SYSTEMD-BOOT(7)

📦
version 261.3 · 16 versions
, in ubuntu stonking

🌸 SYSTEMD-BOOT(7) 🌸

okay bestie, so this is the systemd-boot / sd-boot page from the systemd manual — yes THAT systemd — and it's lowkey a whole vibe. strap in 💅

✨ NAME ✨

systemd-boot, sd-boot — a simple UEFI boot manager. that's it. that's the name. iconic, brief, slays.

🦋 DESCRIPTION 🦋

so systemd-boot (friends call it sd-boot) is a simple UEFI boot manager. it gives you a textual menu where you pick which entry to boot, plus an editor for the kernel command line. heads up though: systemd-boot supports systems with UEFI firmware only — no legacy BIOS energy here, sorry 🫶

systemd-boot loads its boot entry info from the EFI system partition (aka the ESP, usually mounted at /efi/, /boot/, or /boot/efi/ while the OS is running), and also from the Extended Boot Loader partition (XBOOTLDR) if that exists (usually mounted to /boot/). configuration file fragments, kernels, initrds and other EFI images you wanna boot generally need to live on the ESP or the Extended Boot Loader partition. and Linux kernels must be built with CONFIG_EFI_STUB to be directly executable as an EFI image — no cap, that's mandatory.

during boot, systemd-boot automatically assembles a list of boot entries from these sources:

  • Boot entries defined with UAPI.1 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 their associated initrd images, but they can also describe other arbitrary EFI executables — flexible queen ✨
  • Unified kernel images, UAPI.1 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 the UEFI firmware supports it.
  • Secure Boot variable enrollment, if the UEFI firmware is in setup mode and files are provided on the ESP.

and systemd-boot supports these features (a whole resume, honestly):

  • Basic boot manager configuration changes (like timeout config, default boot entry selection, ...) can be made straight from the boot loader UI at boot-time, and also during system runtime with EFI variables. convenient much?
  • The boot manager integrates with the systemctl command to implement features like 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 tells the OS about the EFI System Partition used during boot. that's then used to automatically mount the correct EFI System Partition to /efi/ or /boot/ during OS runtime. see systemd-gpt-auto-generator(8) for the deets.
  • The boot manager gives you info about how much boot time was spent in UEFI firmware, via the Boot Loader Interface[2]. you can see it with systemd-analyze(1).
  • The boot manager implements boot counting and automatic fallback to older, working boot entries on failure. self-healing behavior fr. see Automatic Boot Assessment[3].
  • The boot manager can optionally read a random seed from the ESP partition, combine it with a 'system token' stored in a persistent EFI variable, and derive a random seed for the OS to use as entropy pool initialization — giving a full entropy pool during early boot. entropy queen 🎀
  • The boot manager lets Secure Boot variables be enrolled if the UEFI firmware is in setup mode. plus, variables can be automatically enrolled if configured.

bootctl(1) can be used from a running system to locate the ESP and the Extended Boot Loader Partition, list the available entries, and install systemd-boot itself. very handy.

kernel-install(8) can be used to copy kernel images onto the ESP or the Extended Boot Loader Partition and to generate description files that comply with the Boot Loader Specification.

systemd-stub(7) can be used as the UEFI boot stub for executed kernels, which is useful for showing graphical boot splashes before transitioning into the Linux world 🦋. it's 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 🎀

these keys work 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 the selected entry Added in version 239.
  • d — make the selected entry the preferred boot entry An EFI variable is set so this setting persists. Added in version 260.
  • D — make the selected entry the default boot entry An EFI variable is set so this setting persists. Added in version 260.
  • e — edit the kernel command line for the selected entry Added in version 239.
  • +, t — increase the timeout before the default entry is booted An EFI variable is set so this setting persists. Added in version 239.
  • -, T — decrease the timeout An EFI variable is set so this setting persists. Added in version 239.
  • r — change the screen resolution, skipping any unsupported modes. An EFI variable is set so this setting persists. Added in version 250.
  • R — reset the screen resolution to the firmware or configuration file default. An EFI variable is set so this setting persists. Added in version 250.
  • p — print status Added in version 250.
  • h, ?, F1 — show a help screen Added in version 239.
  • f — reboot into the firmware interface. for compatibility with the keybindings of several firmware implementations, this operation can 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.

and these keys can 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.

