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 bootloader. Using efibootmgr boot entries can be created, reshuffled and removed.
CONFIG_EFI_VARS support needs to be enabled:
Firmware Drivers ---> EFI (Extensible Firmware Interface) Support ---> <*> EFI Variable Support via sysfs
The sys-boot/efibootmgr package 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 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
To list the current boot entries by using the
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 /boot/efi. If an ESP was created by another OS, begin the EFI Boot entry using this directory name, which immediately follows /boot.
If this is the folder structure:
tree /boot/ -L 3
/boot/ └── EFI ├── gentoo │ └── grubx64.efi └── gentoo_2 └── bzImage-4.14.83.efi
then the loader paths will be:
efibootmgr -c -L "Grub" -l '\EFI\gentoo\grubx64.efi'
efibootmgr -c -L "Gentoo" -l '\EFI\gentoo_2\bzImage-4.14.83.efi'
efibootmgr -c -d /dev/sda -p 2 -L "Gentoo" -l '\efi\boot\bootx64.efi'
It is also possible to add parameters - maybe not supported by all UEFI implementations - to the kernel's command line:
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
emerge --ask --depclean --verbose sys-boot/efibootmgr