childprocess.stdout.on('data', callback) not firing

Question

childprocess.stdout.on('data', callback) not firing

I'm trying to capture the output from a child process and send that through a websocket but I'm having problems capturing the on.(data, callback) event.

var subscriberProc = spawn('./stdoutsub', 
                      ['mytopic', '--clientid', 'node-dispatcher'], 
                      {cwd: '/Users/oskar/projects/others/rsmb_1/mac/'});

subscriberProc.stdout.on('data', function(data) {
  console.log('-> "' + data + '"');
  wsServer.send(data);
});

If I change to the following, the stdoutsub process prints to the node process stdout with the expected data.

var subscriberProc = spawn('./stdoutsub', 
                      ['mytopic', '--clientid', 'node-dispatcher'], 
                      {cwd: '/Users/oskar/projects/others/rsmb_1/mac/', 
                       stdio: 'inherit'});

I'm running node version 0.8.1 on OSX.

Anything I'm missing here?

node.js
stdio
spawn

Answer 1

I have the same problem. I am guessing the problem might be that spawn starts the command before stdout stream has that data event handler and because of that it doesn't get the data. At the moment I haven't figured out any way to setup the stream first to test my theory...

Answer 2

I was having similar issue (where I was missing the output in certain cases) and concluded that the spawned process is spewing output before I get a chance to start listening for it.

I solved it by using code similar to below from Node.js documentation :

var fs = require('fs'),
    spawn = require('child_process').spawn,
    out = fs.openSync('./out.log', 'a'),
    err = fs.openSync('./out.log', 'a');

var child = spawn('prg', [], {
    detached: true,
    stdio: [ 'ignore', out, err ]
});

And then I read out.log and so far things have worked out fine for me.