Pipe a stream to a child process

[thomaspeklak]

a child process started with the fork method has only a send method for communication. It would be much nicer if you could pipe directly to the child_process like this:

PARENT

var fs = require('fs');
var fork = require('child_process').fork;

var cp = fork('./child');

fs.createReadStream('./example.txt').pipe(cp);

CHILD

process.write = function(buf){
  process.stdout(buf);
};

process.end = function(buf){
  process.write(buf);
};

Alternatively it would be fine, if you could use fs.createReadStream('./example.txt').pipe(cp.stdin) to achieve the same result. But the forked child process has no stdin.

[TooTallNate]

Why not just use child_process.spawn()?

[thomaspeklak]

It seems to me that spawn should be used for node externals like ls, grep, ... and if you use it you would have to do something like this spawn("node", ['child.js']) if understand it right. Rather ugly compared to the simple fork.

As the node community is embracing streams, I thought that it would be much nicer to have this right baked into fork.

[TooTallNate]

Well fork() is really just a light wrapper around spawn(), only it doesn't let you specify the "stdio" option, which is what you're wanting to alter.

Try something like this:

var args = [ './child.js', /* command arguments */ ];
var child = spawn(process.execPath, args, { stdio: ['pipe', 1, 2, 'ipc'] });
file.pipe(child.stdin);

Which is basically saying "open a new pipe for stdin so I can write to it, share stdout and stderr with this parent process, and open an IPC channel for child.send()/process.send() on FD 4".

[thomaspeklak]

This works as expected. Still I am not completely satisfied with the situation.

1. I don't think that this works if the child is a node_module. I think that fork uses the standard way to look for modules but this does not happen in spawn, or am I wrong?
2. It took me a while to figure out that I could not pipe to a forked child process. I think it should be at least mentioned in the documentation, that this is not possible.

[ypjin]

Hello, if spawning the child process like this:
var child = spawn(process.execPath, args, { stdio: [0, 1, 2, 'ipc', 'pipe'] });
How could I reference the pipe in the child process?

[trevnorris]

@ypjin Here's an example: http://blog.trevnorris.com/2013/07/child-process-multiple-file-descriptors.html

[jacobkolind]

@trevnorris Can you get it to work on Ubuntu? See: http://stackoverflow.com/questions/24490399/node-js-child-processes-and-pipes-osx-vs-ubuntu

[trevnorris]

@jacobkolind looks like you've already found a solution.

I'm not still seeing an issue here. If there is one, ping and will discuss.