systemd/man/systemd-boot.xml

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<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://www.freedesktop.org/wiki/Software/systemd/BootLoaderInterface">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>
        <listitem><para>The menu time-out. Read by the boot loader. (Also, modified by it when the
        <keycap>t</keycap>/<keycap>T</keycap> keys are used, 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>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://www.freedesktop.org/wiki/Software/systemd/BootLoaderInterface">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://www.freedesktop.org/wiki/Software/systemd/BootLoaderInterface">Boot Loader Interface</ulink>
    </para>
  </refsect1>
</refentry>