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34511ca7b1
Make sure the man page titles are similar in style and capitalization so that our man page index looks pretty.
227 lines
12 KiB
XML
227 lines
12 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 the
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system boot. Immediately after power-up, the system
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BIOS will do minimal hardware initialization, and hand
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control over to a boot loader stored on a persistent
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storage device. This boot loader will then invoke an
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OS kernel from disk (or the network). In the Linux
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case this kernel now (optionally) extracts and
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executes an initial RAM disk image (initrd) such as
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<citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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which looks for the root file system. After the root
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file system is found and mounted the initrd hands over
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control to the 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 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><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 follow 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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v | v
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(various | rescue.service
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sockets...) | |
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| | v
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v | <emphasis>rescue.target</emphasis>
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sockets.target |
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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>System Manager Shutdown</title>
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<para>System shutdown also consists of various target
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units with some minimal ordering structure
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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><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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</para>
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</refsect1>
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</refentry>
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