gistfile1.js

// in REPL

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


var uid = 123;

child = exec('id -u root', function(error, stdout, stderr) {
    // trying to update the uid variable outside this function
    uid = parseInt(stdout, 10);
    console.log(uid) // prints 0
});

// ...
// afterwards, when the child process is good and done:
console.log(uid) // still prints 123