print_usermeta_shortcode.php

<?php
/**
 * Create [usermeta] shortcode
 *
 * @param $atts An array of attributes
 *
 * @return string A single user meta field
 *
 * @link https://codex.wordpress.org/Shortcode_API
 * @link https://codex.wordpress.org/Function_Reference/shortcode_atts
 * @link https://codex.wordpress.org/Function_Reference/get_user_meta
 */
function wpmu_user_meta_shortcode( $atts ){
	if ( !is_user_logged_in() ) {
		return;
	}

	$a = shortcode_atts( array( 'field' => 'nickname' ), $atts, 'usermeta' );

	$user = wp_get_current_user();

	$field = $user->get( $a['field'] );

	if( null !== $field && is_string( $field ) ){
		return esc_attr( $field );
	}else{
		return '';
	}
}