How do I run update-grub from a LiveCD

update-grub_from_LiveCD.md

Howto update-grub from LiveCD

1. List partitions to know which partition is root (/) and mount it

lsblk
sudo mount /dev/sda5 /mnt

Then mount a few more directories that are needed:

sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo mount --bind /proc /mnt/proc

Also, if you have a separate Ubuntu boot partition (pretty uncommon these days, but it may be the case):

sudo mount /dev/sdaX /mnt/boot

Start using the mounted directory as the root partition:

sudo chroot /mnt

2. Edit grub

sudo vi /etc/default/grub

3. Update grub

sudo update-grub