javascript - Stream child process output in flowing mode - Stack Overflow

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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).

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).

• javascript
• python
• node.js

Share Improve this question Follow asked Sep 17, 2013 at 11:50 iFadeyiFadey 32033 silver badges1111 bronze badges

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3 Answers 3

Sorted by: Reset to default 12

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)

The best way is to use unbuffered mode of python standard output. It will force python to write output to output streams without need to flush yourself.

For example:

var spawn = require('child_process').spawn,
child = spawn('python',['-u', 'myscript.py']); // Or in custom_cli add python -u myscript.py

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

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

In my case in Python I'm using sys.stdin.readline and yielding last line:

def read_stdin():
    '''
        read standard input
        yeld next line
    '''
    try:
        readline = sys.stdin.readline()
        while readline:
            yield readline
            readline = sys.stdin.readline()
    except:
        # LP: avoid to exit(1) at stdin end
        pass

 for line in read_stdin():
     out = process(line)
     ofp.write(out)
     sys.stdout.flush()

and when in Node.js

var child = spawn(binpath, args);

    // register child process signals
    child.stdout.on('data', function (_data) {
        var data = Buffer.from(_data, 'utf-8').toString().trim();
        console.log(data);
    });
    child.stderr.on('data', function (data) {
        console.warn('pid:%s stderr:%s', child.pid, data);
    });
    child.stdout.on('exit', function (_) {
        console.warn('pid:%s exit', child.pid);
    });
    child.stdout.on('end', function (_) {
        console.warn('pid:%s ended', child.pid);
    });
    child.on('error', function (error) {
        console.error(error);
    });
    child.on('close', (code, signal) => { // called after `end`
        console.warn('pid:%s terminated with code:%d due to receipt of signal:%s with ', child.pid, code, signal);
    });
    child.on('uncaughtException', function (error) {
        console.warn('pid:%s terminated due to receipt of error:%s', child.pid, error);
    });

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