so here's the tea on the menu itself ☕ the boot menu is shown when a non-zero menu timeout has been configured. if the menu timeout has been set to zero, hold down a key (space is recommended) before the boot loader initializes to bring up the boot menu. note that depending on the firmware implementation, the time window where key presses are accepted before the boot loader initializes might be short 😭. if you miss the window, reboot and try again, possibly repeatedly pressing a suitable key; on most systems it should be possible to hit the time window after a few attempts. keys other than the space bar can be used, except for the keys listed above. if showing the menu on demand doesn't work well, consider setting a non-zero timeout to show the boot menu unconditionally. some desktop environments might offer an option to boot directly into the boot menu, which also avoids the problem altogether. alternatively, use the command line systemctl reboot --boot-loader-menu= with a non-zero value from the shell. problem solved bestie 💅

in the editor, most keys just insert themselves, but these keys do extra stuff:

  • ← (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 the entry with the edited command line Added in version 239.
ngl, unless configured otherwise in the UEFI firmware, systemd-boot uses the US keyboard layout, so key labels might not match for keys like +/-. don't come for me, come for the firmware 🙈

🧚 FILES 🧚

the files systemd-boot processes generally live 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 — like 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 UAPI.1 Boot Loader Specification[1] are read from /loader/entries/ on the ESP and the Extended Boot Loader partition.

Unified kernel boot entries following the UAPI.1 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. ⚠️ but if the read-only file attribute is set on this file, it is neither updated nor used: a seed that cannot be updated would be identical on every boot, and hence must not be credited. the file attribute can thus be used to explicitly turn off random seed handling, for example in OS images that are replicated to multiple systems. makes sense, no notes.

during initialization, systemd-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 can be used e.g. to add support for hardware not supported natively by the firmware. cute.

Enrollment of Secure Boot variables can be performed manually or automatically if files are available under /loader/keys/NAME/{db,dbx,KEK,PK}.auth, with 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 the execution environment and the value of the "secure-boot-enroll" option. see loader.conf(5).

💖 COMPANION FILES 💖

Okay so for Type #1 boot loader entries (as defined in the UAPI.1 Boot Loader Specification[1]), systemd-boot will collect extra companion resources declared via the extra key in the entry, dynamically generate cpio initrd archives from them, and register those archives via the Linux initrd EFI protocol so they get passed to the kernel together with the entry's own initrd. Slay. ✨

This is supported for entries referencing a Unified Kernel Image (UKI) via the uki or uki-url keys. Each extra key references a single regular file (relative to the root of the file system containing the entry snippet), and the key may be specified multiple times. Not me explaining file paths like my life depends on it, but here we are. 💅

Companion resources are recognized by file name suffix:

  • .cred — files with this suffix are packed into a cpio archive placed in the /.extra/credentials/ directory of the initrd file hierarchy. This is intended to convey auxiliary, encrypted, authenticated credentials for use with LoadCredentialEncrypted=. See systemd.exec(5) and systemd-creds(1) for details on encrypted credentials. The generated cpio archive is measured into TPM PCR 12 (if a TPM is present). 🔐
  • .sysext.raw — files with this suffix are packed into a cpio archive placed in the /.extra/sysext/ directory of the initrd file hierarchy. This is intended to pass additional entry-specific system extension images to the initrd. See systemd-sysext(8) for details on system extension images. The generated cpio archive is measured into TPM PCR 13 (if a TPM is present). 🧚
  • .confext.raw — files with this suffix are packed into a cpio archive placed in the /.extra/confext/ directory of the initrd file hierarchy. This is intended to pass additional entry-specific configuration extension images to the initrd. See systemd-confext(8) for details on configuration extension images. The generated cpio archive is measured into TPM PCR 12 (if a TPM is present). 🎀

When the booted kernel is a UKI, the systemd-stub UEFI stub embedded in it will combine the companion resources injected here with any companion files it itself collects from the UKI's .extra.d/ drop-in directory and from /loader/credentials/ and /loader/extensions/, so that all sources are merged uniformly into /.extra/ in the initrd. See systemd-stub(7) for details. The way all these paths merge is honestly iconic. 🌈

Example Type #1 entry making use of the extra key:

title       My OS
version     1.2.3
machine-id  6a9857a393724b7a981ebb5b8495b9ea
uki         /6a9857a393724b7a981ebb5b8495b9ea/1.2.3/img.efi
extra       /6a9857a393724b7a981ebb5b8495b9ea/1.2.3/foo.cred
extra       /6a9857a393724b7a981ebb5b8495b9ea/1.2.3/bar.sysext.raw
extra       /6a9857a393724b7a981ebb5b8495b9ea/1.2.3/baz.confext.raw

🦋 EFI VARIABLES 🦋

The following EFI variables are defined, and may be set or read by systemd-boot for communication between the boot loader and the OS. The vendor UUID 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f is used in all cases. Yes bestie, same UUID every single time. 💅

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 an EFI System Partition). Set by the boot loader. (Note that systemd-stub(7) will set this too, if not set yet, to support systems that boot directly 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. Literally the “which disk am I on” receipt. 🧾

Added in version 220.

