child_process.exec()'s stdout in outerscope

[atombug]

• Version: 10.2.1
• Platform: Windows 10 64-bit
• Subsystem: child_process.exec()

Scenario:
I was creating a nodejs library now that is nearly complete i just want to export it but problem is how to access stdout of exec() outside of it's scope.

• Code:

let data = "";
const { exec } = require('child_process');
exec('echo test', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  data = stdout;
  console.log(`stdout: ${stdout}`);
});
console.log(data);

• OUTPUT:

stdout: test

[ORESoftware]

You will have to use callbacks or promises

const { exec } = require('child_process');

const foo = function(cb){

  let data = "";
  exec('echo test', (err, stdout, stderr) => {
    if (err) {
      console.error(`exec error: ${err}`);
      return cb(err);
    }
    console.log(`stdout: ${stdout}`);
    cb(null, {stdout,stderr});
  });

};

then use it like so:

foo(function(err, {stdout,stderr}) {
   // check for err first
  // then use stdout/stderr
});

I am sure your library will be great success :)

[gireeshpunathil]

#cat 1303.js

let data = "";
const { exec } = require('child_process');
exec('echo test', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  data = stdout;
  console.log(`stdout: ${stdout}`);
});
setTimeout(() => {
console.log(`outer: ${data}`);
}, 1000)

#node 1303.js
stdout: test
outer: test

It is not the problem with the scope, instead with the timing. The console.log at the outer scope gets executed first before the child process execution (essence of asynchronous programming), so the data was empty. So you need to follow the sequence of execution while managing your data.

Hope this helps.

[atombug]

@gireeshpunathil that may be inefficient as i wanted to publish library (module) on npm and relying on settimeout doesn't seems to good for that, what is your opinion?

[gireeshpunathil]

agree, please treat it as a sample only to show where, and how the data vanished, and how and from where it is replenished.

To export the data to outside, the proposal that @ORESoftware showed looks great to me!

[atombug]

i am bit confused now @ORESoftware @gireeshpunathil

how to export solution of @ORESoftware with module.exports or exports.name and how to use it with require()?

[ORESoftware]

@atombug you can't export it synchronously, unless you use execSync.

Generally speaking you want to use exec or spawn and use a callback. However in some cases using execSync / spawnSync is OK.

[atombug]

Agreed to use execSync @ORESoftware , but how stderr is handled in it?

nodejs docs needs to give at least one example as it only states:

stdio <string> | <Array> Child's stdio configuration. 
stderr by default will be output to the parent process' stderr unless stdio is specified. 
Default: 'pipe'.

i would request @gireeshpunathil regarding giving one example of execsync in nodejs docs.

[gireeshpunathil]

#cat 1303.js

module.exports.echo = function(cb){
  const { execSync } = require('child_process');
  const out = execSync('echo test')
  console.log(`stdout: ${out}`);
}

#node

> var module = require('./1303.js')
undefined
> module
{ echo: [Function] }
> module.echo()
stdout: test

undefined
>

Basically the sync methods do not call back, instead return with the intended data. Hope this example helps you with that. Please free to come up with PRs if doc needs improvements!

[atombug]

so how can i handle errors / what is proper way to handle errors with execSync? @ORESoftware @gireeshpunathil

I will comeup with PR asap. @gireeshpunathil

[gireeshpunathil]

errors if any, will be returned to the caller, in the return value.

[atombug]

and that isn't good can we supress it or how to tell if isnt worked?

[gireeshpunathil]

sorry for misleading: that was an incorrect information.

excerpt from the doc:
If the process times out or has a non-zero exit code, this method will throw. The Error object will contain the entire result

So the error does not return to the caller, it throws. the return value is populated with the child's stdout data only.

[atombug]

if you don't mind can you give me one example of it (handling error object).

[gireeshpunathil]

#cat 1303.js

module.exports.echo = function(cb){
  let data = "";
  const { execSync } = require('child_process');
  const out = execSync('echos test')
  console.log(`stdout: ${out}`);
}

#cat 1303_1.js

var x = require('./1303.js')
try {
  x.echo()
}
catch(e) {
  console.log(e.toString())
}

#node 1303_1.js

/bin/sh: echos: command not found
Error: Command failed: echos test
/bin/sh: echos: command not found

[atombug]

Thank you for your suggestion.
btw i didnt enjoyed how it handles error (as i wanted it to be done in my module) so i found alternative:

const util = require('util');
const exec = util.promisify(require('child_process').exec);

async function test() {
  const { stdout, stderr } = await exec('echo test');
  if (stderr) {
    return {"error": stderr};
  }
  return {"data": stdout};
};

module.exports = test();

• Now, a simple call

let test = require('./test');

test.then(x => {
    console.log(x);
});

• While researching i also found a way to handle unhandled promises (added in module)

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', reason.stack || reason)
});

Is this good way ?? if not suggest me a good one. @ORESoftware @gireeshpunathil