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systemd-stable/man/systemd-boot.xml
Lennart Poettering 14c4da2ffe
Merge pull request #10805 from poettering/migrate-boot-loader-interface 
migrate boot loader interface doc from wiki into markdown (split out of #10495)
2018-11-16 17:55:08 +01:00

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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->
<refentry id="systemd-boot" conditional='ENABLE_EFI'
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>systemd-boot</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-boot</refentrytitle>
<manvolnum>7</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-boot</refname>
<refname>sd-boot</refname>
<refpurpose>A simple UEFI boot manager</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<para><command>systemd-boot</command> (short: <command>sd-boot</command>) is a simple UEFI boot manager. It
provides a graphical menu to select the entry to boot and an editor for the kernel command line. systemd-boot
supports systems with UEFI firmware only.</para>
<para>systemd-boot loads boot entry information from the EFI system partition (ESP), usually mounted at
<filename>/boot</filename>, <filename>/efi</filename>, or <filename>/boot/efi</filename> during OS
runtime. Configuration file fragments, kernels, initrds and other EFI images to boot generally need to reside on
the ESP. Linux kernels must be built with <option>CONFIG_EFI_STUB</option> to be able to be directly executed as an
EFI image. During boot systemd-boot automatically assembles a list of boot entries from the following
sources:</para>
<itemizedlist>
<listitem><para>Boot entries defined with <ulink
url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader
Specification</ulink> description files located in <filename>/loader/entries/</filename> on the ESP. These
usually describe Linux kernel images with associated initrd images, but alternatively may also describe
arbitrary other EFI executables.</para></listitem>
<listitem><para>Unified kernel images following the <ulink
url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader
Specification</ulink>, as executable EFI binaries in <filename>/EFI/Linux/</filename> on the ESP.
</para></listitem>
<listitem><para>The Microsoft Windows EFI boot manager, if installed</para></listitem>
<listitem><para>The Apple MacOS X boot manager, if installed</para></listitem>
<listitem><para>The EFI Shell binary, if installed</para></listitem>
<listitem><para>A reboot into the UEFI firmware setup option, if supported by the firmware</para></listitem>
</itemizedlist>
<para><citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry> may be
used to copy kernel images onto the ESP and to generate description files compliant with the Boot Loader
Specification. <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
used from a running system to locate the ESP, list available entries, and install systemd-boot itself.</para>
<para>systemd-boot will provide information about the time spent in UEFI firmware using the <ulink
url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. This information can be displayed
using <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
</refsect1>
<refsect1>
<title>Key bindings</title>
<para>The following keys may be used in the boot menu:</para>
<variablelist>
<varlistentry>
<term><keycap>↑</keycap> (Up)</term>
<term><keycap>↓</keycap> (Down)</term>
<term><keycap>j</keycap></term>
<term><keycap>k</keycap></term>
<term><keycap>PageUp</keycap></term>
<term><keycap>PageDown</keycap></term>
<term><keycap>Home</keycap></term>
<term><keycap>End</keycap></term>
<listitem><para>Navigate up/down in the entry list</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>↵</keycap> (Enter)</term>
<listitem><para>Boot selected entry</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>d</keycap></term>
<listitem><para>Make selected entry the default</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>e</keycap></term>
<listitem><para>Edit the kernel command line for selected entry</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>+</keycap></term>
<term><keycap>t</keycap></term>
<listitem><para>Increase the timeout before default entry is booted</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>-</keycap></term>
<term><keycap>T</keycap></term>
<listitem><para>Decrease the timeout</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>v</keycap></term>
<listitem><para>Show systemd-boot, UEFI, and firmware versions</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>P</keycap></term>
<listitem><para>Print status</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>Q</keycap></term>
<listitem><para>Quit</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>h</keycap></term>
<term><keycap>?</keycap></term>
<listitem><para>Show a help screen</para></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Ctrl</keycap><keycap>l</keycap></keycombo></term>
<listitem><para>Reprint the screen</para></listitem>
</varlistentry>
</variablelist>
<para>The following keys may be used during bootup or in the boot menu to
directly boot a specific entry:</para>
<variablelist>
<varlistentry>
<term><keycap>l</keycap></term>
<listitem><para>Linux</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>w</keycap></term>
<listitem><para>Windows</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>a</keycap></term>
