efibootmgr is a tool for managing UEFI boot entries.
It is not a bootloader. It is a tool that interacts with the EFI firmware of the system, which itself is acting as a boot manager. Using efibootmgr boot entries can be created, reshuffled and removed.
CONFIG_EFIVAR_FS support needs to be enabled.
It is not possible to use efivarfs without the EFI runtime services, which (in case they have been disabled by default, i.e.
CONFIG_EFI_DISABLE_RUNTIME=y) can also be enabled by the kernel command-line option efi=runtime.
Thepackage does not have any USE flags. All that is needed is to install it:
emerge --ask sys-boot/efibootmgr
In order to successfully use efibootmgr the EFI variables filesystem must be accessible. This requires that the system has been booted in EFI mode (and not through the firmware's MBR mode) as otherwise the EFI variables themselves cannot be accessed. If the system is in MBR mode, reboot and do what is necessary in order to tell the system firmware to boot in EFI mode. Usually this involved either changing an option in the firmware's settings or selecting an EFI boot entry in the system's boot menu.
When the system is in EFI mode, run the following command to check for the existence of a mounted efivarfs:
mount | grep efivars
efivarfs on /sys/firmware/efi/efivars type efivarfs (ro,relatime)
It is mounted read-only (ro) through the sysfs init script), so it needs to be remounted read-write (rw) manually using the following command:
mount -o remount,rw -t efivarfs efivarfs /sys/firmware/efi/efivars
If an EFI System Partition (ESP) does not exist, one needs to be created, see EFI System Partition.
Listing boot entries
When using an older version of efibootmgr the option
-v is needed:
BootCurrent: 0002 Timeout: 3 seconds BootOrder: 0003,0003,0002,0000,0004 Boot0000* CD/DVD Drive BIOS(3,0,00) Boot0001* Hard Drive BIOS(2,0,00) Boot0002* Gentoo HD(1,800,61800,6d98f360-cb3e-4727-8fed-5ce0c040365d)File(\EFI\boot\bootx64.efi) Boot0003* Hard Drive BIOS(2,0,00)P0: ST1500DM003-9YN16G
Creating a boot entry
To create an EFI boot entry, a couple of arguments are passed to efibootmgr:
-cto create a new entry;
-pfollowed by the partition number on which the EFI System Partition is hosted;
-dfollowed by the disk on which the EFI System Partition is hosted;
-Lfollowed by the label to use as the boot entry;
-lfollowed by the path of the EFI image to boot
The path of the EFI image to boot must use \ (backslash) instead of / (forward slash) as path separator.
Additionally, if the ESP was already created by another OS, it might be named differently than /efi/EFI. If an ESP was created by another OS, begin the EFI Boot entry using this directory name, which immediately follows /efi.
Below are some examples of how a UEFI entry can be created. If this is the folder structure:
tree /efi/ -L 3
/efi/ └── EFI ├── Grub │ └── grubx64.efi └── Gentoo └── bzImage.efi
then the loader paths will be:
efibootmgr -c -L "Grub" -l '\EFI\Grub\grubx64.efi'
efibootmgr -c -L "Gentoo" -l '\EFI\Gentoo\bzImage.efi'
efibootmgr -c -d /dev/sda -p 2 -L "Gentoo" -l '\efi\boot\bootx64.efi'
efibootmgr -c -d /dev/sda -p 2 -L "Gentoo" -l '\efi\boot\bootx64.efi' -u 'root=/dev/sda3 initrd=\efi\boot\initramfs.img quiet'
Optionally, additional kernels can be installed and made known to the UEFI firmware. This is especially useful when wanting to test more kernels or to dual-boot with another operating system. These will be shown in the boot selection prompt, normally after a keyboard hotkey is pressed at the right time during system initialization. The latest added entry always gets highest boot priority, so it will be default. If the hotkey combination is unknown, search for official documentation from the computer manufacturer. This information is usually not difficult to find.
Deleting a boot entry
Before deleting an entry, first figure out what ID the entry has.
To delete the Gentoo entry as shown above (which has Boot0002 as the identifier), ask efibootmgr to delete the entry with id 2, passing the arguments
-b with the identifier, and
-B to delete the entry:
efibootmgr -b 2 -B 2
EFI bootloaders on removable media are not configured as boot entries, so tools like efibootmgr are not required. Instead the computer firmware identifies removable boot options by looking for a predefined file name unique to the system architecture in use, in a predefined path.
Some (U)EFI implementations support the use of the removable media boot path as a fallback on internal drives. This is against specification and should be avoided, but may help on a system with a buggy (U)EFI. Only one such fallback bootloader is possible on a specific system (i.e. architecture), due to the standardized boot path and file names.
|PE executable machine type
To boot from removable media, the firmware has to look for compatible bootloaders on supported devices, which can be configured in the firmware setup. Like most firmwares, (U)EFI provides a hotkey to show a boot selection (formally known as "BIOS Boot Selection" or BBS), otherwise (U)EFI will use the configured default boot entry. However, a buggy (U)EFI may default to the removable media boot path even on internal drives.
To use the removable media boot path it is sufficient to copy the EFI bootloader to the required path and file name. Should the firmware make use of this fallback, this may also remove the ability to select between configured boot entries, meaning that boot options (as listed with efibootmgr) through (U)EFI will not work. Even though every EFI bootloader can be used as fallback, it may be advisable to use a boot manager that itself has the ability to select between boot options, such as GRUB. From the above example for x64 (amd64):
cp /efi/EFI/Grub/grubx64.efi /efi/EFI/boot/bootx64.efi
The FAT file system of the EFI System Partition is not case-sensitive, but case-preserving (VFAT), while Unix and Linux is strictly case-sensitive at all times. Because of this, the path /efi/EFI/boot/bootx64.efi may not work on all systems, as it could be using capital letters, e.g. /efi/EFI/BOOT or /efi/EFI/Boot; the same goes for bootx64.efi. Running tree -L 3 /efi, when the ESP is mounted under /efi, will show the names in use on the individual system, to which the above command has to be changed accordingly.
The fallback bootloader is not automatically updated! Every time e.g. GRUB is re-installed (like after a version update, which may contain security fixes), it has to be copied to the fallback boot path again, overwriting (updating) the previous version!
The boot manager included in systemd, systemd-boot (formally Gummiboot), will automatically install to the removable media boot path. When with the
boot USE flag is updated, it is necessary to run bootctl again in order to update both bootloader files.
emerge --ask --depclean --verbose sys-boot/efibootmgr
- REFInd — a boot manager for EFI and UEFI platforms forked from and successor to rEFIt.
- EFI stub — provides instructions on configuring and installing kernels in the EFI System Partition (ESP) of a computer running in EFI mode
- Efivarfs — a filesystem in the Linux kernel that enables users to create, delete, and modify (U)EFI variables
- Alternative 2: efibootmgr in the Gentoo Handbook
- At least for Dell EFI firmware, a workaround was implemented in kernel 5.10: https://lkml.org/lkml/2020/9/18/228
- UEFI 2.10 Specification, 188.8.131.52 Removable Media Boot Behavior
- Debian Wiki – UEFI: Force grub-efi installation to the removable media path