systemd-boot(7) — Linux manual page


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

NAME         top

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

DESCRIPTION         top

       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

       ·   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.

       ·   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.

       ·   The Microsoft Windows EFI boot manager, if installed

       ·   The Apple MacOS X boot manager, if installed

       ·   The EFI Shell binary, if installed

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

       systemd-boot supports the following features:

       ·   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.

       ·   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.

       ·   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.

       ·   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).

       ·   The boot manager implements boot counting and automatic fallback
           to older, working boot entries on failure. See Automatic Boot

       ·   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         top

       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

           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 pressed 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

       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 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

           Abort the edit and quit the editor

           Clear the command line

       Ctrl+w, Alt+Backspace
           Delete word backwards

           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         top

       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.

EFI VARIABLES         top

       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 above.

           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 data.

           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 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 Boot Loader Interface[2].

BOOT COUNTING         top

       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 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 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         top

       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]

NOTES         top

        1. Boot Loader Specification

        2. Boot Loader Interface

        3. Automatic Boot Assessment

        4. Random Seeds

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ on 2020-07-14.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-07-14.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to

systemd 246                                                  SYSTEMD-BOOT(7)

Pages that refer to this page: bootctl(1)loader.conf(5)org.freedesktop.login1(5)30-systemd-environment-d-generator(7)bootup(7)systemd.directives(7)systemd.index(7)kernel-install(8)systemd-bless-boot(8)systemd-bless-boot-generator(8)systemd-bless-boot.service(8)systemd-boot-system-token.service(8)systemd-random-seed(8)systemd-random-seed.service(8)