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// 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' );