linux - How to reinstall GRUB2 EFI?

After successfully updating my bios, something went wrong and I ended up with a blinking cursor on the top left corner of a black screen. No errors, no nothing. The bios now only listed a SATA:  boot option in place of the usual UEFI ubuntu one. I'm using a GPT partitioning scheme.

I eventually found that the working solution was to properly reinstall grub-efi-amd64. So, how do I do this ?

PS: Actually, i succeeded to reinstall GRUB2 EFI on my own and will post my answer here as I was unable to find any complete how-to on this.

Answer

• 
  

  Boot your computer with a live-USB/CD in UEFI mode. I had two boot options and UEFI: , the second is needed to expose the efi variables in /sys/firmware/efi/ so that efibootmgr don't fail later on. Booting with the first option gives me the following error:

  
  

  Fatal: Couldn't open either sysfs or procfs directories for accessing EFI variables.
  Try 'modprobe efivars' as root.
  

  
  

  modprobe efivars did'nt work for me.

  
  


• 
  

  chroot into the broken system (similar to the ubuntu grub2 help but with efi specificities):

  
  

  sudo mount /dev/sda2 /mnt #sda2 is the root partition
  sudo mount /dev/sda1 /mnt/boot/efi #sda1 is the efi partition
  for i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt$i; done
  sudo cp /etc/resolv.conf /mnt/etc/ #makes the network available after chrooting
  modprobe efivars # make sure this is loaded
  sudo chroot /mnt
  



• 
  

  Depending on your linux distribution, you now do different things.

  
  
  
  
  • 
    

    For Ubuntu/Debian:

    
    

    apt-get install --reinstall grub-efi-amd64
    

    
    

    or alternatively:

    
    

    apt-get install --reinstall grub-efi
    update-grub
    

    
    

    should the above give you a grub, but not a bootable one

    
    
  
  
  • 
    

    For Fedora (up to 16, may work for others):

    
    

    yum reinstall grub-efi
    

    
    

    In the following command, you have to replace sdX with the device which has the EFI partition you want to boot from. In --part Y you have to replace the Y with the number of the EFI partition (as in /dev/sdXY).

    
    

    efibootmgr -c --disk /dev/sdX --part Y
    efibootmgr -v # verify a new record called Linux is there
    

  
  
  
  


• 
  

  Now type Ctrl+D to exit chroot, unmount everything and reboot:

  
  

  for i in /sys /proc /dev/pts /dev; do sudo umount /mnt$i; done
  sudo umount /mnt/boot/efi #please do this. Corrupted efi partitions are not nice
  sudo umount /mnt
  sudo reboot
  

You may need to adapt this to your needs (different partition table, separate /boot partition, etc.) and it may not be the only option but this worked just fine for me.

A suitable live-system for fixing things is grml. There is also an extensive guide on how to setup a bootable USB device, of which the Mac section is the most useful actually (just create a FAT32 partition, copy the files, reboot, done).