Issues with creating test in Mocha/Node to test if argument is being passed in Command Line

I am creating a simple test in Mocha to see if the command-line arguments are being passed properly. I'm using "child_process" to access the stdout in buffer but mocha gives me the error that my stdout is undefined.

Here is the error:

 main function
    1) should take argument for greetings
    2) should take argunent for goodbye


  2 passing (19ms)
  2 failing

  1) main function should take argument for greetings:

      AssertionError: expected 'hello undefined' to equal 'hello Rahul'
      + expected - actual

      +hello Rahul
      -hello undefined

      at Context.<anonymous> (/Users/rahulsharma/Desktop/CF-HW/simpltest/test/main_test.js:14:40)
      at callFn (/usr/local/lib/node_modules/mocha/lib/runnable.js:251:21)
      at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:244:7)
      at Runner.runTest (/usr/local/lib/node_modules/mocha/lib/runner.js:374:10)
      at /usr/local/lib/node_modules/mocha/lib/runner.js:452:12
      at next (/usr/local/lib/node_modules/mocha/lib/runner.js:299:14)
      at /usr/local/lib/node_modules/mocha/lib/runner.js:309:7
      at next (/usr/local/lib/node_modules/mocha/lib/runner.js:248:23)
      at Object._onImmediate (/usr/local/lib/node_modules/mocha/lib/runner.js:276:5)
      at processImmediate [as _immediateCallback] (timers.js:345:15)

  2) main function should take argunent for goodbye:

      AssertionError: expected 'goodbye undefined' to equal 'goodbye Rahul'
      + expected - actual

      +goodbye Rahul
      -goodbye undefined

      at Context.<anonymous> (/Users/rahulsharma/Desktop/CF-HW/simpltest/test/main_test.js:25:38)
      at callFn (/usr/local/lib/node_modules/mocha/lib/runnable.js:251:21)
      at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:244:7)
      at Runner.runTest (/usr/local/lib/node_modules/mocha/lib/runner.js:374:10)
      at /usr/local/lib/node_modules/mocha/lib/runner.js:452:12
      at next (/usr/local/lib/node_modules/mocha/lib/runner.js:299:14)
      at /usr/local/lib/node_modules/mocha/lib/runner.js:309:7
      at next (/usr/local/lib/node_modules/mocha/lib/runner.js:248:23)
      at Object._onImmediate (/usr/local/lib/node_modules/mocha/lib/runner.js:276:5)
      at processImmediate [as _immediateCallback] (timers.js:345:15)

And here is the test file:

var expect = require('chai').expect;
var main = require('../lib/main');

describe('main function', function (){

    it('should take argument for greetings', function() {
       var exec = require('child_process').exec;
       var name;

       exec('Rahul', function (error, stdout, stderr){
       name = stdout;
       });

       expect(main.greetings(name)).to.equal('hello Rahul');
    });

    it('should take argunent for goodbye', function() {
       var exec = require('child_process').exec;
       var name;

       exec('Rahul', function (error, stdout, stderr){
       name = stdout;
       });

       expect(main.goodbye(name)).to.equal('goodbye Rahul');
    });
});

And file being tested:

var greetme = require('./greet.js');

greetme.greetings('Rahul');
greetme.goodbye('Rahul');



exports.greetings = function(name) {
    this.name = name;
    return ('hello ' + this.name);
}

exports.goodbye = function(name) {
    this.name = name;
    return ('goodbye ' + this.name);
}

process.argv.forEach(function(x){
    greetme.greetings(x);
});

process.argv.forEach(function(x){
    greetme.goodbye(x);
});

Any ideas on what's happening here?