function ask_user_if_empty {
  # If variable with name $1 is zero, then ask user for input.
  # Only one line user input is allowed.
  # User input must not be empty.
  #
  # arg $1: name of variable to store user input
  # arg $2: text to display (e.g. "Enter hostname:")
  if [ "$#" -ne 2 ]; then
    echo 'ask_user_if_empty requires two args!';
    return 1
  fi
  for i in "$@"; do
    if [ -z "${i}" ]; then
      echo 'ask_user_if_empty: all given args must not be empty';
      return 1;
    fi
  done

  local -n ptr=$1 || return $?
  if [ -z "${ptr}" ]; then
    # if ptr has no value yet, ask user for input!
    echo "${2}"
    read -r ptr || return $?
  fi

  # check string length to be greater than zero!
  if [ "${#ptr}" -lt 1 ]; then
    echo 'The input must not be empty!';
    return 1;
  fi
}

function get_single_choice {
  # If variable with name $1 is zero, then ask user to select one option.
  #
  # arg $1: name of variable to store the selected option
  # arg $2: text to display
  # arg $3: name of variable with array of options to display (for each option there must be two entries in the array: Item and description)
  if [ "$#" -ne 3 ]; then
    echo 'get_single_choice requires three args!';
    return 1
  fi
  for i in "$@"; do
    if [ -z "${i}" ]; then
      echo 'get_single_choice: all given args must not be empty';
      return 1;
    fi
  done


  local -n ptr=$1 || return $?
  if [ -z "${ptr}" ]; then
    # if ptr has no value yet, ask user for input!

    local -n MENU_OPTIONS=$3 || return $?
    ptr=$(dialog --stdout --menu "${2}" 0 0 0 "${MENU_OPTIONS[@]}") || {
      echo 'Error during menu selection!'
      exit 1
    }
    clear
  fi
}

function get_multi_choice {
  # If variable with name $1 is zero, then ask user to select one ore more options.
  #
  # arg $1: name of variable to store array of selected options
  # arg $2: text to display
  # arg $3: name of variable with array of options to display (for each option there must be three entries in the array: Item, description, on/off)
    if [ "$#" -ne 3 ]; then
    echo 'get_multi_choice requires three args!';
    return 1
  fi
  for i in "$@"; do
    if [ -z "${i}" ]; then
      echo 'get_multi_choice: all given args must not be empty';
      return 1;
    fi
  done


  local -n ptr=$1 || return $?
  if [ -z "${ptr}" ]; then
    # if ptr has no value yet, ask user for input!

    local -n MENU_OPTIONS=$3 || return $?
    TMP1=$(dialog --stdout --checklist "${2}" 0 0 0 "${MENU_OPTIONS[@]}") || {
      echo 'Error during menu selection!'
      exit 1
    }
    clear

    # Result of dialog is space separated list
    # Store this as an array
    # Without newline at last array element: https://unix.stackexchange.com/a/519917/315162
    # readarray -d " " -t ptr < <(printf '%s' "$TMP1")
    #
    space_separated_to_array TMP1 "$1"
  fi
}