node.js - nodejs和gulp一起使用碰到的问题？

Question

var gulp = require('gulp');
var child = require('child_process');
var express = require('express');
var app = express();

//......

app.use(express.static('public'));
app.use(bodyParser.json()); 
app.use(bodyParser.urlencoded({ extended: true })); 

app.get('/', function (req, res) {
  //......
});
//此处我想当post时，执行默认的gulp task，但是出现了错误，错误如下
app.post("/", function(req, res){
  //.....
  child.exec('gulp', function(err){
    if(err)
      console.log(err);
  });
});

var server = app.listen(3000, function (req, res){
  console.log("程序正在运行......");
  // gulp.start('default');
});

node app.js文件后，错误如下：

{ [Error: Command failed: /bin/sh -c gulp
events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE :::3000
    at Object.exports._errnoException (util.js:837:11)
    at exports._exceptionWithHostPort (util.js:860:20)
    at Server._listen2 (net.js:1231:14)
    at listen (net.js:1267:10)
    at Server.listen (net.js:1363:5)
    at EventEmitter.listen (/home/quoniam/nodeStudy/gulp-musheng/node_modules/express/lib/application.js:617:24)
    at Object.<anonymous> (/home/quoniam/nodeStudy/gulp-musheng/gulpfile.js:49:18)
    at Module._compile (module.js:434:26)
    at Object.Module._extensions..js (module.js:452:10)
    at Module.load (module.js:355:32)
] killed: false, code: 1, signal: null, cmd: '/bin/sh -c gulp' }

我想问一下这个问题怎么解决？（问题：我想当post时，执行默认的gulp task，但是出现了错误）