NodeJS parse process.stdout to a variable

Question

I am trying to read from stdout and put it to a variable in NodeJS. This is what I've tried:

process.stdout.on('data', function(data) {
    console.log(data.toString())
});

console.log('hello')

I expect it to output hello two times, but it only outputs it one time. Why is the callback not triggered? It works when using stdin..

Answer 1

You are trying to use the Readable Stream API on a Writable Stream . Specifically, the process.stdout stream is for writing to stdout not reading from stdout . What you are trying to do isn't possible, would create an infinite loop if it did work, and breaks the definition of what stdin and stdout are .

Answer 2

You can't do it. Read @mattr's answer .

Instead, you can hook into the process.stdout.write and get all the text printed to process.stdout . I'm using the code below with my Node.js server.

global.serverLog = "";
process.stdout.write = (function(write) {
        return function(string, encoding, fileDescriptor) {
                global.serverLog += string;
                write.apply(process.stdout, arguments);
            };
    })(process.stdout.write);

Note that you can do the same thing to process.stderr , too.

Answer 3

A less confusing version of the above would be:

process.stdout._orig_write = process.stdout.write;
process.stdout.write = (data) => {
  mylogger.write(data);
  process.stdout._orig_write(data);
}

Answer 4

I'd suggest the following:

1. Pipe stdout from the process to a duplexstream where you can create a readstream
2. You parse the data, and then you can close it.