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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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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>The 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 persistant
                storage device. This boot loader will then invoke an
                OS kernel from disk (or the network). In the Linux
                case this kernel now (optionally) extracts and
                executes an initial RAM disk image (initrd) such as
                <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                which looks for the root file system. After the root
                file system is found and mounted the initrd hands over
                control to the 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 last step the system is powered down.</para>

                <para>Additional information about the system boot
                process may be found in
                <citerefentry><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 follow 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
                        (various               |              rescue.service
                       sockets...)             |                    |
                            |                  |                    v
                            v                  |              <emphasis>rescue.target</emphasis>
                     sockets.target            |
                            |                  |
                            \_________________ |
                                              \|
                                               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>
        </refsect1>

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

                <para>System shutdown 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><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>
                </para>
        </refsect1>

</refentry>