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add_shortcode('field', 'shortcode_field');

function shortcode_field($atts){
  extract(shortcode_atts(array(
   'post_id' => NULL,
  ), $atts));

  if(!isset($atts[0])) return;
  $field = esc_attr($atts[0]);

  global $post;
  $post_id = (NULL === $post_id) ? $post->ID : $post_id;

  return get_post_meta($post_id, $field, true);
}

//Usage:

//[field "my_key"]
//[field "my_key" post_id=1]