function display_excerpt_shortcode($atts, $content = null) {
    global $post;
   
	$content = get_the_content();
	$content = strip_shortcodes($content);
	$content = wp_strip_all_tags($content);
	$excerpt = mb_substr($content, 0, 80);
	if (mb_strlen($content) > 80) {
		$excerpt .= '...';
	}
	return $excerpt;
    
    return '';
}

add_shortcode('post_excerpt', 'display_excerpt_shortcode');