Ubuntu Manpage: systemd-boot, sd-boot - A simple UEFI boot manager

Provided by: systemd_249.11-0ubuntu3.12_amd64

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 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 EFI System Partition
used during boot. This is then used to automatically mount the correct EFI System
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
           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 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-b6c7-440b29bb8c4f", 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.

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.

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