SYSTEMD-BOOT(7)                  systemd-boot                  SYSTEMD-BOOT(7)

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

       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 Boot Loader Specification[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 Boot Loader Specification[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 X boot manager, if installed

       o   The EFI Shell binary, if installed

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

       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.

       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 Boot
           Loader Interface[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 Boot Loader Interface[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 Automatic Boot

       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 initializaton, providing a full entropy pool during early

       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.

       The following keys may be used in the boot menu:

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

       <cr> (Enter)
           Boot selected entry

           Make selected entry the default

           Edit the kernel command line for selected entry

       +, t
           Increase the timeout before default entry is booted

       -, T
           Decrease the timeout

           Show systemd-boot, UEFI, and firmware versions

           Print status


       h, ?
           Show a help screen

           Reprint the screen

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



           OS X

           EFI shell

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

       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

           Abort the edit and quit the editor

           Clear the command line

       Ctrl+w, Alt+Backspace
           Delete word backwards

           Delete word forwards

       <cr> (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 +/-.

       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 Boot Loader Specification[1] are read from /loader/entries/ on the
       ESP and the Extended Boot Loader partition. Unified kernel boot entries
       following the Boot Loader Specification[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.

       The following EFI variables are defined, set and read by systemd-boot,
       under the vendor UUID "4a67b082-0a4c-41cf-b6c7-440b29bb8c4", for
       communication between the OS and the boot loader:

           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
  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.)

           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.

           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

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

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

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

           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.

           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

           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 Random Seeds[4] for further information.

           A binary random data field, that is used for generating the random
           see 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 Boot Loader Interface[2].

       systemd-boot implements a simple boot counting mechanism on top of the
       Boot Loader Specification[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

        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

        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 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 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.

       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.

       bootctl(1), loader.conf(5), systemd-bless-boot.service(8), systemd-
       boot-system-token.service(8), kernel-install(8), Boot Loader
       Specification[1], Boot Loader Interface[2]

        1. Boot Loader Specification

        2. Boot Loader Interface

        3. Automatic Boot Assessment

        4. Random Seeds

systemd 245                                                    SYSTEMD-BOOT(7)
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