Booting from UEFI shell

As a longtime Linux user, I have worked with a long list of distros (starting with the original Mandrake). But for at least the past decade, I've been an Arch Linux user.

Regardless of the distro, though, I've always found it interesting that Linux requires a bootloader, which today usually means Grub2. I never liked it, finding the need for menus and complicated lines to be cumbersome. More recently, I tried to move to efibootmgr, and found that easier and faster. Then I tried Refind, which is better still, I think.

And yet, I wasn't satisfied. Which is what drove me to explore the UEFI shell. And it turns out, it's very easy, fast, and, I think, perhaps the most flexible way of booting Linux or Windows (for those who have dual boots).

This, then, is a guide to making your system boot from the UEFI shell.

Introduction:

Virtually all computers made in the past eight years use UEFI. In Linux, that means that there are at least two partitions on any drive with the Linux operating system. One is the root partition, typically formatted in ext3 or ext4. The other is the EFI partition, which must be formatted as a vfat drive (preferably Fat 32). The EFI partition can be located anywhere on the drive (not like the BIOS that requires a Master Boot Record (MBR) at the beginning of the drive. The EFI partition can be as small as 100 MB (although I usually make mine 500...because space is rarely at a premium these days).

Making the EFI Partition

I use gdisk to make my paritions. The command are simple and the process is fast. Commands are:

sudo gdisk /dev/sdXX (sda, sdb or whatever)

n (for new partition)

<enter> for the default partition number

<enter> for the default start point on the drive

+500M (for the size of the partition)

ef00 (for the EFI type)

w (to write the new partition to the table

After exiting gdisk, format the new partition with:

sudo mkfs.fat -F32 /dev/sdaX (where X is the number of the partition)

And you now have your EFI partition

Putting the necessary files on the EFI partition

In order to drop into the UEFI shell, you must have a shell.efi file on your EFI partition. You will be creating a boot order that tells UEFI to boot from a file...that file is the shell.efi file, and it will drop you into the UEFI shell. The shell.efi file can be found in lots of places. See wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface#Obtaining_UEFI_Shell for where it is in Arch.

Copy the shell efi file to the EFI parition (NOTE: there are no directories necessary on the EFI partition.)

In addition to the shell file, you will need to put the vmlinuz-linux (in Arch...for other distros, the name might vary slightly, but it always starts with vmlinuz) and the initrd (again, plenty of variations...in Arch , it's initramfs-linux.img) on the EFI partition. And finally, you will need to put an nsh file there.

The NSH file

The UEFI shell is going to initiate booting based on the nsh file. If there is a file called startup.nsh, it will automatically run that five seconds after you drop into the shell. If you hit the Escape key before the five seconds are up, the Shell prompt will appear and wait for you to enter the name of an nsh file for it to use.

The nsh file can have as little as one line (the one that tells it the vmlinuz file name and path, the root id (UUID or PartUUID or device name) and the initrd file name and path).

But it is better to use a bit longer file to tell UEFI to search all EFI partitions for the nsh file you've named. A sample nsh file is here:

Booting from UEFI shell

This is the simple process then:

Tell UEFI to boot from a file (the shellx64.efi file)

When you are dropped into the UEFI shell, either let it run your startup.nsh or tell it to run your named nsh.

And it will boot.