Question

I have been tinkering around with node a little, and while trying to learn the child_process module, I ran into a problem. I was attempting to serialize many calls to ‘ps -eF | grep ssh’, but it crashes on my system with the error below. So two questions. First, is there a better way to do what I am attempting without a library? Second, why isn’t it working 🙂

events.js:71
        throw arguments[1]; // Unhandled 'error' event
                       ^
Error: This socket is closed.
    at Socket._write (net.js:519:19)
    at Socket.write (net.js:511:15)
    at Socket.<anonymous> (/home/me/tmp/test.js:10:16)
    at Socket.EventEmitter.emit (events.js:96:17)
    at Pipe.onread (net.js:397:14)

function callpsgrep(callback) {
  var spawn = require('child_process').spawn,
      ps    = spawn('ls', ['-la']),
      grep  = spawn('grep', ['bananas']);

  ps.stdout.on('data', function (data) {
    grep.stdin.write(data);
  });

  ps.stderr.on('data', function (data) {
    console.log('ps stderr: ' + data);
  });

  ps.on('exit', function (code) {
    if (code !== 0) {
      console.log('ps process exited with code ' + code);
    }
    grep.stdin.end();
  });

  grep.stdout.on('data', function (data) {
    console.log('' + data);
  });

  grep.stderr.on('data', function (data) {
    console.log('grep stderr: ' + data);
  });

  grep.on('exit', function (code) {
    if (code !== 0) {
      console.log('grep process exited with code ' + code);
    }
    callback();
  });
}

function series(i) {
  if (i < 1000) {
    callpsgrep( function() {
      return series(i+1);
    });
  }
}
series(0);

Answer 1

Close grep’s stdin on the close event instead of the exit event.

ps.on('exit', function (code) {
  if (code !== 0) {
    console.log('ps process exited with code ' + code);
  }
});

ps.on('close', function (code) {
  grep.stdin.end();
});

---

Although not very well documented I read the following in the help file.

Event: ‘exit’

Note that the child process stdio streams might still be open.

Event: ‘close’#

This event is emitted when the stdio streams of a child process have all terminated. This is distinct from ‘exit’, since multiple processes might share the same stdio streams.