text: var os = require('os');
  var fs = require('fs');
  
  var platform = os.platform();
  var createFile = function (filepath, size, callback) {
      var cb = function (err) {
          callback && callback(err);
      };
      if (fs.existsSync(filepath)) {
          cb('file existed.');
      } else {
          var cmd;
          switch (platform) {
              case 'win32':
                  cmd = 'fsutil file createnew ' + filepath + ' ' + size;
                  break;
              case 'darwin':
              case 'linux':
                  cmd = 'dd if=/dev/zero of=' + filepath + ' count=1 bs=' + size;
                  break;
          }
          var exec = require('child_process').exec;
          exec(cmd, function (err, stdout, stderr) {
              cb(err);
          });
      }
  };
  
  exports.createFile = createFile;
  