#!/usr/bin/env bash

set -e

function print_help() {
    echo "$0: get a chroot shell inside of a Raspbian ISO"
    echo "Usage: $0 target.img"
    echo "    -h            Print this help"
    echo "    -i [target]   Specify an ISO to chroot to (probably a Pi Foundation Raspbian)"
    exit 1
}

args=$(getopt -uo 'ht:i:' -- $*)
[ $? != 0 ] && print_help
set -- $args

for i
do
      case "$i"
      in
            -h)
                    print_help
                    ;;
            -i)
                    target_image="$2"
                    echo "Image = ${2}"
                    shift
                    shift
                    ;;
      esac
done

if [[ $EUID -ne 0 ]]; then
    echo "$0 must be run as root. try: sudo $0"
    exit 1
fi

target_mnt="/media/raspbian-target"

# assemble a mostly-legit filesystem by mounting / and /boot from the target
# iso, plus /dev from the host pi (/dev/(u)random seems to be required by
# recent versions of GPG):
echo "Mounting $target_image on $target_mnt"
kpartx -av $target_image
mkdir -p $target_mnt
mount /dev/dm-1 $target_mnt
mount /dev/dm-0 $target_mnt/boot
mkdir -p $target_mnt/dev
mount --bind /dev $target_mnt/dev

echo "Dropping you into Bash - image will be unmounted after exit"
chroot $target_mnt sudo /bin/bash

echo "Unmounting $target_image"
umount $target_mnt/boot
umount $target_mnt/dev
umount $target_mnt
kpartx -d $target_image