intercept-stdout.js

var _ = require('underscore'),
    util = require('util');

// intercept stdout, passes thru callback
// also pass console.error thru stdout so it goes to callback too
// (stdout.write and stderr.write are both refs to the same stream.write function)
// returns an unhook() function, call when done intercepting
module.exports = function interceptStdout(callback) {
  var old_stdout_write = process.stdout.write,
      old_console_error = console.error;

  process.stdout.write = (function(write) {
    return function(string, encoding, fd) {
      var args = _.toArray(arguments);
      write.apply(process.stdout, args);

      // only intercept the string
      callback.call(callback, string);
    };
  }(process.stdout.write));

  console.error = (function(log) {
    return function() {
      var args = _.toArray(arguments);
      args.unshift('[ERROR]');
      console.log.apply(console.log, args);
      
      // string here encapsulates all the args
      callback.call(callback, util.format(args));
    };
  }(console.error));

  // puts back to original
  return function unhook() {
    process.stdout.write = old_stdout_write;
    console.error = old_console_error;
  };
};

pipe stdout to telnet socket

  var server = require('net').createServer();

  server.listen(5000, function() {
    console.log('Telnet server running on port', server.address().port);
  });

  server.on('connection', function(socket) {
    // pipe logs to telnet for the duration of connection
    var unhookStdout = require('./intercept-stdout')(function intercept(string){
      socket.write(string);
    });
    
    console.log('is this going to telnet?');

    socket.on('end', function(){
      unhookStdout();
    });

    socket.on('data', function(data) {
      socket.setEncoding('utf8');

      // should only be 1 line at a time
      data = data.toString().replace(/(\r\n|\n|\r)/gm,"");

      switch(data) {
        case 'quit':
          socket.write('Goodbye.\n');
          socket.end();
          break;

        case 'test':
          setInterval(function() { console.log(Date()); }, 500);

          break;

        default:
          socket.write('\nUnknown command: ' + data + '\n');
      }
    });
  });

test-intercept.js

// test intercept w/ terminal

var _ = require('underscore'),
    util = require('util');

// to stdout
console.log('a', 1, {a:'b'});
console.log('b', 1, {a:'b'});

util.debug('intercept on');

var unhook = require('./intercept-stdout')(function intercept(){
  var args = _.toArray(arguments);
  util.debug.apply(this, args);
});

// intercepted
console.log('c', 1, {a:'b'});
console.log('d', 1, {a:'b'});

console.error('error e', 1, {a:'b'});

// back to stdout
unhook();
util.debug('Unhooked');

console.log('f back', 1, {a:'b'});
console.error('g error back', 1, {a:'b'});