NAME<\/a> systemd-boot, sd-boot \u2212 A simple UEFI boot manager<\/p>\n systemd\u2212boot<\/b> (short: sd\u2212boot<\/b>) 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\u2212boot<\/b> supports systems with UEFI firmware only.<\/p>\n systemd\u2212boot<\/b> 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<\/b> to be able to be directly executed as an EFI image. During boot systemd\u2212boot<\/b> automatically assembles a list of boot entries from the following sources:<\/p>\n \u2022 Boot entries defined with Boot Loader Specification<\/font><\/b> [1]<\/font><\/small> 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.<\/font><\/p>\n \u2022 Unified kernel images following the<\/font> Boot Loader Specification<\/font><\/b> [1]<\/font><\/small> , as executable EFI binaries in \/EFI\/Linux\/ on the ESP and the Extended Boot Loader Partition.<\/font><\/p>\n \u2022 The Microsoft Windows EFI boot manager, if installed<\/font><\/p>\n \u2022 The Apple macOS boot manager, if installed<\/font><\/p>\n \u2022 The EFI Shell binary, if installed<\/font><\/p>\n \u2022 A reboot into the UEFI firmware setup option, if supported by the firmware<\/font><\/p>\n systemd\u2212boot<\/b> supports the following features:<\/font><\/p>\n \u2022 Basic boot manager configuration changes (such as timeout configuration, default boot entry selection, …) may be made directly from the boot loader UI at boot\u2212time, as well as during system runtime with EFI variables.<\/font><\/p>\n \u2022 The boot manager integrates with the systemctl<\/b> command to implement features such as systemctl reboot \u2212\u2212boot\u2212loader\u2212entry=…<\/b> (for rebooting into a specific boot menu entry, i.e. “reboot into Windows”) and systemctl reboot \u2212\u2212boot\u2212loader\u2212menu=…<\/b> (for rebooting into the boot loader menu), by implementing the<\/font> Boot Loader Interface<\/font><\/b> [2]<\/font><\/small> . See systemctl<\/b>(1) for details.<\/font><\/p>\n \u2022 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<\/b>(8) for details.<\/font><\/p>\n \u2022 The boot manager provides information about the boot time spent in UEFI firmware using the<\/font> Boot Loader Interface<\/font><\/b> [2]<\/font><\/small> . This information can be displayed using systemd-analyze<\/b>(1).<\/font><\/p>\n \u2022 The boot manager implements boot counting and automatic fallback to older, working boot entries on failure. See<\/font> Automatic Boot Assessment<\/font><\/b> [3]<\/font><\/small> .<\/font><\/p>\n \u2022 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.<\/font><\/p>\n bootctl<\/b>(1) may be used from a running system to locate the ESP and the Extended Boot Loader Partition, list available entries, and install systemd\u2212boot<\/b> itself.<\/font><\/p>\n kernel-install<\/b>(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.<\/font><\/p>\n The following keys may be used in the boot menu:<\/font><\/p>\n \u00e2 (Up), \u00e2 (Down), j, k, PageUp, PageDown, Home, End<\/font><\/p>\n Navigate up\/down in the entry list<\/font><\/p>\n \u00e2\u00b5 (Enter), \u00e2 (Right)<\/font><\/p>\n Boot selected entry<\/font><\/p>\n d<\/font><\/p>\n Make selected entry the default<\/font><\/p>\n e<\/font><\/p>\n Edit the kernel command line for selected entry<\/font><\/p>\n +, t<\/font><\/p>\n Increase the timeout before default entry is booted<\/font><\/p>\n \u2212, T<\/font><\/p>\n Decrease the timeout<\/font><\/p>\n v<\/font><\/p>\n Show systemd\u2212boot, UEFI, and firmware versions<\/font><\/p>\n P<\/font><\/p>\n Print status<\/font><\/p>\n Q<\/font><\/p>\n Quit<\/font><\/p>\n h, ?, F1<\/font><\/p>\n Show a help screen<\/font><\/p>\n Ctrl+l<\/font><\/p>\n Reprint the screen<\/font><\/p>\n The following keys may be pressed during bootup or in the boot menu to directly boot a specific entry:<\/font><\/p>\n l<\/font><\/p>\n Linux<\/font><\/p>\n w<\/font><\/p>\n Windows<\/font><\/p>\n a<\/font><\/p>\n macOS<\/font><\/p>\n s<\/font><\/p>\n EFI shell<\/font><\/p>\n 1, 2, 3, 4, 5, 6, 7, 8, 9<\/font><\/p>\n Boot entry number 1 … 9<\/font><\/p>\n The boot menu is shown when a non\u2212zero menu timeout has been configured. If the menu timeout has been set to zero, it is sufficient to press any key \u2014 before the boot loader initializes \u2014 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\u2212zero 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 \u2212\u2212boot\u2212loader\u2212menu=0<\/b> from the shell.<\/font><\/p>\n In the editor, most keys simply insert themselves, but the following keys may be used to perform additional actions:<\/font><\/p>\n \u00e2 (Left), \u00e2 (Right), Home, End<\/font><\/p>\n Navigate left\/right<\/font><\/p>\n Esc<\/font><\/p>\n Abort the edit and quit the editor<\/font><\/p>\n Ctrl+k<\/font><\/p>\n Clear the command line<\/font><\/p>\n Ctrl+w, Alt+Backspace<\/font><\/p>\n Delete word backwards<\/font><\/p>\n Alt+d<\/font><\/p>\n Delete word forwards<\/font><\/p>\n \u00e2\u00b5 (Enter)<\/font><\/p>\n Boot entry with the edited command line<\/font><\/p>\n Note that unless configured otherwise in the UEFI firmware, systemd\u2212boot will use the US keyboard layout, so key labels might not match for keys like +\/\u2212.