A quick shortcode to get a user's username injected into the page

mg_username_shortcode.php

add_shortcode('mg_username', 'mg_username_shortcode');

function mg_username_shortcode($args) {
    $user = wp_get_current_user();
    if ($user && !empty($user->user_login)) {
        return $user->user_login;
    }
    else {
        return '';
    }
}