This is a continuation from gist:5409137 but based off of Euca linux image creation instructions. So as long as you have created your raw image with the three partitions correctly this script will pull out the root image.

rip_rootfs_from_raw_img.sh

#!/usr/bin/env bash

# set -x
# DEBUG='echo'

get_next_multiplier() {
  current_multiplier=$1
  multiplier=$(( ${current_multiplier} * 2 ))
}

set_variables() {
  start=$1
  count=$2
  bs=$3
}

calc_max_divisor() {
  multiplier=$1
  start_block=$2
  block_count=$3
  
  while true; do
    start_divided=$(( ${start_block} / ${multiplier} ))
    start_remainder=$(( ${start_block} % ${multiplier} ))
    count_divided=$(( ${block_count} / ${multiplier} ))
    count_remainder=$(( ${block_count} % ${multiplier} ))

    [ "${start_divided}" -eq "0" ] && break
    [ "${start_remainder}" -ne "0" ] && break
    [ "${count_divided}" -eq "0" ] && break
    [ "${count_remainder}" -ne "0" ] && break
    [ "${start_remainder}" -eq "0" ] && [ "${count_remainder}" -eq "0" ] && set_variables ${start_divided} ${count_divided} ${multiplier}

    get_next_multiplier ${multiplier}
  done
}

get_output_device() {
  device=$1
  [ -n "$( echo anImage.img | sed -n '/.img/p' )" ] && output_device=$( echo "${device}" | sed 's/.img/-root.img/g' ) || exit 3
}

while getopts ":h" opt; do
  case ${opt} in
    h)
      echo "rip_rootfs_from_raw_img INPUT_FILE <OUTPUT_FILE>"
      exit 0
      ;;
    \?)
      echo "Fail"
      exit 1
      ;;
  esac
done

device=$1
[ ! -f "${device}" ] && exit 1
output_device=$2
[ -n "${output_device}" ] && [ -f "${output_device}" ] && exit 2 || get_output_device "${device}"
block_size=1
start_block=$( parted ${device} unit b print | grep ' 2 ' | awk '{print $2}' | sed 's/B//g' )
block_count=$( parted ${device} unit b print | grep ' 2 ' | awk '{print $4}' | sed 's/B//g' )

bs=0
start=0
count=0

calc_max_divisor "${block_size}" "${start_block}" "${block_count}"

${DEBUG} dd if="${device}" of="${output_device}" bs="${bs}" skip="${start}" count="${count}"