Redirect stdout/stderr from a child_process to /dev/null or something similar

Question

I am creating some child_processes with Node.js (require('child_process')) and I want to ensure that the stdout/stderr from each child_process does not go to the terminal, because I want only the output from the parent process to get logged. Is there a way to redirect the stdout/stderr streams in the child_processes to /dev/null or some other place that is not the terminal?

https://nodejs.org/api/child_process.html

perhaps it's just:

var n = cp.fork('child.js',[],{
   stdio: ['ignore','ignore','ignore']
});

I just tried that, and that didn't seem to work.

Now I tried this:

var stdout, stderr;

if (os.platform() === 'win32') {
    stdout = fs.openSync('NUL', 'a');
    stderr = fs.openSync('NUL', 'a');
}
else {
    stdout = fs.openSync('/dev/null', 'a');
    stderr = fs.openSync('/dev/null', 'a');
}

and then this option:

stdio: ['ignore',  stdout, stderr],

but that didn't do it, but it seems like using the "detached:true" option might make this work.

Kelvinkelwen

how about just hijacking process.stdout.write in your forked process? – Ring

I can't unfortunately overwrite that, there must be a way with child_process to do this! – Kelvinkelwen

I suppose I could overwrite that actually, but there are advantages to doing this via the child_process call – Kelvinkelwen

In the docs it looks like the stdio option is for spawn, not fork, maybe you want the silent option? – Home

@Home i think that the silent option might be the one – Kelvinkelwen

FYI https://mcmap.net/q/2033105/-how-to-correctly-use-node-js-child_process-spawn-redirection-on-windows/319204 – Joh

Answer 1

Solution:

To throw away the stdout and stderr of a forked childprocess:

1. setup a pipe i.e. use silent = True when forking.

2. And redirect the stdout and stderr pipes on the parent process into /dev/null.

---

Explanation:

The node.js documentation states :

For convenience, options.stdio may be one of the following strings:

'pipe' - equivalent to ['pipe', 'pipe', 'pipe'] (the default)
'ignore' - equivalent to ['ignore', 'ignore', 'ignore']
'inherit' - equivalent to [process.stdin, process.stdout, process.stderr] or [0,1,2]

Apparently childprocess.fork() does NOT support ignore; Only childprocess.spawn() does.

fork does support a silent option that allows one to choose between pipe OR inherit.

When forking a child process:
If silent = True, then stdio = pipe.
If silent = False, then stdio = inherit.

silent
_Boolean

If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent.

See the 'pipe' and 'inherit' options for child_process.spawn()'s stdio for more details.

Joh

thanks this answer is good, I upvoted awhile ago...I am currently wondering if there is a way to pipe the stderr/stdout of a child_process directly to a file instead of back to the parent process and then to the file. Any idea? – Kelvinkelwen

I guess just call process.stderr.pipe() in the child_process – Kelvinkelwen