Title : Rescuing GRUB
Author: cyr4x3
Date : 10 September 2023
Tags : linux
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.
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.
value of an environment variable. If invoked with no arguments, all
environment variables and their values will be print out.
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).
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.
[^1]: The number sign, hash or pound sign shown at the beginning of every
command means that the command should be ran as a superuser. Either precede
every command with the word `sudo` (or `doas` or any equivalent) or switch to
the root use with the `su` command and then type the commands shown in the
code block.