gistfile1.js

// in REPL

var exec = require('child_process').exec;
var child;

var uid = 123;

child = exec('id -u root', function(error, stdout, stderr) {

	uid = parseInt(stdout, 10);
	console.log(uid) // prints 0

	verifyOutput();
});

function verifyOutput()
{
	console.log(uid);
}