Ubuntu – Recovering moved .mod files using grubrescue

grub2grubrescue

I did a dumb thing… I forgot that Ubuntu 10.04 (Lucid Lynx) switched to GRUB 2 which puts a ton of *.mod files (kernel modules) in /boot/grub. I thought they were soundtrack files put there erroneously, and I moved them. Needless to say, the next reboot was traumatic. I was presented with something I had no memory of ever seeing… a 'grub rescue>' prompt.

With the help of Fixing GRUB error: “error: unknown filesystem” however, I was able to recover…

  • I discovered that GRUB rescue does not have 'cd', 'cp' or any other filesystem commands except its own variation of 'ls'.

  • So first I had to find the partition with the /boot directory containing vmlinuz file and other boot image files… (approximation from memory of failed attempts, as well as blank lines for clarity, added 2014-07-10 by docsalvage) 

    grub rescue>  ls  
(hd0,4) (hd0,3) (hd0,2) (hd0,1)  

grub rescue>  ls (hd0,4)/boot
... some kind of 'not found' message

grub rescue>  ls (hd0,3)/boot
... some kind of 'not found' message

grub rescue>  ls (hd0,2)/boot
... grub ... initrd.img-2.6.32-33-generic ... vmlinuz-2.6.32-33-generic

  • I found a /boot directory containing the vmlinuz file vmlinuz-2.6.32-33-generic on partition (hd0,2).

  • Then I manually booted from the 'grub rescue>' prompt. The following commands will…

    • Set the root to use the /boot directory on partition (hd0,2).
    • Load kernel module linux.
    • Set that module to use the kernel image vmlinuz-2.6.32-33-generic.
    • Set initrd(initialize RAM disk) to use the image initrd.img-2.6.32-33-generic.
    • Boot Linux.
  • grub rescue>  set root=(hd0,2)/boot  
grub rescue>  insmod linux  
grub rescue>  linux (hd0,2)/boot/vmlinuz-2.6.32-33-generic root=/dev/sda2
grub rescue>  initrd (hd0,2)/boot/initrd.img-2.6.32-33-generic  
grub rescue>  boot

  • This boots and crashes to the BusyBox prompt which DOES have some rudimentary filesystem commands.

  • Then I moved the *.mod files back to the /boot/grub directory…

    busybox>  cd /boot  
busybox>  mv mod/* grub
busybox>  reboot

  • The reboot was successful, but that was a lot of work.

Is there an easier way?

Best Answer

No. I think you pretty much found the easiest way to recover from the state of your system using grub rescue; it is a very minimalistic system giving just enough capability to boot the system.

BTW, I believe you must have found the .mod files and executed insmod linux or the linux command would have failed.

The only other way, as mentioned, would be to boot a Live CD and reinstall grub2 after chroot'ing to the 'broken' system.

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