How to read child_process.spawnSync stdout with stdio option 'inherit'

Question

var childProcess = cp.spawnSync(command, args, {
    cwd: process.cwd(),
    env: process.env,
    stdio: 'inherit',
    encoding: 'utf-8'
});

childProcess.output always eq [null, null, null]

process.stdout.write hook doesn't give me any output

Solution 1

If you don't use 'pipe' then childProcess.output will not contain the output.

var cp = require('child_process');

var command = 'echo';
var args = ['hello', 'world'];

var childProcess = cp.spawnSync(command, args, {
    cwd: process.cwd(),
    env: process.env,
    stdio: 'pipe',
    encoding: 'utf-8'
});

console.log(childProcess.output); // [ null, 'hello world\n', '' ]

This is sorta kinda indicated in the documentation for child.stdout and elsewhere, but it's not entirely unambiguous. (By all means, if you wish to see it improved, open a pull request against the Node.js repo.)

Solution 2

Use this for in-process displaying of progress:

var cp = require('child_process');

var command = 'echo';
var args = ['hello', 'world'];

var childProcess = cp.spawnSync(command, args, {
    cwd: process.cwd(),
    env: process.env,
    stdio: [process.stdin, process.stdout, process.stderr],
    encoding: 'utf-8'
});

So you replace string 'pipe' with the array [process.stdin, process.stdout, process.stderr].

Comments

[Fran Dios]

Have you found any solution to this? I have the same problem, I need to use 'inherit' in order to keep the progress display but I cannot hook stdout.write or listen for data event...

[nitro-n]

@FranDios My workaround is to use pipe to catch process output stdio: [0, isOutputNeeded ? 'pipe' : 1, 2],

[nitro-n]

This way I will miss child_process progress displaying. It's important for me.

[mvermand]

@nitro-n: see to my solution to get in-progress output

[mvermand]

Please comment on down-vote. We are all here to learn and help learning, right?

[Matt Kahl]

My guess is the downvote is due to the fact that this solution does not include a way to read the value of stdout. It does show how to inspect/display stdout, but I believe @nitro-n was asking for a way to store stdout and display it (via stdio: 'inherit'). stdio: [process.stdin, process.stdout, process.stderr] is equivalent to stdio: 'inherit'. So, effectively, this solution is the same as the example in the original question.

[Bluu]

stdio: [process.stdin, process.stdout, process.stderr] is equivalent to the OP's stdio: 'inherit'.

[Eduardo Montoya]

Voted up only because this discussion allowed to clarify what it was said by @Bluu

[Joshua Pinter]

Thanks for this. For others, you can get the output string with childProcess.output[1] #=> 'hello world\n'