How to detect if a node process/script is already running?

There are OS-specific ways of looking at all the processes running on the current computer so you could use that (by running an external process as a child_process) to see if your server is already running.

But, for a cross platform mechanism, you can just put a little http server (I call a probe server) into your process and then see if that server responds. If it does, then your app is already running. If not, then it's not.

So, put this into your app:

// pick some port not already in use locally const probePort = 8001; const probePath = '/probe'; const http = require('http'); const rp = require('request-promise'); // see if our probe server is already running rp({ uri: `http://localhost:${probePort}${probePath}`, resolveWithFullResponse: true }).then(res => { if (res.statusCode === 200) { console.log(`Our server already running in another process`); process.exit(1); } else { throw new Error(`statusCode ${res.statusCode}`) } }).catch(err => { // our process is not already running // start our own copy of the probeServer const server = http.createServer((req, res) => { if (req.url === probePath) { res.statusCode = 200; res.end("ok"); } else { res.statusCode = 404; res.end("err"); } }); server.listen(probePort); // make sure this server does not keep the process running server.unref(); // proceed with the rest of your process initialization here console.log("Our server not already running, so we can now start ours"); setInterval(() => { console.log("still running"); }, 1000); }); 

Note: If you already have an http server in your app, then you can just create a special probePath route in that server that responds with a 200 status and test that rather than creating another http server.

If you're looking to monitor the nodejs process, there are plenty of good libraries out there such as foreverjs, pm2, etc. However, to answer your question more directly, you could do this by forking your process from within a node script (another process effectively):

var exited = false; var cp = require('child_process'); var superProcess = cp.fork('./my-super-process'); superProcess.on('exit', (code, signal) => { exited = true; }); setInterval(function() { if (exited) { console.log('is gone :(') } else { console.log('still running ...') } }, 1000); 

If you can somehow know the process id (PID) of your targeting PID (e.g., your server.js may always store the PID name in server.pid file), you can use is-running package to know if it is running.

var _ = require('underscore') var sh = require('shelljs') var isRunning = require('is-running') if(! _.contains(sh.ls("."), 'server.pid')){ // If PID file does not exist console.log("PID file does not exist.") } else { // PID file exists let pid = sh.cat('server.pid').stdout if(!isRunning(pid)){ console.log("server.js is not running") } else { console.log("server.js is running") } } 

Or if you target any arbitrary process, how about this?

if(sh.exec('ps aux', {silent: true}).grep('npm').stdout != "\n"){ console.log("some npm process is running") }