LoaderDeviceURL

If the boot loader has been invoked via network booting, this variable contains the originating URL. This may be used to automatically acquire additional resources from the same source.

Added in version 258.

LoaderEntries

A list of the identifiers of all discovered boot loader entries. Set by the boot loader.

Added in version 240.

LoaderEntryPreferred, LoaderEntryDefault, LoaderEntrySysFail, LoaderEntryOneShot

The identifier of the default boot loader entry. Can be set in the OS and the boot loader. LoaderEntryOneShot sets the default entry for the next boot only, while LoaderEntryDefault sets it persistently for all future boots. LoaderEntryPreferred is like LoaderEntryDefault but additionally takes into account boot assessment and skips boot entries with a tries-left counter equal to zero. 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 altogether 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. Do not use it as a success receipt, bestie. It is not that. 💀

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.

LoaderTpm2ActivePcrBanks

Hexadecimal string representation of a bitmask with values defined by the TCG EFI Protocol Specification for TPM 2.0 as EFI_TCG2_BOOT_HASH_ALG_*. If no TPM2 support or no active banks were detected, will be set to 0. Set by the boot loader. Use systemd-analyze(1) to view this data.

Added in version 258.

LoaderPcrSMBIOS

The PCR register index select SMBIOS structures are measured into — type 1 (system information, with the volatile "Wake-up Type" field zeroed out), type 2 (baseboard information) and type 11 (OEM strings). Formatted as decimal ASCII string (e.g. "1"). Set by the boot loader if a measurement was successfully completed, and remains unset otherwise. (Note that systemd-stub performs the same measurement when booted directly, bypassing the boot loader.)

Added in version 261.

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. One-time vibe, forever token. 🎟️

Added in version 243.

Many of these variables are defined by the Boot Loader Interface[2].

SMBIOS TYPE 11 STRINGS 🧬

okay bestie, systemd-boot can be configured using SMBIOS Type 11 strings, no cap. 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.
  • io.systemd.boot-entries.extra:ID=DEFINITION — this allows inserting additional entries into the systemd-boot menu. Take a pair of menu entry identifier and menu entry definition string. The former should be suitable for use as a filename of a Boot Loader Specification Type #1 entry filename (note that it is used for identification purposes only, no file of this name is actually accessed), the latter shall follow the syntax of the contents of a Type #1 entry. Any menu entry defined this way is processed and shown in pretty much the same way as a Type #1 entry read from the ESP or XBOOTLDR partition. Example:
io.systemd.boot-entries.extra:fooos-current.conf=title FooOS (Current)
uki-url http://example.com/somedir/fooos.efi

Note that this example contains a newline character. When generating this string from a shell care must be taken to encode it correctly. 🫶

Pass multiple strings formatted this way to generate multiple menu entries. Added in version 258.

  • io.systemd.boot.loglevel — if set, the value of this string is used as log level. Valid values (from most to least critical) are emerg, alert, crit, err, warning, notice, info, and debug. 🔥 Added in version 259.

BOOT COUNTING 🔁

systemd-boot implements a simple boot counting mechanism on top of the UAPI.1 Boot Loader Specification[1], for automatic and unattended fallback to older kernel versions/boot loader entries when a specific entry continuously fails. bestie, this is the safety net energy. 😭

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 has not 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). periodt.

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. iykyk 💅

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[4] with OVMF[5] (UEFI Firmware for virtual machines) the -kernel switch works not only for linux kernels, but for any EFI binary, including systemd-boot and unified linux kernels (UKIs). Example command line for loading systemd-boot on x64:

qemu-system-x86_64 -drive if=pflash,format=qcow2,readonly=on,file=/usr/share/edk2/ovmf/OVMF_CODE_4M.qcow2 -kernel /usr/lib/systemd/boot/efi/systemd-bootx64.efi -drive file=... [ ... ]

(The path to the firmware file might need to be adjusted depending on the distribution.) 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). Note that for this to yield a useful result, another -drive argument needs to be used to attach an actual disk image with an ESP. 🧚

SEE ALSO 👀

NOTES 📝

  1. UAPI.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. qemu — https://www.qemu.org/docs/master/
  5. OVMF — https://www.linux-kvm.org/downloads/lersek/ovmf-whitepaper-c770f8c.txt
  6. TPM2 PCR Measurements Made by systemd — https://systemd.io/TPM2_PCR_MEASUREMENTS

that's the whole page, bestie 🫶 this one's footer has systemd 261.3 chillin' on the left and SYSTEMD-BOOT(7) on the right — iconic source/version + page-tag energy, periodt. 💖

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