>
 970         </varlistentry>
 971         <varlistentry>
 972           <term><filename>getty-pre.target</filename></term>
 973           <listitem>
 974             <para>A special passive target unit. Users of this target
 975             are expected to pull it in the boot transaction via
 976             a dependency (e.g. <varname>Wants=</varname>). Order your
 977             unit before this unit if you want to make use of the console
 978             just before <filename>getty</filename> is started.
 979             </para>
 980           </listitem>
 981         </varlistentry>
 982         <varlistentry>
 983           <term><filename>local-fs-pre.target</filename></term>
 984           <listitem>
 985             <para>This target unit is
 986             automatically ordered before
 987             all local mount points marked
 988             with <option>auto</option>
 989             (see above). It can be used to
 990             execute certain units before
 991             all local mounts.</para>
 992           </listitem>
 993         </varlistentry>
 994         <varlistentry>
 995           <term><filename>network.target</filename></term>
 996           <listitem>
 997             <para>This unit is supposed to indicate when network functionality is available, but it is only
 998             very weakly defined what that is supposed to mean. However, the following should apply at
 999             minimum:</para>
1000
1001             <itemizedlist>
1002               <listitem><para>At start-up, any configured synthetic network devices (i.e. not physical ones
1003               that require hardware to show up and be probed, but virtual ones like bridge devices and
1004               similar which are created programmatically) that do not depend on any underlying hardware
1005               should be allocated by the time this target is reached. It is not necessary for these
1006               interfaces to also have completed IP level configuration by the time
1007               <filename>network.target</filename> is reached.</para></listitem>
1008
1009               <listitem><para>At shutdown, a unit that is ordered after <filename>network.target</filename>
1010               will be stopped before the network — to whatever level it might be set up by then — is shut
1011               down. It is hence useful when writing service files that require network access on shutdown,
1012               which should order themselves after this target, but not pull it in. Also see <ulink
1013               url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is Up</ulink> for
1014               more information.</para></listitem>
1015             </itemizedlist>
1016
1017             <para>It must emphasized that at start-up there's no guarantee that hardware-based devices have
1018             shown up by the time this target is reached, or even acquired complete IP configuration. For that
1019             purpose use <filename>network-online.target</filename> as described above.</para>
1020           </listitem>
1021         </varlistentry>
1022         <varlistentry>
1023           <term><filename>network-pre.target</filename></term>
1024           <listitem>
1025             <para>This passive target unit may be pulled in by services that want to run before any network
1026             is set up, for example for the purpose of setting up a firewall. All network management software
1027             orders itself after this target, but does not pull it in. Also see <ulink
1028             url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is Up</ulink> for more
1029             information.</para>
1030           </listitem>
1031         </varlistentry>
1032         <varlistentry>
1033           <term><filename>nss-lookup.target</filename></term>
1034           <listitem>
1035             <para>A target that should be used as synchronization point for all host/network name
1036             service lookups. Note that this is independent of UNIX user/group name lookups for which
1037             <filename>nss-user-lookup.target</filename> should be used.  All services for which the
1038             availability of full host/network name resolution is essential should be ordered after
1039             this target, but not pull it in. systemd automatically adds dependencies of type
1040             <varname>After=</varname> for this target unit to all SysV init script service units
1041             with an LSB header referring to the <literal>$named</literal> facility.</para>
1042           </listitem>
1043         </varlistentry>
1044         <varlistentry>
1045           <term><filename>nss-user-lookup.target</filename></term>
1046           <listitem>
1047             <para>A target that should be used as synchronization point for all regular UNIX
1048             user/group name service lookups. Note that this is independent of host/network name
1049             lookups for which <filename>nss-lookup.target</filename> should be used. All services
1050             for which the availability of the full user/group database is essential should be
1051             ordered after this target, but not pull it in. All services which provide parts of the
1052             user/group database should be ordered before this target, and pull it in. Note that this
1053             unit is only relevant for regular users and groups — system users and groups are
1054             required to be resolvable during earliest boot already, and hence do not need any
1055             special ordering against this target.</para>
1056           </listitem>
1057         </varlistentry>
1058         <varlistentry>
1059           <term><filename>remote-fs-pre.target</filename></term>
1060           <listitem>
1061             <para>This target unit is automatically ordered before all
1062             mount point units (see above) and cryptsetup/veritysetup devices
1063             marked with the <option>_netdev</option>. It can be used to run
1064             certain units before remote encrypted devices and mounts are established.
