Shortcode to show the code

get_footer() echos the contents of the footer.php file. add_shortcode() requires that the callback return the contents of the shortcode and not echo it. I suppose you could do something like this if you really want.

function show_footer() {
  ob_start();
  get_footer();
  return ob_get_clean();
}
add_shortcode( 'show_f', 'show_footer' );