app.js

...

http.createServer(function (request, response) {
    response.writeHead(200, {'Content-Type': 'text/html'});
    response.end(htmlFile);

    // Take a screenshot asynchronously
    var handler = function(erro, stdout, stderr) {
        console.log(stdout);
    }

    exec('./snapshot.sh', handler);

}).listen(3000, "127.0.0.1");

...