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<refentry id="bootup">

  <refentryinfo>
    <title>bootup</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>bootup</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>bootup</refname>
    <refpurpose>System bootup process</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Description</title>

    <para>A number of different components are involved in the system
    boot. Immediately after power-up, the system BIOS will do minimal
    hardware initialization, and hand control over to a boot loader
    stored on a persistent storage device. This boot loader will then
    invoke an OS kernel from disk (or the network). In the Linux case,
    this kernel (optionally) extracts and executes an initial RAM disk
    image (initrd), such as generated by
    <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
    which looks for the root file system (possibly using
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    for this). After the root file system is found and mounted, the
    initrd hands over control to the host's system manager (such as
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
    stored on the OS image, which is then responsible for probing all
    remaining hardware, mounting all necessary file systems and
    spawning all configured services.</para>

    <para>On shutdown, the system manager stops all services, unmounts
    all file systems (detaching the storage technologies backing
    them), and then (optionally) jumps back into the initrd code which
    unmounts/detaches the root file system and the storage it resides
    on. As a last step, the system is powered down.</para>

    <para>Additional information about the system boot process may be
    found in
    <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>System Manager Bootup</title>

    <para>At boot, the system manager on the OS image is responsible
    for initializing the required file systems, services and drivers
    that are necessary for operation of the system. On
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    systems, this process is split up in various discrete steps which
    are exposed as target units. (See
    <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for detailed information about target units.) The boot-up process
    is highly parallelized so that the order in which specific target
    units are reached is not deterministic, but still adheres to a
    limited amount of ordering structure.</para>

    <para>When systemd starts up the system, it will activate all
    units that are dependencies of <filename>default.target</filename>
    (as well as recursively all dependencies of these dependencies).
    Usually, <filename>default.target</filename> is simply an alias of
    <filename>graphical.target</filename> or
    <filename>multi-user.target</filename>, depending on whether the
    system is configured for a graphical UI or only for a text
    console. To enforce minimal ordering between the units pulled in,
    a number of well-known target units are available, as listed on
    <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

    <para>The following chart is a structural overview of these
    well-known units and their position in the boot-up logic. The
    arrows describe which units are pulled in and ordered before which
    other units. Units near the top are started before units nearer to
    the bottom of the chart.</para>

<programlisting>local-fs-pre.target
         |
         v
(various mounts and   (various swap   (various cryptsetup
 fsck services...)     devices...)        devices...)       (various low-level   (various low-level
         |                  |                  |             services: udevd,     API VFS mounts:
         v                  v                  v             tmpfiles, random     mqueue, configfs,
  local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
         |                  |                  |                    |                    |
         \__________________|_________________ | ___________________|____________________/
                                              \|/
                                               v
                                        sysinit.target
                                               |
          ____________________________________/|\________________________________________
         /                  |                  |                    |                    \
         |                  |                  |                    |                    |
         v                  v                  |                    v                    v
     (various           (various               |                (various          rescue.service
    timers...)          paths...)              |               sockets...)               |
         |                  |                  |                    |                    v
         v                  v                  |                    v              <emphasis>rescue.target</emphasis>
   timers.target      paths.target             |             sockets.target
         |                  |                  |                    |
         v                  \_________________ | ___________________/
                                              \|/
                                               v
                                         basic.target
                                               |
          ____________________________________/|                                 emergency.service
         /                  |                  |                                         |
         |                  |                  |                                         v
         v                  v                  v                                 <emphasis>emergency.target</emphasis>
     display-        (various system    (various system
 manager.service         services           services)
         |             required for            |
         |            graphical UIs)           v
         |                  |           <emphasis>multi-user.target</emphasis>
         |                  |                  |
         \_________________ | _________________/
                           \|/
                            v
                  <emphasis>graphical.target</emphasis></programlisting>

    <para>Target units that are commonly used as boot targets are
    <emphasis>emphasized</emphasis>. These units are good choices as
    goal targets, for example by passing them to the
    <varname>systemd.unit=</varname> kernel command line option (see
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
    or by symlinking <filename>default.target</filename> to them.
    </para>

