Stream child process output in flowing mode

Question

I have custom command line written using Python which prints its output using "print" statement. I am using it from Node.js by spawning a child process and sending commands to it using child.stdin.write method. Here's source:

var childProcess = require('child_process'),
    spawn = childProcess.spawn;

var child = spawn('./custom_cli', ['argument_1', 'argument_2']);

child.stdout.on('data', function (d) {
  console.log('out: ' + d);
});

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

//execute first command after 1sec
setTimeout(function () {
  child.stdin.write('some_command' + '\n');
}, 1000);

//execute "quit" command after 2sec
//to terminate the command line
setTimeout(function () {
  child.stdin.write('quit' + '\n');
}, 2000);

Now the issue is I am not receiving the output in flowing mode. I want get the output from child process as soon as it's printed but I am receiving the output of all the commands only when child process is terminated (using custom cli's quit command).

Answer 1

You need to flush the output in the child process.

Probably you think this isn't necessary because when testing and letting the output happen on a terminal, then the library flushes itself (e. g. when a line is complete). This is not done when printing goes to a pipe (due to performance reasons).

Flush yourself:

#!/usr/bin/env python

import sys, time

while True:
  print "foo"
  sys.stdout.flush()
  time.sleep(2)