Display current template filename in WordPress administration menu bar

functions.admin_bar_menu.template_name.php

<?php

/**
 * code to be placed in theme functions.php
 * add_filter priority of 200 or greater to display last.
 */
if ( ! function_exists( 'mytheme_admin_bar_menu' ) ) {
	function mytheme_admin_bar_menu( $wp_admin_bar ) {
		if ( defined( 'WP_DEBUG' ) && WP_DEBUG && ! is_admin() ) {
			global $template;

			$wp_admin_bar->add_menu( array(
				'parent' => false,
				'id' => 'template',
				'title' => preg_replace( '#^' . $_SERVER['DOCUMENT_ROOT'] .'#', '~', $template ),
				'href' => false,
			));
		}
	}

	add_filter( 'admin_bar_menu', 'mytheme_admin_bar_menu', 200 );
}