Meteor callback to sys.exec inside a Meteor.call callback

I have an event triggering a Metor.call():

Meteor.call("runCode", myCode, function(err, response) {
  Session.set('code', response);
  console.log(response);
});

But my runCode function inside the server's Metheor.methods has inside it a callback too and I can't find a way to make it return something to response in the above code.

runCode: function(myCode) {
  var command = 'pwd';

  child = exec(command, function(error, stdout, stderr) {
    console.log(stdout.toString());
    console.log(stderr.toString());

    // I Want to return stdout.toString()
    // returning here causes undefined because runCode doesn't actually return
  });

  // I can't really return here because I don't have yet the valuer of stdout.toString();
}

I'd like a way to have the exec callback return something as runCode without setInterval which would work, but as a hacky way in my opinion.