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https://github.com/systemd/systemd.git
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98118c44ae
Let's remove some explicit references to dracut as we prefer initrds built with mkosi these days.
357 lines
19 KiB
XML
357 lines
19 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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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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</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 boot of a Linux system. Immediately after
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power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot
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loader (e.g.
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
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<ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This
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boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types
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of firmware, this firmware may also load the kernel directly.</para>
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<para>The kernel (optionally) mounts an in-memory file system, which looks for the root file system.
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Nowadays this is implemented as an "initramfs" — a compressed CPIO archive that the kernel extracts into
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a tmpfs. In the past normal file systems using an in-memory block device (ramdisk) were used, and the
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name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads
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both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file
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system.
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be used
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to manage services in the initrd, similarly to the real system.</para>
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<para>After the root file system is found and mounted, the initrd hands over control to the host's system
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manager (such as
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) stored in
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the root file system, which is then responsible for probing all remaining hardware, mounting all
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necessary file systems and spawning all configured services.</para>
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<para>On shutdown, the system manager stops all services, unmounts
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all file systems (detaching the storage technologies backing
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them), and then (optionally) jumps back into the initrd code which
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unmounts/detaches the root file system and the storage it resides
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on. As a last step, the system is powered down.</para>
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<para>Additional information about the system boot process may be
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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 responsible
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for initializing the required file systems, services and drivers
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that are necessary for 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 steps which
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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 boot-up process
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is highly parallelized so that the order in which specific target
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units are reached is not deterministic, but still adheres to a
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limited amount of ordering structure.</para>
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<para>When systemd starts up the system, it will activate all
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units that are dependencies of <filename>default.target</filename>
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(as well as recursively all dependencies of these dependencies).
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Usually, <filename>default.target</filename> is simply an alias of
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<filename>graphical.target</filename> or
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<filename>multi-user.target</filename>, depending on whether the
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system is configured for a graphical UI or only for a text
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console. To enforce minimal ordering between the units pulled in,
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a number of well-known 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 these
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well-known units and their position in the boot-up logic. The
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arrows describe which units are pulled in and ordered before which
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other units. Units near the top are started before units nearer to
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the bottom of the chart.</para>
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<!-- note: do not use unicode ellipsis here, because docbook will replace that
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with three dots anyway, messing up alignment -->
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<programlisting> cryptsetup-pre.target veritysetup-pre.target
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(various low-level v
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API VFS mounts: (various cryptsetup/veritysetup devices...)
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mqueue, configfs, | |
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debugfs, ...) v |
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| cryptsetup.target |
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| (various swap | | remote-fs-pre.target
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| devices...) | | | |
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| | | | | v
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| v local-fs-pre.target | | | (network file systems)
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| swap.target | | v v |
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| | v | remote-cryptsetup.target |
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| | (various low-level (various mounts and | remote-veritysetup.target |
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| | services: udevd, fsck services...) | | |
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| | tmpfiles, random | | | remote-fs.target
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| | seed, sysctl, ...) v | | |
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| | | local-fs.target | | _____________/
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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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| | | | | |
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v v | v | |
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(various (various | (various | |
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timers...) paths...) | sockets...) | |
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| | | | | |
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v v | v | |
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timers.target paths.target | sockets.target | |
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| | | | v |
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v \_______ | _____/ rescue.service |
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\|/ | |
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v v |
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basic.target <emphasis>rescue.target</emphasis> |
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| |
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________v____________________ |
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/ | \ |
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| | | |
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v v v |
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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 v
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| | <emphasis>multi-user.target</emphasis>
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emergency.service | | |
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| \_____________ | _____________/
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v \|/
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<emphasis>emergency.target</emphasis> v
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<emphasis>graphical.target</emphasis></programlisting>
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<para>Target units that are commonly used as boot targets are
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<emphasis>emphasized</emphasis>. These units are good choices as
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goal targets, for example by passing them to the
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<varname>systemd.unit=</varname> kernel command line 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> to them.
