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 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.

KEY BINDINGS

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 +/-.

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.

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.

EFI VARIABLES

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 partition the boot loader has been started from on the current boot (usually a EFI System Partition). Set by the boot loader. (Note that systemd-stub will set this too, if not set yet, to support systems that directly boot into a unified kernel image, bypassing any 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 220.

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 file system path to the EFI executable of the boot loader for the current boot, relative to the partition's root directory (i.e. relative to the partition indicated by LoaderDevicePartUUID, see above). Set by the boot loader. (Note that systemd-stub will set this too, if not set yet, to support systems that directly boot into a unified kernel image, bypassing any boot loader.) Use bootctl(1) to view this data.

    Added in version 220.

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].

SMBIOS TYPE 11 STRINGS

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.

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 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.

USING SYSTEMD-BOOT IN VIRTUAL MACHINES

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).

SEE ALSO

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]

NOTES

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