reinstall grub2 with EFI

install_grub.sh

#!/bin/bash

# https://superuser.com/questions/376470/how-to-reinstall-grub2-efi
# and some of mine

if ! [ $(id -u) = 0 ]; then
   echo "I am not root!"
   exit 1
fi

# prepare for chroot
mount /dev/sda4 /mnt #sda2 is the root partition
mount /dev/sda1 /mnt/boot/efi #sda1 is the efi partition
for i in /dev /dev/pts /proc /sys; do mount -B $i /mnt$i; done
cp /etc/resolv.conf /mnt/etc/ #makes the network available after chrooting

modprobe efivars # make sure this is loaded

# dump script to run in chroot, run it - does the install
cat <<EOF > /mnt/install_grub_chroot.sh
#!/bin/bash
apt-get update
apt-get install --reinstall grub-efi-amd64 
EOF
chmod +x /mnt/install_grub_chroot.sh

sudo chroot /mnt ./install_grub_chroot.sh

# clean up
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