Alright, let's work through this GRUB issue you're facing after a botched Ubuntu update—this is a pretty common scenario, and we can get your system back up and running with a few targeted steps.

Step 1: Locate Your Ubuntu Installation Partition #

When stuck at the GRUB prompt, even if the ls command throws weird errors, we need to find which partition holds your Ubuntu files. Here's how:

• Run ls first to see all detected disks (you'll see entries like (hd0) or (hd1)).
• For each disk, list its partitions: e.g., ls (hd0) will show partitions such as (hd0,1) or (hd0,2).
• Check each partition with ls (hd0,2)—look for core system directories like boot, bin, etc, or home. That's your target partition (the one with your Ubuntu install).

Step 2: Boot into Ubuntu Directly from GRUB #

Once you've identified the correct partition (we'll use (hd0,2) as an example), run these commands in sequence:

1. set root=(hd0,2)
2. set prefix=(hd0,2)/boot/grub
3. insmod normal
4. normal

This should load the standard GRUB menu. Select your Ubuntu entry to boot into the system like normal.

Step 3: Permanently Repair GRUB to Avoid Future Loops #

After successfully booting into Ubuntu, open a terminal and run these commands to fix GRUB for good:

• First, update the GRUB configuration with your current system setup:

  sudo update-grub
  

• Then reinstall GRUB to your system's main disk (replace /dev/sda with your actual disk—use lsblk to check if you're unsure; it's the disk name without a partition number, e.g., /dev/nvme0n1 for NVMe drives):

  sudo grub-install /dev/sda
  

Handling "Unknown Filesystem" Errors #

If running ls (hdX,n) returns an "unknown filesystem" error, your partition might be corrupted. Here's how to resolve that:

1. Boot from an Ubuntu installation USB drive, select "Try Ubuntu without installing".
2. Open a terminal and run lsblk to find your Ubuntu partition (look for the one labeled ext4).
3. Run a filesystem check to repair errors (replace /dev/sdXn with your partition path):

   sudo fsck /dev/sdXn
   

   Follow the on-screen prompts to fix any detected issues, then repeat Steps 1-3 above to repair GRUB.

内容的提问来源于stack exchange，提问作者Clark Allen