Some years ago I ran into a problem where, when starting my computer, I got thrown into a GRUB rescue command prompt instead of GRUB showing up and letting me choose which OS to boot in directly. I was running Debian 11 “Bullseye” (the stable version at that time) and this started happening after performing a dist-upgrade. Probably this “breakage” had something to do with me fiddling with my GRUB config sometime before the update and making changes that were incompatible with the new version of GRUB installed by the update (at least, that’s what I think may’ve happened as of today, though I may be completely wrong). In other words, I’m pretty sure it was my fault.

When this happened I had never seen a GRUB rescue prompt before, so I freaked out. It turns out that the problem wasn’t as bad as I first thought.

Initial fix

After the initial shock after upgrading my system, I had to find a way to fix this problem. I found a temporary solution: in order for my computer to boot up normally I just had to enter the series of commands

    set prefix=(hd0,gpt2)/boot/grub
    set root=(hd0,gpt2)
    insmod linux
    insmod normal
    normal

and my computer booted up perfectly fine. Let’s see what these commands actually mean.

• The set command, as described in the GNU GRUB Manual 2.06, lets us set the value of an environment variable. If invoked with no arguments, all environment variables and their values will be print out.

• The insmod command is used to insert dynamic GRUB modules (which we can understand as loading or activating them).

Therefore, the first two commands set the value of the prefix and root environment variables, defining the path to the GRUB directory and the bootable partition respectively.

Later on, the linux and normal modules are inserted. The linux module is a loader for Linux images, while normal provides us with “Normal Mode” (the opposite of “Rescue Mode”, the one in which we enter these commands).

Once these modules are loaded one can switch to “Normal Mode” by simply typing normal. Then, the usual GRUB menu, that I’m sure you’re familiar with, is shown (the one that lets you choose the OS you want to boot in).

Final solution

It was a very busy season, so I kept postponing finding a better and definitive solution to this problem and I had to enter these series of commands every time I had to boot.

One day I got tired and decided to find a better solution. It was quite simple. Once my computer booted up, I ran the commands shown in the previous section so I could start Debian and, once I was logged in into my user account, I simply ran ¹:

    # update-grub
    # grub-mkconfig -o /boot/grub/grub.cfg
    # grub-install /dev/sda

As described in the grub-mkconfig(8) manpage, by default the grub-mkconfig command prints the generated GRUB configuration file to stdout (standard output). The -o flag outputs the generated config to a file, so the path following that flag should correspond to the location were GRUB expects its config file to be (in my case it was /boot/grub/grub.cfg).

The command grub-install will install GRUB to a device. The path you specify must refer to a disk and not one of its partitions. Tools like lsblk can help you identify the disk path on which you should ask grub-install to install GRUB.

Resources and references

GNU GRUB Manual 2.06

Musings: GRUB2 Modules

grub-mkconfig Manpage

grub-install Manpage

lsblk Manpage