gistfile1.js

  var run_cmd = function(cmd, args) {
    var promise = require('bluebird');
    return new promise(function (resolve, reject) {
      var spawn = require('child_process').spawn;
      var child = spawn(cmd, args);
      var resp = "";
  
      child.stdout.on('data', function (buffer) { resp += buffer.toString() });
      //child.stdout.on('end', function(){ resolve(resp); });
      child.on('close', function(exitCode){ resolve(resp); }); // < This might be more approperate
      child.on('error', function(err){ reject(resp); });
    });
  }

  // Example of use
  run_cmd(...)
    .then(function(resp){})

  // Adding to a existing Promise chain (recommended)
  Promise...
    .then(...)
    ...
    .then(function(){
      return run_cmd(...);
    })
    .then(function(resp){
      ...
    })

  // Adding to a existing Promise chain
  Promise...
    .then(...)
    ...
    .then(function(){
      return run_cmd(...);
        .then(function(resp){
          ...
        });
    })