Child Process Output Not Shown in Node.JS

Question

I am executing below code in NodeJS.

As seen below parent process spawns a child process and sets env variable. This variable is used to decide if process is parent or child when executing the file.

const {IS_CHILD} = process.env if(IS_CHILD){ console.log('CHILD'); console.log('child pid = ',process.pid); console.log('child env values = ',process.env); }else{ const {parse} = require('path') const {root} = parse(process.cwd()); console.log('PARENT'); console.log('parent pid = ',process.pid) const {spawn} = require('child_process'); const sp = spawn(process.execPath,[__filename], { cwd: root, env: {IS_CHILD : true} }); sp.stdout.pipe(process.stdout); // if this is commented }

The issue I am facing is , if I comment out code sp.stdout.pipe(process.stdout) inside parent process , the child process output is not shown on console. (Three lines inside IS_CHILD if block )

If sp.stdout.pipe(process.stdout) line is commented out , does that mean that process.env for child process is also not written ?

Can anybody please help here ?

Am I missing anything here ?

I was assuming that even if sp.stdout.pipe(process.stdout) line is commented out , the child process should have env variable set in it as we have executed spawn command

srknzl · Accepted Answer · 2021-04-17 07:56:12Z

If sp.stdout.pipe(process.stdout) line is commented out , does that mean that process.env for child process is also not written ?

No. Environment variable is still there.

Explanation

The output you get is expected.

When you run your script, it starts parent process. So what you will see in the console is output of parent process. Then, via spawn you are spawning a child process. The child process is a completely different process. In order to see its output in the parent process, you can use piping as you do.

Another option is to use events of child process. You can attach a handler to child process' stdout data event. e.g

sp.stdout.on('data', (data) => { console.log(`stdout: ${data}`); });

For more info about child processes, see child process reference.

If you omit piping, the child process will write output to its own stdout which is different from parent process' stdout. Therefore, you don't see it.

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