<listitem><para>OS X</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>s</keycap></term>
<listitem><para>EFI shell</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>1</keycap></term>
<term><keycap>2</keycap></term>
<term><keycap>3</keycap></term>
<term><keycap>4</keycap></term>
<term><keycap>5</keycap></term>
<term><keycap>6</keycap></term>
<term><keycap>7</keycap></term>
<term><keycap>8</keycap></term>
<term><keycap>9</keycap></term>
<listitem><para>Boot entry number 1 … 9</para></listitem>
</varlistentry>
</variablelist>
<para>In the editor, most keys simply insert themselves, but the following keys
may be used to perform additional actions:</para>
<variablelist>
<varlistentry>
<term><keycap>←</keycap> (Left)</term>
<term><keycap>→</keycap> (Right)</term>
<term><keycap>Home</keycap></term>
<term><keycap>End</keycap></term>
<listitem><para>Navigate left/right</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>Esc</keycap></term>
<listitem><para>Abort the edit and quit the editor</para></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Ctrl</keycap><keycap>k</keycap></keycombo></term>
<listitem><para>Clear the command line</para></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Ctrl</keycap><keycap>w</keycap></keycombo></term>
<term><keycombo><keycap>Alt</keycap><keycap>Backspace</keycap></keycombo></term>
<listitem><para>Delete word backwards</para></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Alt</keycap><keycap>d</keycap></keycombo></term>
<listitem><para>Delete word forwards</para></listitem>
</varlistentry>
<varlistentry>
<term><keycap>↵</keycap> (Enter)</term>
<listitem><para>Boot entry with the edited command line</para></listitem>
</varlistentry>
</variablelist>
<para>Note that unless configured otherwise in the UEFI firmware, systemd-boot will
use the US keyboard layout, so key labels might not match for keys like +/-.
</para>
</refsect1>
<refsect1>
<title>Files</title>
<para>The files systemd-boot reads generally reside on the UEFI ESP which is usually mounted to
<filename>/boot/</filename>, <filename>/efi/</filename> or <filename>/boot/efi</filename> during OS
runtime. systemd-boot reads runtime configuration such as the boot timeout and default entry from
<filename>/loader/loader.conf</filename> on the ESP (in combination with data read from EFI variables). See
<citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. Boot entry
description files following the <ulink
url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader
Specification</ulink> are read from <filename>/loader/entries/</filename> on the ESP. Unified kernel boot entries
following the <ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot
Loader Specification</ulink> are read from <filename>/EFI/Linux/</filename> on the ESP.</para>
</refsect1>
<refsect1>
<title>EFI Variables</title>
<para>The following EFI variables are defined, set and read by <command>systemd-boot</command>, under the vendor
UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4</literal>, for communication between the OS and the boot
loader:</para>
<variablelist>
<varlistentry>
<term><varname>LoaderBootCountPath</varname></term>
<listitem><para>If boot counting is enabled, contains the path to the file in whose name the boot counters are
encoded. Set by the boot
loader. <citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
uses this information to mark a boot as successful as determined by the successful activation of the
<filename>boot-complete.target</filename> target unit.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderConfigTimeout</varname></term>
<term><varname>LoaderConfigTimeoutOneShot</varname></term>
<listitem><para>The menu time-out in seconds. Read by the boot loader. <varname>LoaderConfigTimeout</varname>
is maintained persistently, while <varname>LoaderConfigTimeoutOneShot</varname> is a one-time override which is
read once (in which case it takes precedence over <varname>LoaderConfigTimeout</varname>) and then
removed. <varname>LoaderConfigTimeout</varname> may be manipulated with the
<keycap>t</keycap>/<keycap>T</keycap> keys, see above.)</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderDevicePartUUID</varname></term>
<listitem><para>Contains the partition UUID of the EFI System Partition the boot loader was run from. Set by
the boot
loader. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
uses this information to automatically find the disk booted from, in order to discover various other partitions
on the same disk automatically.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntries</varname></term>
<listitem><para>A list of the identifiers of all discovered boot loader entries. Set by the boot
loader.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntryDefault</varname></term>
<term><varname>LoaderEntryOneShot</varname></term>
<listitem><para>The identifier of the default boot loader entry. Set primarily by the OS and read by the boot
loader. <varname>LoaderEntryOneShot</varname> sets the default entry for the next boot only, while
<varname>LoaderEntryDefault</varname> sets it persistently for all future
boots. <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<option>set-default</option> and <option>set-oneshot</option> commands make use of these variables. The boot
loader modifies <varname>LoaderEntryDefault</varname> on request, when the <keycap>d</keycap> key is used, see
above.)</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntrySelected</varname></term>
