systemd-bootsystemdsystemd-boot7systemd-bootsd-bootA simple UEFI boot managerDescriptionsystemd-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 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 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 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. See
systemctl1 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-generator8
for details.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-analyze1.
The boot manager implements boot counting and automatic fallback to older, working boot
entries on failure. See Automatic Boot
Assessment.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.bootctl1
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-install8
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-stub7
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.Key bindingsThe following keys may be used in the boot menu:↑ (Up)↓ (Down)jkPageUpPageDownHomeEndNavigate up/down in the entry list↵ (Enter)→ (Right)Boot selected entrydMake selected entry the defaulteEdit the kernel command line for selected entry+tIncrease the timeout before default entry is booted-TDecrease the timeoutrChange screen resolution, skipping any unsupported modes.RReset screen resolution to firmware or configuration file default.pPrint statush?F1Show a help screenfReboot into firmware interface.For compatibility with the keybindings of several firmware implementations this operation
may also be reached with F2, F10, Del and
Esc.ShiftoPower off the system.ShiftbReboot the system.The following keys may be pressed during bootup or in the boot menu to directly boot a specific
entry:lLinuxwWindowsamacOSsEFI shell123456789Boot entry number 1 … 9The 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)HomeEndNavigate left/rightEscCtrlcAbort the edit and quit the editorCtrlkClear the command line forwardsCtrlwAltBackspaceDelete word backwardsCtrlDelAltdDelete word forwards↵ (Enter)Boot entry with the edited command lineNote 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 +/-.
FilesThe 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.conf5.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 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,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.EFI VariablesThe 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:LoaderBootCountPathIf 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.service8
uses this information to mark a boot as successful as determined by the successful activation of the
boot-complete.target target unit.LoaderConfigTimeoutLoaderConfigTimeoutOneShotThe 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.LoaderConfigConsoleModeThe numerical menu console mode. Read by the boot loader. LoaderConfigConsoleMode
is maintained persistently. LoaderConfigConsoleMode may be manipulated with the
r/R keys, see above.LoaderDevicePartUUIDContains the partition UUID of the EFI System Partition the boot loader was run from. Set by
the boot
loader. systemd-gpt-auto-generator8
uses this information to automatically find the disk booted from, in order to discover various other partitions
on the same disk automatically.LoaderEntriesA list of the identifiers of all discovered boot loader entries. Set by the boot
loader.LoaderEntryDefaultLoaderEntryOneShotThe 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. bootctl1's
and commands make use of these variables. The boot
loader modifies LoaderEntryDefault on request, when the d key is used, see
above.LoaderEntryLastBootedThe 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.conf5. 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.LoaderEntrySelectedThe identifier of the boot loader entry currently being booted. Set by the boot
loader.LoaderFeaturesA set of flags indicating the features the boot loader supports. Set by the boot loader. Use
bootctl1 to view this
data.LoaderFirmwareInfoLoaderFirmwareTypeBrief firmware information. Set by the boot loader. Use
bootctl1 to view this
data.LoaderImageIdentifierThe 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
bootctl1 to view this
data.LoaderInfoBrief information about the boot loader. Set by the boot loader. Use
bootctl1 to view this
data.LoaderTimeExecUSecLoaderTimeInitUSecLoaderTimeMenuUsecInformation about the time spent in various parts of the boot loader. Set by the boot
loader. Use systemd-analyze1
to view this data. LoaderSystemTokenA 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.SMBIOS Type 11 Stringssystemd-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-extraIf 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.Boot Countingsystemd-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: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.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.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.service8
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-install8 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.Using systemd-boot in virtual machinesWhen using qemu with OVMF (UEFI Firmware for virtual machines) the 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.efisystemd-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).See Alsobootctl1loader.conf5systemd-bless-boot.service8systemd-boot-random-seed.service8kernel-install8systemd-stub7Boot Loader SpecificationBoot Loader InterfaceTPM2 PCR Measurements Made by systemd