1065             Note that this unit is generally not part of the initial
1066             transaction, unless the unit that wants to be ordered before
1067             all remote mounts pulls it in via a
1068             <varname>Wants=</varname> type dependency. If the unit wants
1069             to be pulled in by the first remote mount showing up, it
1070             should use <filename>network-online.target</filename> (see
1071             above).</para>
1072           </listitem>
1073         </varlistentry>
1074         <varlistentry>
1075           <term><filename>rpcbind.target</filename></term>
1076           <listitem>
1077             <para>The portmapper/rpcbind pulls in this target and orders
1078             itself before it, to indicate its availability. systemd
1079             automatically adds dependencies of type
1080             <varname>After=</varname> for this target unit to all SysV
1081             init script service units with an LSB header referring to
1082             the <literal>$portmap</literal> facility.</para>
1083           </listitem>
1084         </varlistentry>
1085         <varlistentry>
1086           <term><filename>time-set.target</filename></term>
1087           <listitem>
1088             <para>Services responsible for setting the system clock (<constant>CLOCK_REALTIME</constant>)
1089             from a local source (such as a maintained timestamp file or imprecise real-time clock) should
1090             pull in this target and order themselves before it. Services where approximate, roughly monotonic
1091             time is desired should be ordered after this unit, but not pull it in.</para>
1092
1093             <para>This target does not provide the accuracy guarantees of
1094             <filename>time-sync.target</filename> (see below), however does not depend on remote clock
1095             sources to be reachable, i.e. the target is typically not delayed by network problems and
1096             similar. Use of this target is recommended for services where approximate clock accuracy and
1097             rough monotonicity is desired but activation shall not be delayed for possibly unreliable network
1098             communication.</para>
1099
1100             <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
1101             this target unit to all timer units with at least one <varname>OnCalendar=</varname>
1102             directive.</para>
1103
1104             <para>The
1105             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1106             service is a simple daemon that pulls in this target and orders itself before it. Besides
1107             implementing the SNTP network protocol it maintains a timestamp file on disk whose modification
1108             time is regularly updated. At service start-up the local system clock is set from that modification time,
1109             ensuring it increases roughly monotonically.</para>
1110
1111             <para>Note that ordering a unit after <filename>time-set.target</filename> only has effect if
1112             there's actually a service ordered before it that delays it until the clock is adjusted for rough
1113             monotonicity. Otherwise, this target might get reached before the clock is adjusted to be roughly
1114             monotonic. Enable
1115             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1116             or an alternative NTP implementation to delay the target.</para>
1117           </listitem>
1118         </varlistentry>
1119         <varlistentry>
1120           <term><filename>time-sync.target</filename></term>
1121           <listitem>
1122             <para>Services indicating completed synchronization of the system clock
1123             (<constant>CLOCK_REALTIME</constant>) to a remote source should pull in this target and order
1124             themselves before it. Services where accurate time is essential should be ordered after this
1125             unit, but not pull it in.</para>
1126
1127             <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
1128             this target unit to all SysV init script service units with an LSB header referring to the
1129             <literal>$time</literal> facility, as well to all timer units with at least one
1130             <varname>OnCalendar=</varname> directive.</para>
1131
1132             <para>This target provides stricter clock accuracy guarantees than
1133             <filename>time-set.target</filename> (see above), but likely requires
1134             network communication and thus introduces unpredictable delays.
