adding user shortcode to display any user data

gistfile1.txt

// Add Shortcode
function user_shortcode( $atts ) {
    $a = shortcode_atts(
        array(
            'user_id'   => get_current_user_id(),
            'field'     => '',
		), $atts );
​
    $user = get_user_by( 'id', $a['user_id'] );
    
    $output = !empty($user->$a['field']) ? $user->$a['field'] : '';
​
    return $output;
}
add_shortcode( 'user', 'user_shortcode' );

Now you can display any user data via shortcode.

[user field="user_login" user_id="1"]

will display the user login for user id 1. if user_id is omitted, defaults to current_user.

[user field="first_name" user_id="1"]

will display the first name for user_id 1.