function helloFunction($atts, $content) {
     extract(shortcode_atts(array(
        "id" => "",
    ), $atts));

    //if $id isnt empty
    if($id != "") {
        //i created a new variable called $theid which = id="whatever you put in"
        $theid = ' id="'.$id.'"';
    }
            //$theid wont equal anything unless you write [writehello id="something"]
    return '<div'.$theid.'>'.$content.'</div>';
}

add_shortcode("writehello", "helloFunction");