systemd-boot

SYSTEMD-BOOT(7)                   systemd-boot                   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:

       •   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
           firmware

       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
           Assessment[3].

       •   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
           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
           OS X

       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 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 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
       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:

       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.

       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 Random Seeds[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 Boot Loader Interface[2].

BOOT COUNTING
       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
       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
        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



systemd 246                                                      SYSTEMD-BOOT(7)