Somehow I managed to completely mangle grub2 to the point even grub's self-repair couldn't work things out.
The usual method of fixing grub2 is to dnf reinstall grub2-common shim and maybe some other packages however this wasn't helping at all. I also use grub-btrfs (which, now that I think of it, I could have used to repair in this scenario too 🤔).
Anyway, I can never find one guide that's complete or up-to-date on fixing a completely messed up grub2. So here it is. Mostly for myself when I do it the next time! 😊
Note: This guide is intended for UEFI and Fedora systems only. The order is similar and can be adapted to other systems, if you know their peculiarities.
Where is stuff?
Step 1. Boot from the Fedora Live CD...
First of all, run lsblk to remember your partition structure which you haven't looked at for 10 months. You can also use Gnome Disks:
> lsblk
nvme0n1 259:0 0 1.8T 0 disk
├─nvme0n1p1 259:1 0 100M 0 part
├─nvme0n1p2 259:2 0 16M 0 part
├─nvme0n1p3 259:3 0 117.2G 0 part
├─nvme0n1p4 259:4 0 707M 0 part
├─nvme0n1p5 259:5 0 512M 0 part
└─nvme0n1p6 259:6 0 1.1T 0 part
Get set up
Great, so now I remember nvme0n1p6 is my Fedora install (Btrfs), and nvme0n1p5 is my Linux EFI partition (FAT32).
Note also here: My Windows has its nice own EFI partition—nvme0n1p1 FAT32 of 100MB. If you have separate EFI partitions for Windows & Linux, they are unlikely to ever fight and kill each others' boot systems, and if they do, it's a lot simpler to fix it.
Let's mount my filesystem. Thankfully I no longer need to remember Btrfs commands, mount is smart enough to recognise it's mounting Btrfs.
You will need to examine the following commands, because your mount points might be different from mine.
# Mount my main root '/' folder the new mount point:
sudo mount /dev/nvme0n1p6 /mnt
# If your `/boot` is its own partition, mount it now with:
sudo mount /dev/nvme0n1p4
# Otherwise if your `/boot` is a just a folder within root `/`, mount it like this:
sudo mount -o bind /boot /mnt/boot
# Tell chroot where other important folders are:
sudo mount -o bind /dev /mnt/dev
sudo mount -o bind /proc /mnt/proc
sudo mount -o bind /sys /mnt/sys
sudo mount -o bind /run /mnt/run
# And tell the kernel how to access to the efivars directory and mount the EFI system partition:
sudo mount -o bind /sys/firmware/efi/efivars /mnt/sys/firmware/efi/efivars
# Then lastly, mount the EFI partition inside /boot
sudo mount /dev/nvme0n1p5 /mnt/boot/efi
Okay, we are ready to switch control to your normal Fedora install! We do this with chroot:
sudo chroot /mnt
Resetting and reinstalling grub2
Now you're chroot, you no longer need to use sudo for things. You are su already. So excercise appropriate care.
For grub to completely regenerate itself, you need to clear several files.
**Important. If you've customised any files in /etc/grub.d/ (for grub-btrfs, or chainloading, etc.) copy these files somewhere safe (e.g. /home/<yourusername>/backup. For the reinstall to work, only copy the minimum of files you are sure are correct/working.
What we need to do now is delete ALL of grub's configuration files and shim (except your /etc/default/grub config). This is what triggers grub to re-create them for you. If you miss one, reinstalling grub will do nothing.
We will also delete the symbolic links (grub will recreate these too).
rm /boot/efi/EFI/fedora/grub.cfg
rm /etc/grub2.cfg
rm /etc/grub2-efi.cfg
rm /etc/grub.d/*
rm /etc/sysconfig/grub
With all this gone, and everything mounted correctly, grub should now regenerate correctly them on reinstall:
dnf reinstall grub2-* shim-*
If you backed up any configs in /etc/grub.d/, copy them back now.
Now that all the original configs are reset, you should be able to build working grub2 configs:
grub2-mkconfig -o /boot/grub2/grub.cfg
Note: After this on Feodra just use grub2-mkconfig -o /etc/grub2.cfg.
Some final checks...
We're getting close. Lastly we'll check the symlinks in /etc/ are correct:
ls -la /etc/grub2*
They should look exactly like this:
lrwxrwxrwx. 1 root root 20 Oct 27 23:43 /etc/grub2.cfg -> /boot/grub2/grub.cfg
lrwxrwxrwx. 1 root root 22 Oct 12 13:24 /etc/grub2-efi.cfg -> ../boot/grub2/grub.cfg
If not, you can recreate them manually, however this is an indication grub did not reinstall properly, so it's better to go back and examine your steps (and probably reboot and start from the beginning).
Great. Now list your grub2 menu options and make sure they look correct before you reboot with:
grubby --info=ALL
If some are missing or incorrect, check your /etc/default/grub config, and re-run grub2-mkconfig - /etc/grub2.cfg.
If you want to be really sure GRUB2 was properly reinstalled, you can check the files we deleted were recreated:
cat /boot/efi/EFI/fedora/grub.cfg
cat /boot/grub2/grub.cfg
...and make sure those files both have contents. The first file should only be several lines long (that's the shim).
Unmount stuff
Exit the chroot then unmount things, before finally rebooting 😊:
Note: You could probably get away with not doing this, hoewever it makes sure the file system has actually written all changes to the drive before you reboot, especially for COW (copy-on-write) filesystems like Btrfs.
sync && exit
sudo umount /mnt/boot/efi
sudo umount /mnt/sys/firmware/efi/efivars
sudo umount /mnt/sys
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt/run
sudo umount /mnt
reboot
