function display_user_full_name() {
    if (is_user_logged_in()) {
        $current_user = wp_get_current_user();
        $first_name = $current_user->user_firstname;
        $last_name = $current_user->user_lastname;
		$display_name =  $first_name . ' ' . $last_name;
		if (empty(trim($display_name))) $display_name = $current_user->nickname;
        return $display_name;
    } 
}
add_shortcode('user_full_name', 'display_user_full_name');