function ask_user_if_empty {
  # If variable with name $1 is empty, then ask for user input.
  #
  # Only one line of user input is allowed.
  # The 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 single_choice_if_empty {
  # If variable with name $1 is empty, then let user select one of the given options.
  #
  # 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 'single_choice_if_empty requires three args!';
    return 1
  fi
  for i in "$@"; do
    if [ -z "${i}" ]; then
      echo 'single_choice_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!

    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 multi_choice_if_empty {
  # If variable with name $1 is empty, then let user select one or more of the given 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 'multi_choice_if_empty requires three args!';
    return 1
  fi
  for i in "$@"; do
    if [ -z "${i}" ]; then
      echo 'multi_choice_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!

    local -n MENU_OPTIONS=$3 || return $?
    # shellcheck disable=SC2034 # Variable name used below.
    TMP1=$(dialog --stdout --checklist "${2}" 0 0 0 "${MENU_OPTIONS[@]}") || {
      echo 'Error during menu selection!'
      exit 1
    }
    clear

    # Result of dialog is a 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
}