POSIX script to access EXT4 partitions in img files

copy-out.sh

#!/bin/sh
# Abort script at first error, when a command exits with non-zero status
# (except in until or while loops, if-tests, list constructs)
set -e

# Attempt to use undefined variable outputs error message, and forces an exit
set -u

# Similar to verbose mode (-v), but expands commands
#set -x

# Causes a pipeline to return the exit status of the last command in the pipe
# that returned a non-zero return value.
set -o pipefail

# Causes libguestfs to display (very verbose) debug information
LIBGUESTFS_DEBUG=0

# Causes libguestfs to display a few dozens of traces
LIBGUESTFS_TRACE=0

_confirmOrQuit() {
  QUESTION="${1:-"Do you want to continue?":}"
  read -n1 -s -r -p "${QUESTION}" key
  printf "\n\e[1A\e[K\e[2m%s\e[m" "${QUESTION}"
  if [ "$key" = '' ]; then
    printf "\e[1;32m✔\e[m\n"
  else
    printf "\e[1;91m✘\e[m\n"
    exit 0
  fi
}

_isAbsolute() {
  case "$1" in
  /*)
    return 0
    ;;

  *)
    return 1
    ;;
  esac
}

GUESTFISH_IMAGE="$(find -s . -iname "*.img" -not -path "./shared/*" | head -n 1)"
if [ ! -f "${GUESTFISH_IMAGE}" ]; then
  printf "\e[1;31mNo image found to copy out from.\e[39m\n"
  printf "Please run \e[1;96m%s\e[39m in a directory containing an \e[1;96m%s\e[39m file.\n" "$(basename "$0")" ".img"
  exit 1
fi

printf "\e[40;90m░\e[49;39m\e[46;96m░\e[49;39m\e[44;94m░\e[49;39m\e[42;92m░\e[49;39m\e[43;93m\e[49;39m\e[45;95m░\e[49;39m\e[41;91m░\e[49;39m \e[96m%s\e[39m \e[36m%s\e[39m\n" "COPY-OUT" "from ${GUESTFISH_IMAGE}"

PATH_TO_COPY="${1:-/}"

if ! _isAbsolute "${PATH_TO_COPY}"; then
  _confirmOrQuit "$(printf "An absolute path is required. Use \e[1;96m/%s\e[m instead? " "${PATH_TO_COPY}")"
  PATH_TO_COPY="/${PATH_TO_COPY}"
fi

if [ "${PATH_TO_COPY}" = "/" ] && [ -n "$(find "${GUESTFISH_IMAGE}" -prune -size +200000000c)" ]; then
  _confirmOrQuit "$(printf "The image is rather big. Are you sure you want to copy-out \e[1;96m%s\e[39m ? " "${PATH_TO_COPY}")"
fi

PATH_TO_COPY_TO=$(dirname "${PATH_TO_COPY}")
if [ "${PATH_TO_COPY_TO}" = "/" ]; then
  PATH_TO_COPY_TO=""
fi

printf "Copying out \e[1;96m%s\e[m to \e[1;96m%s\e[m ...\n" "${PATH_TO_COPY}" "shared${PATH_TO_COPY_TO}"

GUESTFISH_SHARED_DIR="$(pwd)/shared"
if [ ! -e "${GUESTFISH_SHARED_DIR}" ]; then
  mkdir "${GUESTFISH_SHARED_DIR}"
fi

docker 2>&1 run \
  --env LIBGUESTFS_DEBUG="${LIBGUESTFS_DEBUG}" \
  --env LIBGUESTFS_TRACE="${LIBGUESTFS_TRACE}" \
  --rm \
  -i \
  --mount type=bind,source="${GUESTFISH_SHARED_DIR}",target=/shared \
  --mount type=bind,source="$(pwd)/${GUESTFISH_IMAGE}",target=/images/disk.img \
  -w / \
  bkahlert/libguestfs \
  /usr/bin/guestfish \
  --ro \
  --add /images/disk.img \
  --mount /dev/sda2:/ \
  --mount /dev/sda1:/boot \
<<COMMANDS
!mkdir -p "shared${PATH_TO_COPY}"
-copy-out "${PATH_TO_COPY}" "shared${PATH_TO_COPY_TO}"
umount-all
exit
COMMANDS

if [ -z "$(ls -A "${GUESTFISH_SHARED_DIR}")" ]; then
  rmdir "${GUESTFISH_SHARED_DIR}"
fi

printf "Successfully copied out \e[1;96m%s\e[39m to \e[1;96m%s\e[39m\n" \
  "${PATH_TO_COPY}" \
  "shared${PATH_TO_COPY_TO}"

Simply type copy-out.
This script searches the current directory automatically for an .img file and copies its content to a newly created directory shared.

Type copy-out <path> to only copy a specific directory or file.

Uses a dockerized guestfish to do its magic. Should work on all POSIX platforms with installed Docker but only tested on macOS.