why Witty Laravel5.0 Db Backup Artisan::call('db:backup') respond only empty files

I've using php artisan to back update database but all files which I got from always empty.

function export_db() {

   Artisan::call('db:backup');
    $dir = public_path('backups');
    $files = scandir($dir);

    foreach($files as $f){
     if($f!='index.html' && strlen($f) > 5) $file_arr[$f] = $f;
    } 
    $ff = end($file_arr);
    return Response::download($dir.'/'.$ff, $ff);
}

Result

-- MySQL dump 10.13  Distrib 5.5.46, for debian-linux-gnu (x86_64)
--
-- Host: localhost    Database: tgo_loansystem
-- ------------------------------------------------------
-- Server version   5.5.46-0ubuntu0.14.04.2-log

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

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