path: root/man/systemd-boot.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<refentry id="systemd-boot" conditional='ENABLE_BOOTLOADER'
    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 textual menu to select the entry to boot and an editor for the kernel command
    line. <command>systemd-boot</command> supports systems with UEFI firmware only.</para>

    <para><command>systemd-boot</command> loads boot entry information from the EFI system partition (ESP),
    usually mounted at <filename>/efi/</filename>, <filename>/boot/</filename>, or
    <filename>/boot/efi/</filename> during OS runtime, as well as from the Extended Boot Loader partition
    (XBOOTLDR) if it exists (usually mounted to <filename>/boot/</filename>). Configuration file fragments,
    kernels, initrds and other EFI images to boot generally need to reside on the ESP or the Extended Boot
    Loader partition. Linux kernels must be built with <option>CONFIG_EFI_STUB</option> to be able to be
    directly executed as an EFI image. During boot <command>systemd-boot</command> automatically assembles a
    list of boot entries from the following sources:</para>

    <itemizedlist>
      <listitem><para>Boot entries defined with <ulink
      url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink> Type #1
      description files located in <filename>/loader/entries/</filename> on the ESP and the Extended Boot
      Loader Partition. These usually describe Linux kernel images with associated initrd images, but
      alternatively may also describe other arbitrary EFI executables.</para></listitem>

      <listitem><para>Unified kernel images, <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot
      Loader Specification</ulink> Type #2, which are executable EFI binaries in
      <filename>/EFI/Linux/</filename> on the ESP and the Extended Boot Loader Partition.</para></listitem>

      <listitem><para>The Microsoft Windows EFI boot manager, if installed.</para></listitem>

      <listitem><para>The Apple macOS boot manager, if installed.</para></listitem>

      <listitem><para>The EFI Shell binary, if installed.</para></listitem>

      <listitem><para>A <literal>Reboot Into Firmware Interface option</literal>, if supported by the UEFI
      firmware.</para></listitem>

      <listitem><para>Secure Boot variables enrollment if the UEFI firmware is in setup-mode and files are provided
      on the ESP.</para></listitem>
    </itemizedlist>

    <para><command>systemd-boot</command> supports the following features:</para>

    <itemizedlist>
      <listitem><para>Basic boot manager configuration changes (such as timeout
      configuration, default boot entry selection, …) may be made directly from the boot loader UI at
      boot-time, as well as during system runtime with EFI variables.</para></listitem>

      <listitem><para>The boot manager integrates with the <command>systemctl</command> command to implement
      features such as <command>systemctl reboot --boot-loader-entry=…</command> (for rebooting into a
      specific boot menu entry, i.e. "reboot into Windows") and <command>systemctl reboot
      --boot-loader-menu=…</command> (for rebooting into the boot loader menu), by implementing the <ulink
      url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. See
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
      details.</para></listitem>

      <listitem><para>An EFI variable set by the boot loader informs the OS about the EFI System Partition used
      during boot. This is then used to automatically mount the correct EFI System Partition to
      <filename>/efi/</filename> or <filename>/boot/</filename> during OS runtime. See
      <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      for details.</para></listitem>

      <listitem><para>The boot manager provides information about the boot 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></listitem>

      <listitem><para>The boot manager implements boot counting and automatic fallback to older, working boot
      entries on failure. See <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot
      Assessment</ulink>.</para></listitem>

      <listitem><para>The boot manager optionally reads a random seed from the ESP partition, combines it
      with a 'system token' stored in a persistent EFI variable and derives a random seed to use by the OS as
      entropy pool initialization, providing a full entropy pool during early boot.</para></listitem>

      <listitem><para>The boot manager allows for Secure Boot variables to be enrolled if the UEFI firmware is
      in setup-mode. Additionally, variables can be automatically enrolled if configured.</para></listitem>
    </itemizedlist>

    <para><citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    may be used from a running system to locate the ESP and the Extended Boot Loader Partition, list
    available entries, and install <command>systemd-boot</command> itself.</para>

    <para><citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    may be used to copy kernel images onto the ESP or the Extended Boot Loader Partition and to generate
    description files compliant with the Boot Loader
    Specification.</para>

    <para><citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    may be used as UEFI boot stub for executed kernels, which is useful to show graphical boot splashes
    before transitioning into the Linux world. It is also capable of automatically picking up auxiliary
    credential files (for boot parameterization) and system extension images, as companion files to the
    booted kernel images.</para>
  </refsect1>

  <refsect1>
    <title>Key bindings</title>
    <para>The following keys may be used in the boot menu:</para>