<listitem><para>The identifier of the boot loader entry currently being booted. Set by the boot
loader.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderFeatures</varname></term>
<listitem><para>A set of flags indicating the features the boot loader supports. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderFirmwareInfo</varname></term>
<term><varname>LoaderFirmwareType</varname></term>
<listitem><para>Brief firmware information. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderImageIdentifier</varname></term>
<listitem><para>The path of executable of the boot loader used for the current boot, relative to the EFI System
Partition's root directory. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderInfo</varname></term>
<listitem><para>Brief information about the boot loader. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderTimeExecUSec</varname></term>
<term><varname>LoaderTimeInitUSec</varname></term>
<term><varname>LoaderTimeMenuUsec</varname></term>
<listitem><para>Information about the time spent in various parts of the boot loader. Set by the boot
loader. Use <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>
to view this data. These variables are defined by the <ulink
url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Boot Counting</title>
<para><command>systemd-boot</command> implements a simple boot counting mechanism on top of the <ulink
url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>, for automatic and unattended
fallback to older kernel versions/boot loader entries when a specific entry continously fails. Any boot loader
entry file and unified kernel image file that contains a <literal>+</literal> followed by one or two numbers (if
two they need to be separated by a <literal>-</literal>), before the <filename>.conf</filename> or
<filename>.efi</filename> suffix is subject to boot counting: the first of the two numbers ('tries left') is
decreased by one on every boot attempt, the second of the two numbers ('tries done') is increased by one (if 'tries
done' is absent it is considered equivalent to 0). Depending on the current value of these two counters the boot
entry is considered to be in one of three states:</para>
<orderedlist>
<listitem><para>If the 'tries left' counter of an entry is greater than zero the entry is considered to be in
'indeterminate' state. This means the entry has not completed booting successfully yet, but also hasn't been
determined not to work.</para></listitem>
<listitem><para>If the 'tries left' counter of an entry is zero it is considered to be in 'bad' state. This means
no further attempts to boot this item will be made (that is, unless all other boot entries are also in 'bad'
state), as all attempts to boot this entry have not completed successfully.</para></listitem>
<listitem><para>If the 'tries left' and 'tries done' counters of an entry are absent it is considered to be in
'good' state. This means further boot counting for the entry is turned off, as it successfully booted at least
once. The
<citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
service moves the currently booted entry from 'indeterminate' into 'good' state when a boot attempt completed
successfully.</para></listitem>
</orderedlist>
<para>Generally, when new entries are added to the boot loader, they first start out in 'indeterminate' state,
i.e. with a 'tries left' counter greater than zero. The boot entry remains in this state until either it managed to
complete a full boot successfully at least once (in which case it will be in 'good' state) — or the 'tries left'
counter reaches zero (in which case it will be in 'bad' state).</para>
<para>Example: let's say a boot loader entry file <filename>foo.conf</filename> is set up for 3 boot tries. The
installer will hence create it under the name <filename>foo+3.conf</filename>. On first boot, the boot loader will
rename it to <filename>foo+2-1.conf</filename>. If that boot does not complete successfully, the boot loader will
rename it to <filename>foo+1-2.conf</filename> on the following boot. If that fails too, it will finally be renamed
<filename>foo+0-3.conf</filename> by the boot loader on next boot, after which it will be considered 'bad'. If the
boot succeeds however the entry file will be renamed to <filename>foo.conf</filename> by the OS, so that it is
considered 'good' from then on.</para>
<para>The boot menu takes the 'tries left' counter into account when sorting the menu entries: entries in 'bad'
state are ordered at the end of the list, and entries in 'good' or 'indeterminate' at the beginning. The user can
freely choose to boot any entry of the menu, including those already marked 'bad'. If the menu entry to boot is
automatically determined, this means that 'good' or 'indeterminate' entries are generally preferred (as the top item of
the menu is the one booted by default), and 'bad' entries will only be considered if there are no 'good' or
'indeterminate' entries left.</para>
<para>The <citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry> kernel
install framework optionally sets the initial 'tries left' counter to the value specified in
<filename>/etc/kernel/tries</filename> when a boot loader entry is first created.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
<ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>
</para>
</refsect1>
</refentry>
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