child_process exec can not response to client broswer

I am trying to execute an command via child_process module and reponse the result to client browser as below, console.log(stdout) work find, but the response.write() doesen't work.

function start(response) {
    dir_list(function(stdout) {
        response.writeHead(200, {"Content-Type": "text/plain"});
        response.write(stdout);
        console.log(stdout);
        response.end();
});     

function dir_list(callback) {
     var exec = require('child_process').exec
     child = exec('ls -la', function(error, stdout, stderr) {
         callback(stdout);
     });
}

Above code is refractored by me which I refer to Node.js: Object value as stdout of child_process.exec, but, unluckly it doesn't work for me.

BTW, my originally code is:

  function start(response) {
      console.log("Request handler 'start' was called.");
      var exec = require('child_process').exec;
      exec("ls -lah", function (error, stdout, stderr) {
      response.writeHead(200, {"Content-Type": "text/plain"});
      response.write(stdout);
      console.log(stdout);
      response.end();
 });