sd-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:
o 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.
o 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.
o The Microsoft Windows EFI boot manager, if installed
o The Apple MacOS X boot manager, if installed
o The EFI Shell binary, if installed
o A reboot into the UEFI firmware setup option, if supported by the
firmware
systemd-boot supports the following features:
o 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.
o 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.
o 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.
o 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).
o The boot manager implements boot counting and automatic fallback to
older, working boot entries on failure. See Automatic Boot
Assessment[3].
o 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 initializaton, 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), v (Down), j, k, PageUp, PageDown, Home, End
Navigate up/down in the entry list
<cr> (Enter)
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, ?
Show a help screen
Ctrl+l
Reprint the screen
The following keys may be used 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
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
<cr> (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
see 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 end of the
list, and entries in 'good' or 'indeterminate' at the beginning. 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 top 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 245 SYSTEMD-BOOT(7)
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