    <!-- Developer commands Q/v/Ctrl+l deliberately not advertised. -->

    <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>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>↵</keycap> (Enter)</term>
        <term><keycap>→</keycap> (Right)</term>
        <listitem><para>Boot selected entry</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>d</keycap></term>
        <listitem><para>Make selected entry the default</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>e</keycap></term>
        <listitem><para>Edit the kernel command line for selected entry</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>+</keycap></term>
        <term><keycap>t</keycap></term>
        <listitem><para>Increase the timeout before default entry is booted</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>-</keycap></term>
        <term><keycap>T</keycap></term>
        <listitem><para>Decrease the timeout</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>r</keycap></term>
        <listitem><para>Change screen resolution, skipping any unsupported modes.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>R</keycap></term>
        <listitem><para>Reset screen resolution to firmware or configuration file default.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>p</keycap></term>
        <listitem><para>Print status</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>h</keycap></term>
        <term><keycap>?</keycap></term>
        <term><keycap>F1</keycap></term>
        <listitem><para>Show a help screen</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>f</keycap></term>
        <listitem><para>Reboot into firmware interface.</para>

        <para>For compatibility with the keybindings of several firmware implementations this operation
        may also be reached with <keycap>F2</keycap>, <keycap>F10</keycap>, <keycap>Del</keycap> and
        <keycap>Esc</keycap>.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycombo><keycap>Shift</keycap><keycap>o</keycap></keycombo></term>
        <listitem><para>Power off the system.</para>

        <xi:include href="version-info.xml" xpointer="v255"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycombo><keycap>Shift</keycap><keycap>b</keycap></keycombo></term>
        <listitem><para>Reboot the system.</para>

        <xi:include href="version-info.xml" xpointer="v255"/></listitem>
      </varlistentry>
    </variablelist>

    <para>The following keys may be pressed 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>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>w</keycap></term>
        <listitem><para>Windows</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>a</keycap></term>
        <listitem><para>macOS</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>s</keycap></term>
        <listitem><para>EFI shell</para>

        <xi:include href="version-info.xml" xpointer="v239"/></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>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>
    </variablelist>

    <para>The boot menu is shown when a non-zero menu timeout has been configured. If the menu timeout has
    been set to zero, it is sufficient to press any key — before the boot loader initializes — to bring up
    the boot menu, except for the keys listed immediately above as they directly boot into the selected boot
    menu item. Note that depending on the firmware implementation the time window where key presses are
    accepted before the boot loader initializes might be short. If the window is missed, reboot and try
    again, possibly pressing a suitable key (e.g. the space bar) continuously; on most systems it should be
    possible to hit the time window after a few attempts. To avoid this problem, consider setting a non-zero
    timeout, thus showing the boot menu unconditionally. Some desktop environments might offer an option to
    directly boot into the boot menu, to avoid the problem altogether. Alternatively, use the command line
    <command>systemctl reboot --boot-loader-menu=0</command> from the shell.</para>

    <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>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>Esc</keycap></term>
        <term><keycombo><keycap>Ctrl</keycap><keycap>c</keycap></keycombo></term>
        <listitem><para>Abort the edit and quit the editor</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycombo><keycap>Ctrl</keycap><keycap>k</keycap></keycombo></term>
        <listitem><para>Clear the command line forwards</para>

        <xi:include href="version-info.xml" xpointer="v239"/></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>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycombo><keycap>Ctrl</keycap><keycap>Del</keycap></keycombo></term>
        <term><keycombo><keycap>Alt</keycap><keycap>d</keycap></keycombo></term>
        <listitem><para>Delete word forwards</para>

        <xi:include href="version-info.xml" xpointer="v239"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><keycap>↵</keycap> (Enter)</term>
        <listitem><para>Boot entry with the edited command line</para>

        <xi:include href="version-info.xml" xpointer="v239"/></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 <command>systemd-boot</command> processes generally reside on the UEFI ESP which is
    usually mounted to <filename>/efi/</filename>, <filename>/boot/</filename> or
    <filename>/boot/efi/</filename> during OS runtime. It also processes files on the Extended Boot Loader
    partition which is typically mounted to <filename>/boot/</filename>, if it
    exists.</para>

    <para><command>systemd-boot</command> 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>.</para>

    <para>Boot entry description files following the <ulink
    url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink> are read from
    <filename>/loader/entries/</filename> on the ESP and the Extended Boot Loader partition.</para>

