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NAME | DESCRIPTION | KEY BINDINGS | FILES | EFI VARIABLES | SMBIOS TYPE 11 STRINGS | BOOT COUNTING | USING SYSTEMD-BOOT IN VIRTUAL MACHINES | SEE ALSO | NOTES | COLOPHON |
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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 textual 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 (XBOOTLDR) 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] Type
#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 other arbitrary EFI
executables.
• Unified kernel images, Boot Loader Specification[1] Type #2,
which are 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 Firmware Interface option", if supported by
the UEFI firmware.
• Secure Boot variables enrollment if the UEFI firmware is in
setup-mode and files are provided on the ESP.
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.
• The boot manager allows for Secure Boot variables to be
enrolled if the UEFI firmware is in setup-mode. Additionally,
variables can be automatically enrolled if configured.
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.
systemd-stub(7) may be used as UEFI boot stub for executed
kernels, which is useful to show graphical boot splashes before
transitioning into the Linux world. It is also capable of
automatically picking up auxiliary credential files (for boot
parameterization) and system extension images, as companion files
to the booted kernel images.
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
Added in version 239.
↵ (Enter), → (Right)
Boot selected entry
Added in version 239.
d
Make selected entry the default
Added in version 239.
e
Edit the kernel command line for selected entry
Added in version 239.
+, t
Increase the timeout before default entry is booted
Added in version 239.
-, T
Decrease the timeout
Added in version 239.
r
Change screen resolution, skipping any unsupported modes.
Added in version 250.
R
Reset screen resolution to firmware or configuration file
default.
Added in version 250.
p
Print status
Added in version 250.
h, ?, F1
Show a help screen
Added in version 239.
f
Reboot into firmware interface.
For compatibility with the keybindings of several firmware
implementations this operation may also be reached with F2,
F10, Del and Esc.
Added in version 250.
Shift+o
Power off the system.
Added in version 255.
Shift+b
Reboot the system.
Added in version 255.
The following keys may be pressed during bootup or in the boot
menu to directly boot a specific entry:
l
Linux
Added in version 239.
w
Windows
Added in version 239.
a
macOS
Added in version 239.
s
EFI shell
Added in version 239.
1, 2, 3, 4, 5, 6, 7, 8, 9
Boot entry number 1 ... 9
Added in version 239.
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
Added in version 239.
Esc, Ctrl+c
Abort the edit and quit the editor
Added in version 239.
Ctrl+k
Clear the command line forwards
Added in version 239.
Ctrl+w, Alt+Backspace
Delete word backwards
Added in version 239.
Ctrl+Del, Alt+d
Delete word forwards
Added in version 239.
↵ (Enter)
Boot entry with the edited command line
Added in version 239.
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.
During initialization, sd-boot automatically loads all driver
files placed in the /EFI/systemd/drivers/ directory of the ESP.
The files placed there must have an extension of the EFI
architecture ID followed by .efi (e.g. for x86-64 this means a
suffix of x64.efi). This may be used to automatically load file
system drivers and similar, to extend the native firmware
support.
Enrollment of Secure Boot variables can be performed manually or
automatically if files are available under
/loader/keys/NAME/{db,dbx,KEK,PK}.auth, NAME being the display
name for the set of variables in the menu. If one of the sets is
named auto then it might be enrolled automatically depending on
whether "secure-boot-enroll" is set to force or not.
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 boot loader and the OS:
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.
Added in version 240.
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.
Added in version 240.
LoaderConfigConsoleMode
The numerical menu console mode. Read by the boot loader.
LoaderConfigConsoleMode is maintained persistently.
LoaderConfigConsoleMode may be manipulated with the r/R keys,
see above.
Added in version 250.
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.
Added in version 240.
LoaderEntries
A list of the identifiers of all discovered boot loader
entries. Set by the boot loader.
Added in version 240.
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.
Added in version 240.
LoaderEntryLastBooted
The identifier of the boot loader entry last attempted. Set
and read by the boot loader, only when /loader/loader.conf
has default set to "@saved". See loader.conf(5).
The boot loader will ensure LoaderEntryLastBooted is up-to
date for every boot, updating it as needed and will omit
changing it all together when LoaderEntryOneShot is set.
The boot loader reads the variable, which takes higher
priority than LoaderEntryDefault. The variable is ignored
when LoaderEntryOneShot is set.
LoaderEntryLastBooted cannot be used as indication that the
last boot was successful or not.
Added in version 250.
LoaderEntrySelected
The identifier of the boot loader entry currently being
booted. Set by the boot loader.
Added in version 240.
LoaderFeatures
A set of flags indicating the features the boot loader
supports. Set by the boot loader. Use bootctl(1) to view this
data.
Added in version 240.
LoaderFirmwareInfo, LoaderFirmwareType
Brief firmware information. Set by the boot loader. Use
bootctl(1) to view this data.
Added in version 240.
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.
Added in version 240.
LoaderInfo
Brief information about the boot loader. Set by the boot
loader. Use bootctl(1) to view this data.
Added in version 240.
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.
Added in version 240.
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.
Added in version 243.
Many of these variables are defined by the Boot Loader
Interface[2].
systemd-boot can be configured using SMBIOS Type 11 strings.
Applicable strings consist of a name, followed by "=", followed
by the value. Unless systemd-boot detects it is running inside a
confidential computing environment, systemd-boot will search the
table for a string with a specific name, and if found, use its
value. The following strings are read:
io.systemd.boot.kernel-cmdline-extra
If set, the value of this string is added to the list of
kernel command line arguments for Boot Loader Specification
Type 1 entries that are measured in PCR12 and passed to the
kernel.
Added in version 256.
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.
When using qemu with OVMF (UEFI Firmware for virtual machines)
the -kernel switch works not only for linux kernels, but for any
EFI binary, including sd-boot and unified linux kernels. Example
command line for loading systemd-boot on x64:
qemu-system-x86_64 [ ... ] -kernel
/usr/lib/systemd/boot/efi/systemd-bootx64.efi
systemd-boot will detect that it was started directly instead of
being loaded from ESP and will search for the ESP in that case,
taking into account boot order information from the hypervisor
(if available).
bootctl(1), loader.conf(5), systemd-bless-boot.service(8),
systemd-boot-random-seed.service(8), kernel-install(8),
systemd-stub(7), Boot Loader Specification[1], Boot Loader
Interface[2], TPM2 PCR Measurements Made by systemd[4]
1. Boot Loader Specification
https://uapi-group.org/specifications/specs/boot_loader_specification
2. Boot Loader Interface
https://systemd.io/BOOT_LOADER_INTERFACE
3. Automatic Boot Assessment
https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT
4. TPM2 PCR Measurements Made by systemd
https://systemd.io/TPM2_PCR_MEASUREMENTS
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 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) 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 257~devel SYSTEMD-BOOT(7)
Pages that refer to this page: bootctl(1), systemd-cryptenroll(1), ukify(1), loader.conf(5), org.freedesktop.login1(5), bootup(7), systemd.directives(7), systemd.index(7), systemd-stub(7), kernel-install(8), systemd-bless-boot-generator(8), systemd-bless-boot.service(8), systemd-boot-random-seed.service(8), systemd-random-seed.service(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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