Add custom shortcode to output user name

welcome-message.php

<?php
// Refer: https://wordpress.stackexchange.com/questions/49686/how-do-i-display-logged-in-username-if-logged-in
// Recommend adding as a code snippet using the code snippets plugin. Can also add to functions.php but will need to remember to transfer if change theme

function mysite_custom_shortcode_makers() {

	if( is_user_logged_in() ) {
		$user = wp_get_current_user();
		return '<h2>Hello ' . $user->first_name . ', welcome to your shop dashboard!</h2>';
	}
}

add_shortcode( 'welcome_message', 'mysite_custom_shortcode_makers' );

?>

If you need to tweak the template you can use <?php echo do_shortcode( '[welcome_message]' ); ?>