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
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 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, as executable EFI binaries in /EFI/Linux/ on the
ESP and the Extended Boot Loader Partition.
•The Microsoft Windows EFI boot manager, if
•The Apple MacOS X boot manager, if
•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
•The boot manager integrates with the
command to implement features such as systemctl reboot
(for rebooting into a specific boot menu entry,
i.e. "reboot into Windows") and systemctl reboot
(for rebooting into the boot loader menu), by
implementing the Boot Loader Interface
. See systemctl(1)
•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)
•The boot manager provides information about the
boot time spent in UEFI firmware using the Boot Loader Interface
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
•The boot manager optionally reads a random seed
from the ESP partition, combines it with a 'system token' stored in a
persistant 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.
The following keys may be used in the boot menu:
↑ (Up), ↓ (Down), j, k, PageUp, PageDown, Home,
Navigate up/down in the entry list
Boot selected entry
Make selected entry the default
Edit the kernel command line for selected entry
Increase the timeout before default entry is booted
Decrease the timeout
Show systemd-boot, UEFI, and firmware versions
Show a help screen
Reprint the screen
The following keys may be used during bootup or in the boot menu
to directly boot a specific entry:
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
Abort the edit and quit the editor
Clear the command line
Delete word backwards
Delete word forwards
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 are read from /loader/entries/ on the
ESP and the Extended Boot Loader partition. Unified kernel boot entries
following the Boot Loader Specification 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
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:
If boot counting is enabled, contains the path to the
file in whose name the boot counters are encoded. Set by the boot loader.
uses this information to mark a boot as
successful as determined by the successful activation of the
boot-complete.target target unit.
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
Contains the partition UUID of the EFI System Partition
the boot loader was run from. Set by the boot loader.
uses this information to automatically
find the disk booted from, in order to discover various other partitions on
the same disk automatically.
A list of the identifiers of all discovered boot loader
entries. Set by the boot loader.
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)
commands make use of these
variables. The boot loader modifies LoaderEntryDefault
on request, when
the d key is used, see above.)
The identifier of the boot loader entry currently being
booted. Set by the boot loader.
A set of flags indicating the features the boot loader
supports. Set by the boot loader. Use bootctl(1)
to view this
Brief firmware information. Set by the boot loader. Use
to view this data.
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.
Brief information about the boot loader. Set by the boot
loader. Use bootctl(1)
to view this data.
Information about the time spent in various parts of the
boot loader. Set by the boot loader. Use systemd-analyze(1)
A binary random seed systemd-boot
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
(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 for further information.
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
Many of these variables are defined by the Boot Loader
systemd-boot implements a simple boot counting mechanism on top of the
Boot Loader Specification, 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
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
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
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'
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.
- Boot Loader Specification
- Boot Loader Interface
- Automatic Boot Assessment
- Random Seeds