SYSTEMD-BOOT(7)                                   SYSTEMD-BOOT(7)

     NAME
          systemd-boot, sd-boot - A simple UEFI boot manager

     DESCRIPTION
          systemd-boot (short: sd-boot) 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.

          systemd-boot loads boot entry information from the EFI
          system partition (ESP), usually mounted at /efi/, /boot/, or
          /boot/efi/ during OS runtime, as well as from the Extended
          Boot Loader partition if it exists (usually mounted to
          /boot/). 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 CONFIG_EFI_STUB 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:

          +o   Boot entries defined with m[blue]Boot Loader
              Specificationm[][1] description files located in
              /loader/entries/ on the ESP and the Extended Boot Loader
              Partition. These usually describe Linux kernel images
              with associated initrd images, but alternatively may
              also describe arbitrary other EFI executables.

          +o   Unified kernel images following the m[blue]Boot Loader
              Specificationm[][1], as executable EFI binaries in
              /EFI/Linux/ on the ESP and the Extended Boot Loader
              Partition.

          +o   The Microsoft Windows EFI boot manager, if installed

          +o   The Apple macOS boot manager, if installed

          +o   The EFI Shell binary, if installed

          +o   A reboot into the UEFI firmware setup option, if
              supported by the firmware

          systemd-boot supports the following features:

          +o   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.

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          +o   The boot manager integrates with the systemctl command
              to implement features such as systemctl reboot
              --boot-loader-entry=...  (for rebooting into a specific
              boot menu entry, i.e. "reboot into Windows") and
              systemctl reboot --boot-loader-menu=...  (for rebooting
              into the boot loader menu), by implementing the
              m[blue]Boot Loader Interfacem[][2]. See systemctl(1) for
              details.

          +o   An EFI variable set by the boot loader informs the OS
              about the ESP partition used during boot. This is then
              used to automatically mount the correct ESP partition to
              /efi/ or /boot/ during OS runtime. See systemd-gpt-
              auto-generator(8) for details.

          +o   The boot manager provides information about the boot
              time spent in UEFI firmware using the m[blue]Boot Loader
              Interfacem[][2]. This information can be displayed using
              systemd-analyze(1).

          +o   The boot manager implements boot counting and automatic
              fallback to older, working boot entries on failure. See
              m[blue]Automatic Boot Assessmentm[][3].

          +o   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.

          bootctl(1) may be used from a running system to locate the
          ESP and the Extended Boot Loader Partition, list available
          entries, and install systemd-boot itself.

          kernel-install(8) 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.

     KEY BINDINGS
          The following keys may be used in the boot menu:

          ↑ (Up), ↓ (Down), j, k, PageUp, PageDown, Home, End
              Navigate up/down in the entry list

          ↵ (Enter), → (Right)
              Boot selected entry

          d
              Make selected entry the default

          e

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              Edit the kernel command line for selected entry

          +, t
              Increase the timeout before default entry is booted

          -, T
              Decrease the timeout

          v
              Show systemd-boot, UEFI, and firmware versions

          P
              Print status

          Q
              Quit

          h, ?, F1
              Show a help screen

          Ctrl+l
              Reprint the screen

          The following keys may be pressed during bootup or in the
          boot menu to directly boot a specific entry:

          l
              Linux

          w
              Windows

          a
              macOS

          s
              EFI shell

          1, 2, 3, 4, 5, 6, 7, 8, 9
              Boot entry number 1 ... 9

          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

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     SYSTEMD-BOOT(7)                                   SYSTEMD-BOOT(7)

          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
          systemctl reboot --boot-loader-menu=0 from the shell.

          In the editor, most keys simply insert themselves, but the
          following keys may be used to perform additional actions:

          ← (Left), → (Right), Home, End
              Navigate left/right

          Esc
              Abort the edit and quit the editor

          Ctrl+k
              Clear the command line

          Ctrl+w, Alt+Backspace
              Delete word backwards

          Alt+d
              Delete word forwards

          ↵ (Enter)
              Boot entry with the edited command line

          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 +/-.

     FILES
          The files systemd-boot processes generally reside on the
          UEFI ESP which is usually mounted to /efi/, /boot/ or
          /boot/efi/ during OS runtime. It also processes files on the
          Extended Boot Loader partition which is typically mounted to
          /boot/, if it exists.  systemd-boot reads runtime
          configuration such as the boot timeout and default entry
          from /loader/loader.conf on the ESP (in combination with
          data read from EFI variables). See loader.conf(5). Boot
          entry description files following the m[blue]Boot Loader
          Specificationm[][1] are read from /loader/entries/ on the
          ESP and the Extended Boot Loader partition. Unified kernel
          boot entries following the m[blue]Boot Loader
          Specificationm[][1] are read from /EFI/Linux/ on the ESP and
          the Extended Boot Loader partition. Optionally, a random
          seed for early boot entropy pool provisioning is stored in
          /loader/random-seed in the ESP.

     EFI VARIABLES
          The following EFI variables are defined, set and read by

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     SYSTEMD-BOOT(7)                                   SYSTEMD-BOOT(7)

          systemd-boot, under the vendor UUID
          "4a67b082-0a4c-41cf-b6c7-440b29bb8c4f", for communication
          between the OS and the boot loader:

          LoaderBootCountPath
              If boot counting is enabled, contains the path to the
              file in whose name the boot counters are encoded. Set by
              the boot loader.  systemd-bless-boot.service(8) uses
              this information to mark a boot as successful as
              determined by the successful activation of the
              boot-complete.target target unit.