    <para><filename>timers.target</filename> is pulled-in by
    <filename>basic.target</filename> asynchronously. This allows
    timers units to depend on services which become only available
    later in boot.</para>
  </refsect1>

  <refsect1>
    <title>Bootup in the Initial RAM Disk (initrd)</title>
    <para>The initial RAM disk implementation (initrd) can be set up
    using systemd as well. In this case, boot up inside the initrd
    follows the following structure.</para>

    <para>The default target in the initrd is
    <filename>initrd.target</filename>. The bootup process begins
    identical to the system manager bootup (see above) until it
    reaches <filename>basic.target</filename>. From there, systemd
    approaches the special target <filename>initrd.target</filename>.
    If the root device can be mounted at
    <filename>/sysroot</filename>, the
    <filename>sysroot.mount</filename> unit becomes active and
    <filename>initrd-root-fs.target</filename> is reached. The service
    <filename>initrd-parse-etc.service</filename> scans
    <filename>/sysroot/etc/fstab</filename> for a possible
    <filename>/usr</filename> mount point and additional entries
    marked with the <emphasis>x-initrd.mount</emphasis> option. All
    entries found are mounted below <filename>/sysroot</filename>, and
    <filename>initrd-fs.target</filename> is reached. The service
    <filename>initrd-cleanup.service</filename> isolates to the
    <filename>initrd-switch-root.target</filename>, where cleanup
    services can run. As the very last step, the
    <filename>initrd-switch-root.service</filename> is activated,
    which will cause the system to switch its root to
    <filename>/sysroot</filename>.
    </para>

<programlisting>                                               : (beginning identical to above)
                                               :
                                               v
                                         basic.target
                                               |                                 emergency.service
                        ______________________/|                                         |
                       /                       |                                         v
                       |                  sysroot.mount                          <emphasis>emergency.target</emphasis>
                       |                       |
                       |                       v
                       |             initrd-root-fs.target
                       |                       |
                       |                       v
                       v            initrd-parse-etc.service
                (custom initrd                 |
                 services...)                  v
                       |            (sysroot-usr.mount and
                       |             various mounts marked
                       |               with fstab option
                       |              x-initrd.mount...)
                       |                       |
                       |                       v
                       |                initrd-fs.target
                       \______________________ |
                                              \|
                                               v
                                          initrd.target
                                               |
                                               v
                                     initrd-cleanup.service
                                          isolates to
                                    initrd-switch-root.target
                                               |
                                               v
                        ______________________/|
                       /                       v
                       |        initrd-udevadm-cleanup-db.service
                       v                       |
                (custom initrd                 |
                 services...)                  |
                       \______________________ |
                                              \|
                                               v
                                   initrd-switch-root.target
                                               |
                                               v
                                   initrd-switch-root.service
                                               |
                                               v
                                     Transition to Host OS</programlisting>
  </refsect1>

  <refsect1>
    <title>System Manager Shutdown</title>

    <para>System shutdown with systemd also consists of various target
    units with some minimal ordering structure applied:</para>

<programlisting>                                  (conflicts with  (conflicts with
                                    all system     all file system
                                     services)     mounts, swaps,
                                         |           cryptsetup
                                         |          devices, ...)
                                         |                |
                                         v                v
                                  shutdown.target    umount.target
                                         |                |
                                         \_______   ______/
                                                 \ /
                                                  v
                                         (various low-level
                                              services)
                                                  |
                                                  v
                                            final.target
                                                  |
            _____________________________________/ \_________________________________
           /                         |                        |                      \
           |                         |                        |                      |
           v                         v                        v                      v
systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
           |                         |                        |                      |
           v                         v                        v                      v
    <emphasis>reboot.target</emphasis>             <emphasis>poweroff.target</emphasis>            <emphasis>halt.target</emphasis>           <emphasis>kexec.target</emphasis></programlisting>

    <para>Commonly used system shutdown targets are
    <emphasis>emphasized</emphasis>.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
