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 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:
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 installed
The Apple macOS boot manager, if installed
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 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 systemctl(1) for details.
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.
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 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.
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:
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
Print status
Quit
Show a help screen
Reprint the screen
The following keys may be pressed during bootup or in the boot menu to directly boot a specific entry:
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:
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 ESP.
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 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 for further information.
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.
Many of these variables are defined by the Boot Loader Interface.
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:
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.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.