          LoaderConfigTimeout, LoaderConfigTimeoutOneShot
              The menu timeout in seconds. Read by the boot loader.
              LoaderConfigTimeout is maintained persistently, while
              LoaderConfigTimeoutOneShot is a one-time override which
              is read once (in which case it takes precedence over
              LoaderConfigTimeout) and then removed.
              LoaderConfigTimeout may be manipulated with the t/T
              keys, see above.

          LoaderDevicePartUUID
              Contains the partition UUID of the EFI System Partition
              the boot loader was run from. Set by the boot loader.
              systemd-gpt-auto-generator(8) uses this information to
              automatically find the disk booted from, in order to
              discover various other partitions on the same disk
              automatically.

          LoaderEntries
              A list of the identifiers of all discovered boot loader
              entries. Set by the boot loader.

          LoaderEntryDefault, LoaderEntryOneShot
              The identifier of the default boot loader entry. Set
              primarily by the OS and read by the boot loader.
              LoaderEntryOneShot sets the default entry for the next
              boot only, while LoaderEntryDefault sets it persistently
              for all future boots.  bootctl(1)'s set-default and
              set-oneshot commands make use of these variables. The
              boot loader modifies LoaderEntryDefault on request, when
              the d key is used, see above.

          LoaderEntrySelected
              The identifier of the boot loader entry currently being
              booted. Set by the boot loader.

          LoaderFeatures
              A set of flags indicating the features the boot loader
              supports. Set by the boot loader. Use bootctl(1) to view
              this data.

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     SYSTEMD-BOOT(7)                                   SYSTEMD-BOOT(7)

          LoaderFirmwareInfo, LoaderFirmwareType
              Brief firmware information. Set by the boot loader. Use
              bootctl(1) to view this data.

          LoaderImageIdentifier
              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 bootctl(1)
              to view this data.

          LoaderInfo
              Brief information about the boot loader. Set by the boot
              loader. Use bootctl(1) to view this data.

          LoaderTimeExecUSec, LoaderTimeInitUSec, LoaderTimeMenuUsec
              Information about the time spent in various parts of the
              boot loader. Set by the boot loader. Use systemd-
              analyze(1) to view this data.

          LoaderRandomSeed
              A binary random seed systemd-boot may optionally pass to
              the OS. This is a volatile EFI variable that is hashed
              at boot from the combination of a random seed stored in
              the ESP (in /loader/random-seed) and a "system token"
              persistently stored in the EFI variable
              LoaderSystemToken (see below). During early OS boot the
              system manager reads this variable and passes it to the
              OS kernel's random pool, crediting the full entropy it
              contains. This is an efficient way to ensure the system
              starts up with a fully initialized kernel random pool -
              as early as the initial RAM disk phase.  systemd-boot
              reads the random seed from the ESP, combines it with the
              "system token", and both derives a new random seed to
              update in-place the seed stored in the ESP, and the
              random seed to pass to the OS from it via SHA256 hashing
              in counter mode. This ensures that different physical
              systems that boot the same "golden" OS image - i.e.
              containing the same random seed file in the ESP - will
              still pass a different random seed to the OS. It is made
              sure the random seed stored in the ESP is fully
              overwritten before the OS is booted, to ensure different
              random seed data is used between subsequent boots.

              See m[blue]Random Seedsm[][4] for further information.

          LoaderSystemToken
              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.

          Many of these variables are defined by the m[blue]Boot

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     SYSTEMD-BOOT(7)                                   SYSTEMD-BOOT(7)

          Loader Interfacem[][2].

     BOOT COUNTING
          systemd-boot implements a simple boot counting mechanism on
          top of the m[blue]Boot Loader Specificationm[][1], 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 "+" followed by one or two
          numbers (if two they need to be separated by a "-"), before
          the .conf or .efi 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:

           1. 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.

           2. 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.

           3. 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
              systemd-bless-boot.service(8) service moves the
              currently booted entry from 'indeterminate' into 'good'
              state when a boot attempt completed successfully.

          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).

          Example: let's say a boot loader entry file foo.conf is set
          up for 3 boot tries. The installer will hence create it
          under the name foo+3.conf. On first boot, the boot loader
          will rename it to foo+2-1.conf. If that boot does not
          complete successfully, the boot loader will rename it to

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     SYSTEMD-BOOT(7)                                   SYSTEMD-BOOT(7)

          foo+1-2.conf on the following boot. If that fails too, it
          will finally be renamed foo+0-3.conf 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
          foo.conf by the OS, so that it is considered 'good' from
          then on.

          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.

          The kernel-install(8) kernel install framework optionally
          sets the initial 'tries left' counter to the value specified
          in /etc/kernel/tries when a boot loader entry is first
          created.

     SEE ALSO
          bootctl(1), loader.conf(5), systemd-bless-boot.service(8),
          systemd-boot-system-token.service(8), kernel-install(8),
          m[blue]Boot Loader Specificationm[][1], m[blue]Boot Loader
          Interfacem[][2]

     NOTES
           1. Boot Loader Specification
              https://systemd.io/BOOT_LOADER_SPECIFICATION

           2. Boot Loader Interface
              https://systemd.io/BOOT_LOADER_INTERFACE

           3. Automatic Boot Assessment
              https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT

           4. Random Seeds
              https://systemd.io/RANDOM_SEEDS

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