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5aded36978
It is annoying when we have dead links on fd.o. Add project='man-pages|die-net|archlinux' to <citerefentry>-ies. In generated html, add external links to http://man7.org/linux/man-pages/man, http://linux.die.net/man/, https://www.archlinux.org/. By default, pages in sections 2 and 4 go to man7, since Michael Kerrisk is the autorative source on kernel related stuff. The rest of links goes to linux.die.net, because they have the manpages. Except for the pacman stuff, since it seems to be only available from archlinux.org. Poor gummiboot gets no link, because gummitboot(8) ain't to be found on the net. According to common wisdom, that would mean that it does not exist. But I have seen Kay using it, so I know it does, and deserves to be found. Can somebody be nice and put it up somewhere?
319 lines
17 KiB
XML
319 lines
17 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="bootup">
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<refentryinfo>
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<title>bootup</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>bootup</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>bootup</refname>
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<refpurpose>System bootup process</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<para>A number of different components are involved in
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the system boot. Immediately after power-up, the
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system BIOS will do minimal hardware initialization,
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and hand control over to a boot loader stored on a
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persistent storage device. This boot loader will then
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invoke an OS kernel from disk (or the network). In the
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Linux case, this kernel (optionally) extracts and
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executes an initial RAM disk image (initrd), such as
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generated by
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<citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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which looks for the root file system (possibly using
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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for this). After the root file system is found and
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mounted, the initrd hands over control to the host's
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system manager (such as
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
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stored on the OS image, which is then responsible for
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probing all remaining hardware, mounting all necessary
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file systems and spawning all configured
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services.</para>
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<para>On shutdown, the system manager stops all
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services, unmounts all file systems (detaching the
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storage technologies backing them), and then
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(optionally) jumps back into the initrd code which
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unmounts/detaches the root file system and the storage
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it resides on. As a last step, the system is powered down.</para>
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<para>Additional information about the system boot
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process may be found in
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<citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
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</refsect1>
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<refsect1>
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<title>System Manager Bootup</title>
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<para>At boot, the system manager on the OS image is
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responsible for initializing the required file
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systems, services and drivers that are necessary for
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operation of the system. On
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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systems, this process is split up in various discrete
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steps which are exposed as target units. (See
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<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for detailed information about target units.) The
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boot-up process is highly parallelized so that the
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order in which specific target units are reached is not
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deterministic, but still adheres to a limited amount
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of ordering structure.</para>
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<para>When systemd starts up the system, it will
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activate all units that are dependencies of
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<filename>default.target</filename> (as well as
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recursively all dependencies of these
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dependencies). Usually,
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<filename>default.target</filename> is simply an alias
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of <filename>graphical.target</filename> or
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<filename>multi-user.target</filename>, depending on
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whether the system is configured for a graphical UI or
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only for a text console. To enforce minimal ordering
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between the units pulled in, a number of well-known
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target units are available, as listed on
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
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<para>The following chart is a structural overview of
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these well-known units and their position in the
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boot-up logic. The arrows describe which units are
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pulled in and ordered before which other units. Units
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near the top are started before units nearer to the
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bottom of the chart.</para>
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<programlisting>local-fs-pre.target
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v
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(various mounts and (various swap (various cryptsetup
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fsck services...) devices...) devices...) (various low-level (various low-level
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| | | services: udevd, API VFS mounts:
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v v v tmpfiles, random mqueue, configfs,
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local-fs.target swap.target cryptsetup.target seed, sysctl, ...) debugfs, ...)