1135             Services that require clock accuracy and where network
1136             communication delays are acceptable should use this target. Services that require a less accurate
1137             clock, and only approximate and roughly monotonic clock behaviour should use
1138             <filename>time-set.target</filename> instead.</para>
1139
1140             <para>Note that ordering a unit after <filename>time-sync.target</filename> only has effect if
1141             there's actually a service ordered before it that delays it until clock synchronization is
1142             reached. Otherwise, this target might get reached before the clock is synchronized to any remote
1143             accurate reference clock. When using
1144             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1145             enable
1146             <citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1147             to delay the target; or use an equivalent service for other NTP implementations.</para>
1148
1149             <table>
1150               <title>Comparison</title>
1151               <tgroup cols='2' align='left' colsep='1' rowsep='1'>
1152                 <colspec colname="time-set" />
1153                 <colspec colname="time-sync" />
1154                 <thead>
1155                   <row>
1156                     <entry><filename>time-set.target</filename></entry>
1157                     <entry><filename>time-sync.target</filename></entry>
1158                   </row>
1159                 </thead>
1160                 <tbody>
1161                   <row>
1162                     <entry>"quick" to reach</entry>
1163                     <entry>"slow" to reach</entry>
1164                   </row>
1165                   <row>
1166                     <entry>typically uses local clock sources, boot process not affected by availability of external resources</entry>
1167                     <entry>typically uses remote clock sources, inserts dependencies on remote resources into boot process</entry>
1168                   </row>
1169                   <row>
1170                     <entry>reliable, because local</entry>
1171                     <entry>unreliable, because typically network involved</entry>
1172                   </row>
1173                   <row>
1174                     <entry>typically guarantees an approximate and roughly monotonic clock only</entry>
1175                     <entry>typically guarantees an accurate clock</entry>
1176                   </row>
1177                   <row>
1178                     <entry>implemented by <filename>systemd-timesyncd.service</filename></entry>
1179                     <entry>implemented by <filename>systemd-time-wait-sync.service</filename></entry>
1180                   </row>
1181                 </tbody>
1182               </tgroup>
1183             </table>
1184
1185           </listitem>
1186         </varlistentry>
1187       </variablelist>
1188     </refsect2>
1189
1190     <refsect2>
1191       <title>Special Slice Units</title>
1192
1193       <para>There are four <literal>.slice</literal> units which form the basis of the hierarchy for
1194       assignment of resources for services, users, and virtual machines or containers. See
1195       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1196       for details about slice units.</para>
1197
1198       <variablelist>
1199         <varlistentry>
1200           <term><filename>-.slice</filename></term>
1201           <listitem>
1202             <para>The root slice is the root of the slice hierarchy. It usually does not contain
1203             units directly, but may be used to set defaults for the whole tree.</para>
1204           </listitem>
1205         </varlistentry>
1206
1207         <varlistentry>
1208           <term><filename>system.slice</filename></term>
1209           <listitem>
1210             <para>By default, all system services started by
1211             <command>systemd</command> are found in this slice.</para>
1212           </listitem>
1213         </varlistentry>
1214
1215         <varlistentry>
1216           <term><filename>user.slice</filename></term>
1217           <listitem>
1218             <para>By default, all user processes and services started on
1219             behalf of the user, including the per-user systemd instance
1220             are found in this slice.  This is pulled in by
1221             <filename>systemd-logind.service</filename>.</para>
1222           </listitem>
1223         </varlistentry>
1224
1225         <varlistentry>
1226           <term><filename>machine.slice</filename></term>
1227           <listitem>
1228             <para>By default, all virtual machines and containers
1229             registered with <command>systemd-machined</command> are
1230             found in this slice. This is pulled in by
1231             <filename>systemd-machined.service</filename>.</para>
1232           </listitem>
1233         </varlistentry>
1234       </variablelist>
1235     </refsect2>
1236   </refsect1>
1237
1238   <refsect1>
1239     <title>Units managed by the user service manager</title>
1240
1241     <refsect2>
1242       <title>Special User Units</title>
1243
1244       <para>When systemd runs as a user instance, the following special
1245       units are available:</para>
1246
1247       <variablelist>
1248         <varlistentry>
1249           <term><filename>default.target</filename></term>
1250           <listitem>
1251             <para>This is the main target of the user session, started by default. Various services that
1252             compose the normal user session should be pulled into this target. In this regard,
1253             <filename>default.target</filename> is similar to <filename>multi-user.target</filename> in the
1254             system instance, but it is a real unit, not an alias.</para>
1255           </listitem>
1256         </varlistentry>
1257       </variablelist>
1258
1259       <para>In addition, the following units are available which have definitions similar to their
1260       system counterparts:
1261       <filename>exit.target</filename>,
1262       <filename>shutdown.target</filename>,
1263       <filename>sockets.target</filename>,
1264       <filename>timers.target</filename>,
1265       <filename>paths.target</filename>,
1266       <filename>bluetooth.target</filename>,
1267       <filename>printer.target</filename>,
1268       <filename>smartcard.target</filename>,
1269       <filename>sound.target</filename>.</para>
1270     </refsect2>
1271
1272     <refsect2>
1273       <title>Special Passive User Units</title>
1274
1275       <variablelist>
1276         <varlistentry>
1277           <term><filename>graphical-session.target</filename></term>
1278           <listitem>
1279             <para>This target is active whenever any graphical session is running. It is used to
1280             stop user services which only apply to a graphical (X, Wayland, etc.) session when the
1281             session is terminated. Such services should have
1282             <literal>PartOf=graphical-session.target</literal> in their [Unit]
1283             section. A target for a particular session (e. g.