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</para>
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<para><filename>timers.target</filename> is pulled-in by
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<filename>basic.target</filename> asynchronously. This allows
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timers units to depend on services which become only available
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later in boot.</para>
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</refsect1>
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<refsect1>
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<title>User manager startup</title>
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<para>The system manager starts the <filename>user@<replaceable>uid</replaceable>.service</filename> unit
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for each user, which launches a separate unprivileged instance of <command>systemd</command> for each
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user — the user manager. Similarly to the system manager, the user manager starts units which are pulled
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in by <filename>default.target</filename>. The following chart is a structural overview of the well-known
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user units. For non-graphical sessions, <filename>default.target</filename> is used. Whenever the user
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logs into a graphical session, the login manager will start the
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<filename>graphical-session.target</filename> target that is used to pull in units required for the
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graphical session. A number of targets (shown on the right side) are started when specific hardware is
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available to the user.</para>
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<programlisting>
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(various (various (various
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timers...) paths...) sockets...) (sound devices)
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| | | |
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v v v v
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timers.target paths.target sockets.target sound.target
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\______________ _|_________________/ (bluetooth devices)
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\ / |
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V v
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basic.target bluetooth.target
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__________/ \_______ (smartcard devices)
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/ \ |
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| | v
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| v smartcard.target
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v graphical-session-pre.target
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(various user services) | (printers)
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| v |
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| (services for the graphical session) v
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| | printer.target
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v v
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<emphasis>default.target</emphasis> graphical-session.target</programlisting>
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</refsect1>
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<refsect1>
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<title>Bootup in the initrd</title>
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<para>Systemd can be used in the initrd as well. It detects the initrd environment by checking for the
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<filename>/etc/initrd-release</filename> file. The default target in the initrd is
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<filename>initrd.target</filename>. The bootup process is identical to the system manager bootup until
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the target <filename>basic.target</filename>. After that, systemd executes the special target
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<filename>initrd.target</filename>.
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Before any file systems are mounted, the manager will determine whether the system shall resume from
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hibernation or proceed with normal boot. This is accomplished by
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<filename>systemd-hibernate-resume.service</filename> which must be finished before
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<filename>local-fs-pre.target</filename>, so no filesystems can be mounted before the check is complete.
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When the root device becomes available,
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<filename>initrd-root-device.target</filename> is reached.
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If the root device can be mounted at
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<filename>/sysroot</filename>, the
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<filename>sysroot.mount</filename> unit becomes active and
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<filename>initrd-root-fs.target</filename> is 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 entries
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marked with the <emphasis>x-initrd.mount</emphasis> option. All
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entries found are mounted below <filename>/sysroot</filename>, and
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<filename>initrd-fs.target</filename> is reached. The service
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<filename>initrd-cleanup.service</filename> isolates to the
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<filename>initrd-switch-root.target</filename>, where cleanup
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services can run. As the very last step, the
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<filename>initrd-switch-root.service</filename> is activated,
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which will cause the system to switch its root to
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<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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| initrd-root-device.target <emphasis>emergency.target</emphasis>
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| |
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| v
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| sysroot.mount
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| |
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| v
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| initrd-root-fs.target
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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 various target
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units with some minimal ordering 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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| veritysetup
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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 | | |
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systemd-reboot.service | | |
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| v | |
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| systemd-poweroff.service | |
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v | v |
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<emphasis>reboot.target</emphasis> | systemd-halt.service |
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v | v
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<emphasis>poweroff.target</emphasis> | systemd-kexec.service
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v |
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<emphasis>halt.target</emphasis> |
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v
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<emphasis>kexec.target</emphasis></programlisting>
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<para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
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<para>Note that
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<citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
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<filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
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phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
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remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
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robust fashion, without taking any service or unit concept into account anymore. At that point, regular
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applications and resources are generally terminated and released already, the second phase hence operates only as
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safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
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shutdown phase described above.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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