#!/bin/sh -x

fail() {
    if [ -n "$*" ]; then
      echo "$*"
    fi
    exit 1
}

get_fs_details() {
  df --block-size 1 "${1}" | \
    while read device size used available use_pct mounted_on
    do
      echo "#${mount_point}# #${mounted_on}#"
      if [ "${mount_point}" = "${mounted_on}" ]; then
        echo "fs_size=${size} fs_device=${device}"
        break
      fi
    done
}

mount_point="${1%%/}"
dest_dir="${2}"

test -z "${mount_point}" && fail "Usage: ${0} mount_point destination_directory"
test -z "${dest_dir}" && fail "Usage: ${0} mount_point destination_directory" 
test -d "${mount_point}" || fail "Error: mount point ${mount_point} does not exist or is not a directory."
test -d "${dest_dir}" || fail "Error: destination ${dest_dir} does not exist or is not a directory."

eval "$(get_fs_details "${mount_point}")"
test -z "${fs_size}" && fail "Error: failed to determine filesystem size for ${mount_point}."
test -z "${fs_device}" && fail "Error: failed to determine device for ${mount_point}."

if ! cd "${dest_dir}"; then
  fail "Error: failed to change directory to ${dest_dir}"
fi

# Try various unpacking commands, exiting as soon as one of them is successful.
# Note that we need to silence dd error, as it will always fail when running out of data.

if (dd if="${fs_device}" skip="${fs_size}" bs=1; true) | tar xzf -; then
  exit 0
fi

if (dd if="${fs_device}" skip="${fs_size}" bs=1; true) | tar xjf -; then
  exit 0
fi

fail "Error: failed to extract additional data."