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:
systemd-boot supports the following features:
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:
↑ (Up), ↓ (Down), j, k, PageUp, PageDown, Home, End
↵ (Enter), → (Right)
h, ?, F1
The following keys may be pressed during bootup or in the boot menu to directly boot a specific entry:
1, 2, 3, 4, 5, 6, 7, 8, 9
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
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:
LoaderTimeExecUSec, LoaderTimeInitUSec, LoaderTimeMenuUsec
See Random Seeds for further information.
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:
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.
- Boot Loader Specification
- Boot Loader Interface
- Automatic Boot Assessment
- Random Seeds