Please use below code
function html_show($atts) {
$makearr = explode(",",$atts['content']);
if(isset($makearr) && !empty($makearr)) {
foreach($makearr as $arrval){
$stringli .= '<li>'. $arrval.'</li>';
}
} else {
$stringli = '';
}
$htmlcode="<div class="notify"><span class="symbol icon-fact"></span> <strong>A kind of a notice box !</strong><ul>".$stringli.'</ul>
</div>';
return $htmlcode;
}
add_shortcode( 'htmltag', 'html_show' );
/*******Using method*******/
echo do_shortcode('[htmltag content="message1,message2,message3,message4"]');