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NAME | DESCRIPTION | KEY BINDINGS | FILES | EFI VARIABLES | BOOT COUNTING | SEE ALSO | NOTES | COLOPHON |
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:
· 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.
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 +/-.
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:
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].
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.
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
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
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) 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 man-pages@man7.org
systemd 246 SYSTEMD-BOOT(7)
Pages that refer to this page: 30-systemd-environment-d-generator(7) , systemd.index(7)