<\/font><\/p>\n The files systemd\u2212boot<\/b> 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\u2212boot<\/b> 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<\/b>(5). Boot entry description files following the<\/font> Boot Loader Specification<\/font><\/b> [1]<\/font><\/small> are read from \/loader\/entries\/ on the ESP and the Extended Boot Loader partition. Unified kernel boot entries following the<\/font> Boot Loader Specification<\/font><\/b> [1]<\/font><\/small> 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\u2212seed in the ESP.<\/font><\/p>\n The following EFI variables are defined, set and read by systemd\u2212boot<\/b>, under the vendor UUID “4a67b082\u22120a4c\u221241cf\u2212b6c7\u2212440b29bb8c4f”, for communication between the OS and the boot loader:<\/font><\/p>\n LoaderBootCountPath<\/i><\/font><\/p>\n 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<\/b>(8) uses this information to mark a boot as successful as determined by the successful activation of the boot\u2212complete.target target unit.<\/font><\/p>\n LoaderConfigTimeout<\/i>, LoaderConfigTimeoutOneShot<\/i><\/font><\/p>\n The menu timeout in seconds. Read by the boot loader. LoaderConfigTimeout<\/i> is maintained persistently, while LoaderConfigTimeoutOneShot<\/i> is a one\u2212time override which is read once (in which case it takes precedence over LoaderConfigTimeout<\/i>) and then removed. LoaderConfigTimeout<\/i> may be manipulated with the t\/T keys, see above.<\/font><\/p>\n LoaderDevicePartUUID<\/i><\/font><\/p>\n Contains the partition UUID of the EFI System Partition the boot loader was run from. Set by the boot loader. systemd-gpt-auto-generator<\/b>(8) uses this information to automatically find the disk booted from, in order to discover various other partitions on the same disk automatically.<\/font><\/p>\n LoaderEntries<\/i><\/font><\/p>\n A list of the identifiers of all discovered boot loader entries. Set by the boot loader.<\/font><\/p>\n LoaderEntryDefault<\/i>, LoaderEntryOneShot<\/i><\/font><\/p>\n The identifier of the default boot loader entry. Set primarily by the OS and read by the boot loader. LoaderEntryOneShot<\/i> sets the default entry for the next boot only, while LoaderEntryDefault<\/i> sets it persistently for all future boots. bootctl<\/b>(1)’s set\u2212default<\/b> and set\u2212oneshot<\/b> commands make use of these variables. The boot loader modifies LoaderEntryDefault<\/i> on request, when the d key is used, see above.<\/font><\/p>\n LoaderEntrySelected<\/i><\/font><\/p>\n The identifier of the boot loader entry currently being booted. Set by the boot loader.<\/font><\/p>\n LoaderFeatures<\/i><\/font><\/p>\n A set of flags indicating the features the boot loader supports. Set by the boot loader. Use bootctl<\/b>(1) to view this data.<\/font><\/p>\n LoaderFirmwareInfo<\/i>, LoaderFirmwareType<\/i><\/font><\/p>\n Brief firmware information. Set by the boot loader. Use bootctl<\/b>(1) to view this data.<\/font><\/p>\n LoaderImageIdentifier<\/i><\/font><\/p>\n 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<\/b>(1) to view this data.<\/font><\/p>\n LoaderInfo<\/i><\/font><\/p>\n Brief information about the boot loader. Set by the boot loader. Use bootctl<\/b>(1) to view this data.<\/font><\/p>\n LoaderTimeExecUSec<\/i>, LoaderTimeInitUSec<\/i>, LoaderTimeMenuUsec<\/i><\/font><\/p>\n Information about the time spent in various parts of the boot loader. Set by the boot loader. Use systemd-analyze<\/b>(1) to view this data.<\/font><\/p>\n LoaderRandomSeed<\/i><\/font><\/p>\n A binary random seed systemd\u2212boot<\/b> 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\u2212seed) and a “system token” persistently stored in the EFI variable LoaderSystemToken<\/i> (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 \u2014 as early as the initial RAM disk phase. systemd\u2212boot<\/b> reads the random seed from the ESP, combines it with the “system token”, and both derives a new random seed to update in\u2212place 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 \u2014 i.e. containing the same random seed file in the ESP \u2014 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.<\/font><\/p>\n See<\/font> Random Seeds<\/font><\/b> [4]<\/font><\/small> for further information.<\/font><\/p>\n LoaderSystemToken<\/i><\/font><\/p>\n 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.<\/font><\/p>\n Many of these variables are defined by the<\/font> Boot Loader Interface<\/font><\/b> [2]<\/font><\/small> .<\/font><\/p>\n
DESCRIPTION<\/a>
KEY BINDINGS<\/a>
FILES<\/a>
EFI VARIABLES<\/a>
BOOT COUNTING<\/a>
SEE ALSO<\/a>
NOTES<\/a> <\/p>\n
\nNAME <\/a> <\/h2>\n
DESCRIPTION <\/a> <\/h2>\n
KEY BINDINGS <\/a> <\/h2>\n
FILES <\/a> <\/h2>\n
EFI VARIABLES <\/a> <\/h2>\n
BOOT COUNTING <\/a> <\/h2>\n