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| | | | |
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\__________________|_________________ | ___________________|____________________/
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\|/
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v
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sysinit.target
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____________________________________/|\________________________________________
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/ | | | \
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| | | | |
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v v | v v
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(various (various | (various rescue.service
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timers...) paths...) | sockets...) |
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| | | | v
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v v | v <emphasis>rescue.target</emphasis>
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timers.target paths.target | sockets.target
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| | | |
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\__________________|_________________ | ___________________/
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\|/
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v
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basic.target
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____________________________________/| emergency.service
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/ | | |
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| | | v
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v v v <emphasis>emergency.target</emphasis>
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display- (various system (various system
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manager.service services services)
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| required for |
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| graphical UIs) v
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| | <emphasis>multi-user.target</emphasis>
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| | |
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\_________________ | _________________/
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\|/
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v
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<emphasis>graphical.target</emphasis></programlisting>
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<para>Target units that are commonly used as boot
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targets are <emphasis>emphasized</emphasis>. These
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units are good choices as goal targets, for
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example by passing them to the
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<varname>systemd.unit=</varname> kernel command line
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option (see
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
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or by symlinking <filename>default.target</filename>
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to them.</para>
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</refsect1>
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<refsect1>
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<title>Bootup in the Initial RAM Disk (initrd)</title>
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<para>The initial RAM disk implementation (initrd) can
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be set up using systemd as well. In this case, boot up
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inside the initrd follows the following
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structure.</para>
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<para>The default target in the initrd is
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<filename>initrd.target</filename>. The bootup process
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begins identical to the system manager bootup (see
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above) until it reaches
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<filename>basic.target</filename>. From there, systemd
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approaches the special target
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<filename>initrd.target</filename>. If the root device
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can be mounted at <filename>/sysroot</filename>, the
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<filename>sysroot.mount</filename> unit becomes active
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and <filename>initrd-root-fs.target</filename> is
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reached. The service
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<filename>initrd-parse-etc.service</filename> scans
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<filename>/sysroot/etc/fstab</filename> for a possible
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<filename>/usr</filename> mount point and additional
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entries marked with the
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<emphasis>x-initrd.mount</emphasis> option. All
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entries found are mounted below
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<filename>/sysroot</filename>, and
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<filename>initrd-fs.target</filename> is reached. The
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service <filename>initrd-cleanup.service</filename>
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isolates to the
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<filename>initrd-switch-root.target</filename>, where
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cleanup services can run. As the very last step, the
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<filename>initrd-switch-root.service</filename> is
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activated, which will cause the system to switch its
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root to <filename>/sysroot</filename>.
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</para>
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<programlisting> : (beginning identical to above)
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:
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v
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basic.target
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| emergency.service
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______________________/| |
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/ | v
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| sysroot.mount <emphasis>emergency.target</emphasis>
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| v
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| initrd-root-fs.target
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| |
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| v
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v initrd-parse-etc.service
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(custom initrd |
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services...) v
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| (sysroot-usr.mount and
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| various mounts marked
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| with fstab option
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| x-initrd.mount...)
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| |
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| v
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| initrd-fs.target
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\______________________ |
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\|
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v
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initrd.target
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v
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initrd-cleanup.service
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isolates to
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initrd-switch-root.target
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v
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______________________/|
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/ v
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| initrd-udevadm-cleanup-db.service
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v |
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(custom initrd |
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services...) |
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\______________________ |
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\|
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v
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initrd-switch-root.target
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v
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initrd-switch-root.service
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v
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Transition to Host OS</programlisting>
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</refsect1>
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<refsect1>
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<title>System Manager Shutdown</title>
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<para>System shutdown with systemd also consists of
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various target units with some minimal ordering
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structure applied:</para>
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<programlisting> (conflicts with (conflicts with
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all system all file system
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services) mounts, swaps,
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| cryptsetup
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| devices, ...)
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v v
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shutdown.target umount.target
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\_______ ______/
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\ /
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v
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(various low-level
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services)
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v
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final.target
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_____________________________________/ \_________________________________
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/ | | \
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v v v v
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systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
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v v v v
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<emphasis>reboot.target</emphasis> <emphasis>poweroff.target</emphasis> <emphasis>halt.target</emphasis> <emphasis>kexec.target</emphasis></programlisting>
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<para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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