1284             <filename>gnome-session.target</filename>) starts and stops
1285             <literal>graphical-session.target</literal> with
1286             <literal>BindsTo=graphical-session.target</literal>.</para>
1287
1288             <para>Which services are started by a session target is determined by the
1289             <literal>Wants=</literal> and <literal>Requires=</literal> dependencies.  For services
1290             that can be enabled independently, symlinks in <literal>.wants/</literal> and
1291             <literal>.requires/</literal> should be used, see
1292             <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
1293             Those symlinks should either be shipped in packages, or should be added dynamically
1294             after installation, for example using <literal>systemctl add-wants</literal>, see
1295             <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
1296             </para>
1297
1298             <example>
1299               <title>Nautilus as part of a GNOME session</title>
1300
1301               <para><literal>gnome-session.target</literal> pulls in Nautilus as top-level service:</para>
1302
1303               <programlisting>[Unit]
1304 Description=User systemd services for GNOME graphical session
1305 Wants=nautilus.service
1306 BindsTo=graphical-session.target</programlisting>
1307
1308               <para><literal>nautilus.service</literal> gets stopped when the session stops:</para>
1309
1310               <programlisting>[Unit]
1311 Description=Render the desktop icons with Nautilus
1312 PartOf=graphical-session.target
1313
1314 [Service]
1315 …</programlisting>
1316             </example>
1317           </listitem>
1318         </varlistentry>
1319
1320         <varlistentry>
1321           <term><filename>graphical-session-pre.target</filename></term>
1322           <listitem>
1323             <para>This target contains services which set up the environment or global configuration
1324             of a graphical session, such as SSH/GPG agents (which need to export an environment
1325             variable into all desktop processes) or migration of obsolete d-conf keys after an OS
1326             upgrade (which needs to happen before starting any process that might use them). This
1327             target must be started before starting a graphical session like
1328             <filename>gnome-session.target</filename>.</para>
1329           </listitem>
1330         </varlistentry>
1331
1332         <varlistentry>
1333           <term><filename>xdg-desktop-autostart.target</filename></term>
1334           <listitem>
1335             <para>The XDG specification defines a way to autostart applications using XDG desktop files.
1336             systemd ships
1337             <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1338             for the XDG desktop files in autostart directories. Desktop Environments can opt-in to use this
1339             service by adding a <varname>Wants=</varname> dependency on
1340             <filename>xdg-desktop-autostart.target</filename>.</para>
1341           </listitem>
1342         </varlistentry>
1343       </variablelist>
1344     </refsect2>
1345
1346     <refsect2>
1347       <title>Special User Slice Units</title>
1348
1349       <para>There are four <literal>.slice</literal> units which form the basis of the user hierarchy for
1350       assignment of resources for user applications and services. See
1351       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1352       for details about slice units and the documentation about
1353       <ulink url="https://systemd.io/DESKTOP_ENVIRONMENTS">Desktop Environments</ulink>
1354       for further information.</para>
1355
1356       <variablelist>
1357         <varlistentry>
1358           <term><filename>-.slice</filename></term>
1359           <listitem>
1360             <para>The root slice is the root of the user's slice hierarchy.
1361             It usually does not contain units directly, but may be used to set defaults for the whole tree.</para>
1362           </listitem>
1363         </varlistentry>
1364
1365         <varlistentry>
1366           <term><filename>app.slice</filename></term>
1367           <listitem>
1368             <para>By default, all user services and applications managed by
1369             <command>systemd</command> are found in this slice.
1370             All interactively launched applications like web browsers and text editors
1371             as well as non-critical services should be placed into this slice.</para>
1372           </listitem>
1373         </varlistentry>
1374
1375         <varlistentry>
1376           <term><filename>session.slice</filename></term>
1377           <listitem>
1378             <para>All essential services and applications required for the
1379             session should use this slice.
1380             These are services that either cannot be restarted easily
1381             or where latency issues may affect the interactivity of the system and applications.
1382             This includes the display server, screen readers and other services such as DBus or XDG portals.
1383             Such services should be configured to be part of this slice by
1384             adding <varname>Slice=session.slice</varname> to their unit files.</para>
1385           </listitem>
1386         </varlistentry>
1387
1388         <varlistentry>
1389           <term><filename>background.slice</filename></term>
1390           <listitem>
1391             <para>All services running low-priority background tasks should use this slice.
1392             This permits resources to be preferentially assigned to the other slices.
1393             Examples include non-interactive tasks like file indexing or backup operations
1394             where latency is not important.</para>
1395           </listitem>
1396         </varlistentry>
1397       </variablelist>
1398     </refsect2>
1399   </refsect1>
1400
1401   <refsect1>
1402       <title>See Also</title>
1403       <para>
1404         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1405         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1406         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1407         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1408         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1409         <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1410         <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1411         <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1412         <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1413       </para>
1414   </refsect1>
1415
1416 </refentry>