grub repair from unbootable disk

from : https://www.suse.com/de-de/support/kb/doc/?id=000018770 https://www.suse.com/support/kb/doc/?id=000019909

Boot with a media/iso (might need LVM tools, ex. SLES 15). At bootprompt mount a CHROOT environment.

# vgdisplay

# mount /dev/rootvg/lv_root /mnt
# mount /dev/rootvg/lv_home /mnt/home
# mount /dev/rootvg/lv_tmp /mnt/tmp
# ...
# ...
# mount --rebind /proc /mnt/proc
# mount --rebind /sys /mnt/sys
# mount --rebind /dev /mnt/dev
#
## if EFI boot partion is needed
# mount /dev/sda1 /mnt/boot/efi
#
# chroot /mnt

## recreate a grub config
# grub2-mkconfig -o /boot/grub2/grub.cfg
#
## install grub bootloader on bootdevice (BIOS)
# grub2-install /dev/sda
#
## if SHIM (EFI needed partition)
# shim-install --config-file=/boot/grub2/grub.cfg

# exit

# reboot