Efficient root fs extraction using dd. As a part of my training I am learning how to create a linux image from a VM installed under KVM. I got to the point of ripping the root fs out of my original image and the instructions had told me to use a block size of 1. It also suggested how it could be sped up and gave the general idea. Finding the gre…

efficent_dd_with_start_and_count.sh

#!/usr/bin/env bash

# set -x
while getopts ":h" opt; do
  case ${opt} in
    h)
      echo "efficent_dd_with_start_and_count.sh INPUT_FILE OUTPUT_FILE START_BYTE BYTE_SIZE"
      exit 0
      ;;
    \?)
      echo "Fail"
      exit 1
      ;;
  esac
done

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

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

multiplier=1 

input_file=$1
output_file=$2
initial_start=$3
initial_count=$4

start=0
count=0
byte_size=${multiplier}

while true; do

  start_divided=$(( ${initial_start} / ${multiplier} ))
  start_remainder=$(( ${initial_start} % ${multiplier} ))
  count_divided=$(( ${initial_count} / ${multiplier} ))
  count_remainder=$(( ${initial_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

echo "dd if=${input_file} of=${output_file} bs=${byte_size} start=${start} count=${count}"