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Reinstall GRUB for Ubuntu

Ezra UNIX/Linux Ubuntu Linux

So I accidentally damaged my Ubuntu EFI data today, which caused my system to fail to start. Now, I have to reinstall GRUB:

  1. Boot from the live CD or live USB, in "Try Ubuntu" mode.

  2. Determine the partition number of your main partition. sudo fdisk -l, sudo blkid or GParted (which should already be installed, by default, on the live session) can help you here. I'm going to assume in this answer that it's /dev/sda2, but make sure you use the correct partition number for your system!

    If your main partition is in an LVM, the device will instead be located in /dev/mapper/, most likely, /dev/mapper/{volume}--{os}-root where {volume} is the LVM volume name and {os} is the operating system. Execute ls /dev/mapper for the exact name.

  3. Mount your partition:

    sudo mount /dev/sda2 /mnt  #Replace sda2 with the partition from step 2

    If you have a separate /boot, /var or /usr partitions, repeat steps 2 and 3 to mount these partitions to /mnt/boot, /mnt/var and /mnt/usr respectively. For example,

    sudo mount /dev/sdXW /mnt/boot
sudo mount /dev/sdXY /mnt/var
sudo mount /dev/sdXZ /mnt/usr

    replacing sdXW, sdXY, and sdXZ with the respective partition numbers.

  4. Bind mount some other necessary stuff:

    for i in /sys /proc /run /dev; do sudo mount --rbind "$i" "/mnt$i"; done

  5. If Ubuntu is installed in EFI mode (see this answer if you're unsure), use sudo fdisk -l | grep -i efi or GParted to find your EFI partition. It will have a label of EFI. Mount this partition, replacing sdXY with the actual partition number for your system:

    sudo mount /dev/sdXY /mnt/boot/efi

  6. chroot into your Ubuntu install:

    sudo chroot /mnt

  7. At this point, you're in your install, not the live session, and running as root. Update grub:

    update-grub

    If you get errors or if going up to step 7 didn't fix your problem, go to step 8. (Otherwise, it is optional.)

  8. Depending on your situation, you might have to reinstall grub:

    grub-install /dev/sda
update-grub # In order to find and add windows to grub menu.

  9. If Ubuntu is installed in EFI mode, and EFI partition UUID has changed, you may need to update it in /etc/fstab. Compare it:

    blkid | grep -i efi
grep -i efi /etc/fstab

    If current EFI partition UUID (from blkid) differs from the one in /etc/fstab, update /etc/fstab with current UUID.

  10. If everything worked without errors, then you're all set:

    exit
sudo reboot

  11. At this point, you should be able to boot normally.

If you cannot boot normally, and didn't do step 8 because there were no error messages, try again with step 8.

  • Sometimes giving GRUB2 the correct configuration for your partitions is not enough, and you must actually install it (or reinstall it) to the Master Boot Record, which step 8 does. Experience helping users in chat has shown that step 8 is sometimes necessary even when no error messages are shown.