    <para>Unified kernel boot entries following the <ulink
    url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink> are read from
    <filename>/EFI/Linux/</filename> on the ESP and the Extended Boot Loader partition.</para>

    <para>Optionally, a random seed for early boot entropy pool provisioning is stored in
    <filename>/loader/random-seed</filename> in the ESP.</para>

    <para>During initialization, <command>sd-boot</command> automatically loads all driver files placed in
    the <filename>/EFI/systemd/drivers/</filename> directory of the ESP. The files placed there must have an
    extension of the EFI architecture ID followed by <filename>.efi</filename> (e.g. for x86-64 this means a
    suffix of <filename>x64.efi</filename>). This may be used to automatically load file system drivers and
    similar, to extend the native firmware support.</para>

    <para>Enrollment of Secure Boot variables can be performed manually or automatically if files are available
    under <filename>/loader/keys/<replaceable>NAME</replaceable>/{db,KEK,PK}.auth</filename>, <replaceable>NAME</replaceable>
    being the display name for the set of variables in the menu. If one of the sets is named <filename>auto</filename>
    then it might be enrolled automatically depending on whether <literal>secure-boot-enroll</literal> is set
    to force or not.</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-b6c7-440b29bb8c4f</literal>, for communication between the boot
    loader and the OS:</para>

    <variablelist class='efi-variables'>
      <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>

        <xi:include href="version-info.xml" xpointer="v240"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LoaderConfigTimeout</varname></term>
        <term><varname>LoaderConfigTimeoutOneShot</varname></term>
        <listitem><para>The menu timeout 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>

        <xi:include href="version-info.xml" xpointer="v240"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LoaderConfigConsoleMode</varname></term>
        <listitem><para>The numerical menu console mode. Read by the boot loader. <varname>LoaderConfigConsoleMode</varname>
        is maintained persistently. <varname>LoaderConfigConsoleMode</varname> may be manipulated with the
        <keycap>r</keycap>/<keycap>R</keycap> keys, see above.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LoaderEntryLastBooted</varname></term>

        <listitem><para>The identifier of the boot loader entry last attempted. Set and read by the boot loader,
        only when <filename>/loader/loader.conf</filename> has default set to <literal>@saved</literal>. See
        <citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>

        <para> The boot loader will ensure <varname>LoaderEntryLastBooted</varname> is up-to date for every boot,
        updating it as needed and will omit changing it all together when <varname>LoaderEntryOneShot</varname>
        is set.</para>

        <para>The boot loader reads the variable, which takes higher priority than
        <varname>LoaderEntryDefault</varname>. The variable is ignored when <varname>LoaderEntryOneShot</varname>
        is set.</para>

        <para><varname>LoaderEntryLastBooted</varname> cannot be used as indication that the last boot was
        successful or not.</para>

        <xi:include href="version-info.xml" xpointer="v250"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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>

        <xi:include href="version-info.xml" xpointer="v240"/></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. </para>

        <xi:include href="version-info.xml" xpointer="v240"/></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>LoaderSystemToken</varname></term>

        <listitem><para>A binary random data field, that is used for generating the random seed to pass to
        the OS (see above). Note that this random data is generally only generated once, during OS
        installation, and is then never updated again.</para>

        <xi:include href="version-info.xml" xpointer="v243"/></listitem>
      </varlistentry>
    </variablelist>

    <para>Many of these variables are defined by the <ulink
    url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
  </refsect1>

  <refsect1>
    <title>Boot Counting</title>

    <para><command>systemd-boot</command> implements a simple boot counting mechanism on top of the <ulink
    url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>, for automatic and unattended
    fallback to older kernel versions/boot loader entries when a specific entry continuously 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 beginning of the list, and entries in 'good' or 'indeterminate' at the end. 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 bottom
    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>Using <command>systemd-boot</command> in virtual machines</title>

    <para>When using qemu with OVMF (UEFI Firmware for virtual machines) the <option>-kernel</option> switch
    works not only for linux kernels, but for any EFI binary, including sd-boot and unified linux
    kernels. Example command line for loading <command>systemd-boot</command> on x64:</para>

    <para>
      <command>qemu-system-x86_64 <replaceable>[ ... ]</replaceable>
      -kernel /usr/lib/systemd/boot/efi/systemd-bootx64.efi</command>
    </para>

    <para>systemd-boot will detect that it was started directly instead of being loaded from ESP and will
    search for the ESP in that case, taking into account boot order information from the hypervisor (if
    available).</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>systemd-boot-random-seed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>,
      <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
      <ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink>
    </para>
  </refsect1>
</refentry>