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I have two scripts A.js and B.js, and a 3rd party application (which is open source). Script A.js launches the 3rd party application using execSync. Script B.js launches script A.js using spawn.

If I run script A.js by itself, I can see the entire output of the 3rd party app. When I run it indirectly by running script B.js, I can only see some of the output.

A.js

const { execSync } = require('child_process');
console.log("Hello");
execSync('mybinary.exe -arg1 -arg2', { stdio: 'inherit' });

B.js

const { spawn } = require('child_process');
const app = spawn(process.execPath, ['A.js'], {
  stdio: ['inherit', null, 'inherit']
});

app.stdout.on('data', (chunk) => {
  process.stdout.write(chunk);
});

When I launch B, I only see "Hello", however the binary is running (it's a server and I can open a TCP connection to it). I'm just not seeing the output. When I launch A by itself, I see both "Hello" and the regular binary's output.

If I change B's stdout to 'inherit', it works.

Why is this happening, and how do I fix this?

UPDATE: Doesn't seem to have anything to do with the use of two scripts. Spawning the binary directly without using stdio inherit exhibits the same issue. Seems to be dependant on the third party application. How could a program affect node if it just prints to stdout?

CONTEXT: I want to launch a server and then run a command when the server is ready. When ready, the server prints out a particular message. The idea was to wait for this message to be printed. After printing out this message, it doesn't print out anything else, but it stays running.

The particular executable I'm attempting to pipe is a ChatScript server.

UPDATE: At this point, I assume it's something with the specific binary I'm attempting to pipe. I want to understand how the binary can affect this behaviour, i.e. works with inherit but not pipe. If someone can recreate this behaviour with a node script (or c program), that will answer the question.

Environment

  • OS: Windows 10
  • Console: Cygwin (and also tried regular Command Prompt).
  • Node v7.10.0

Note that this is the same issue as here: node.js child_process.spawn no stdout unless 'inherit'

noahnu
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2 Answers2

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The writes that chatscript is doing are 'fprintf( stdout, ...)' This is a buffered output, and is saved until like (4k?) is buffered and then flushed. Adding 'fflush(stdout)' after a fprintf( stdout ) in ChatScript/src/os.cpp line 1849 for instance... then output works (somewhat, further changes will be required, but I did get at least 'in cs_init.txt' errors logging now.

Another test is just to redirect chatscript.exe to a file and see what the file gets. 'chatscript > zz' and then ending chatscript with ctrl-c you'll see that zz is blank also... this is an indication that the IO has been buffered and not flushed to output appropriately. 

M:\javascript\test_exec>node b.js
Hello

   in cs_init.txt at 0:
     A subdirectory or file USERS already exists.
Error opening utf8writeappend file LOGS/startlog.txt: No such file or     directory
ChatScript Release Version 7.5 pid: 0 32 bit Windows compiled Jun 24 2017 09:42:13 host=local
Params:   dict:2097151 fact:800000 text:100000kb hash:215127
          buffer:80x80kb cache:1x5000kb userfacts:100 outputlimit:80000     loglimit:80000
Unable to read dictionarySystem.h
Missing 37 word files
read 0 raw words

   in facts.txt at 0:
    A subdirectory or file USERS already exists.
J Decker
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  • I had a feeling that was the problem. Do you know why running chatscript directly (outside of node) prints the stdout 'correctly'? Do ttys set some sort of flag to disable (or lower) buffering? You'll see it prints okay outside of node in a simple windows cmd. – noahnu Jun 24 '17 at 17:03
0

There is a simple and safe workaround for this is to pipe the output from the application to a file and then read it from the same file after, so you don't have to worry if the event fired before or after you tried to listen to it.

Pipe to file:

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

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

Hope it helps, update me on how it goes;

Good luck,

sBanda
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  • Didn't work. When detached is true (tested with `shell: true` and `shell:false`) no output is written to the file. When detached is false, only partial output is written, just like when I pipe in my original question. As a side note, when detached is true (and shell is true) and there's no output to the file, I can see the correct output in a new detached shell which appears on the screen. I am unable to capture that output though. – noahnu Jun 19 '17 at 15:55