gistfile1.js

#package.json
{
  "name": "redis_nutcracker_agent",
  "version": "0.0.1",
  "description": "Redis agent for nutcracker update",
  "dependencies": {
    "node-sentinel": "0.0.3",
    "underscore": "~1.4.4",
    "async": "~0.2.5",
    "nodemailer": "~0.3.42"
  },
  "engine": "node >= 0.8.16"
}

#config_development.js

module.exports = {
    "nutcracker_config_file": "/etc/nutcracker/nutcracker.yml",
	"redis_sentinel_ip": "127.0.0.1",
	"redis_sentinel_port": "26379"
};

#index.js
var path = require("path"),
    Agent = require(path.join(__dirname, "./lib/agent.js"));

var config = require(
	path.join(__dirname, "./config_"+process.env.NODE_ENV)
);

module.exports = new Agent(config).bootstrap();

#lib/agent.js
/*jshint multistr:true */
var fs = require('fs'),
    exec = require('child_process').exec,
	path = require('path'),
	os = require('os');

var Sentinel = require("node-sentinel"),
	_ = require("underscore"),
	async = require("async"),
	nodemailer = require("nodemailer");

function Agent(config){
	if(!_.isObject(config)){
		return console.error("Bad config");
	}

	this.nutcracker_config_file = config.nutcracker_config_file;
	this.redis_sentinel_ip = config.redis_sentinel_ip;
	this.redis_sentinel_port = config.redis_sentinel_port;
	this.log = "";
}

Agent.prototype.restart_nutcracker = function(callback){
	var self = this;
	var child = exec(
		"/etc/init.d/nutcracker restart",
		function(error, stdout, stderr) {
			self.log += "<h2>Nutcracker restarted with outuput:</h2>";
			self.log += "<div><pre>"+stdout+"</pre></div>";

			if (error !== null) {
				self.log += "<h2>Nutcracker failed restarting with error:</h2>";
				self.log += "<div><pre>"+error+"</pre></div>";
			}
			return callback();
		}
	);
};

Agent.prototype.update_nutcracker_config = function(data, callback){
	var old_content = fs.readFileSync(this.nutcracker_config_file, 'utf8');

	var new_content = old_content.replace(
		data.details["old-ip"]+":"+data.details["old-port"],
		data.details["new-ip"]+":"+data.details["new-port"]
	);

	fs.writeFileSync(this.nutcracker_config_file, new_content, 'utf8');

	this.log += "<h2>Nutcracker config updated with new content:</h2>";
	this.log += "<div><pre>"+new_content+"</pre></div>";

	return callback();
};

Agent.prototype.send_mail = function() {
	var self = this;
	var transport = nodemailer.createTransport("Sendmail");

	var mailOptions = {
		from: "from@mail.com",
		to: "to@mail.com",
		subject: "Warning: Nutcracker restarted forced on "+os.hostname(),
		generateTextFromHTML: true,
		html: "<h1>Nutcracker was restarted due to master switch in redis configuration.</h1>" + this.log
	};

	transport.sendMail(mailOptions, function(error, responseStatus){
		self.log = "";
		if(error) {
			return console.error(error);
		}

		return console.info(
			responseStatus.message+"\n"+responseStatus.messageId
		);
	});
};

Agent.prototype.switch_master_handler = function(){
	var self = this;
	return function(data) {
		async.series([
			function(callback) {
				self.update_nutcracker_config(data, callback);
			},
			function(callback) {
				self.restart_nutcracker(callback);
			}
		],
		function(){
			return self.send_mail();
		});
	};
};

Agent.prototype.start_sentinel = function(){
	this.sentinel = new Sentinel(
		this.redis_sentinel_ip, this.redis_sentinel_port
	);

	this.sentinel.on("switch-master", this.switch_master_handler());
};

Agent.prototype.check_nutcracker_config = function(cb){
	fs.appendFile(this.nutcracker_config_file, "", cb);
};

Agent.prototype.bootstrap = function(){
	var self = this;

	this.check_nutcracker_config(
		function(error){
			if(error) {
				return console.error("Nutcracker config file: "+error);
			}

			return self.start_sentinel();
		}
	);
};

module.exports = Agent;

btw, I've started seeing what I can do to generalize this as an npm module. Possibly even for publishing in the main npm repo. Are you okay licensing this under BSD?

Yes! No problem