Send command to running node process, get back data from inside the app

Question

I start a node.js app per commandline in linux. I see the app running, e.g. by entering "top". Is there a way to send some command to the running app (maybe to the pid?) and get back info from inside it (maybe listen for some input and return requested info)?

Answer 1

Use repl module. There are examples in the doco doing exactly what you need: run JS in the context of your application and return output.

Answer 2

One simple solution is to use process signals. You can define a handler for a signal in your program to output some data to the console (or write to a file or to a database, if your application is running as a service not attached to a terminal you can see):

process.on('SIGUSR1', function() {
  console.log('hello. you called?');
});

and then send a signal to it from your shell:

kill --signal USR1 <pid of node app.js>

This will invoke the signal handler you have defined in your node.js application.