Ability to prefix child process stdio lines with a custom string

[callumlocke]

Proposal: add a new option prefix (optional, string). If given, the child's stdout and stderr streams would be transformed so each line is prefixed with the given string.

Use case: when running several child processes in parallel and using inherit to pipe each child's stdio to the parent, prefixing would make it easier to see what's going on:

Promise.all(jobs.map((job, i) => execa(job.cmd, job.args, {
  stdio: 'inherit',
  prefix: `[job ${i + 1} of jobs.length] `,
}))).then(() => {
  // ...
});

[job 1 of 4] blah blah
[job 3 of 4] blah blah
[job 3 of 4] blah blah blah
[job 2 of 4] blah
...

I think the best API would be to have a prefix option which would apply to both stdout and stderr streams, and also prefixStdout and prefixStderr in case you want to override them individually.

Happy to implement this if interested.

[sindresorhus]

It's not possible to intercept the stdout/stderr when you're using inherit (Really wish it was though). We could support it on pipe, but not sure it belongs here.

[callumlocke]

Can't we just reimplement inherit so it doesn't actually use the underlying cp's inherit? AFAICT inherit is just shorthand for [process.stdin, process.stdout, process.stderr].

[sindresorhus]

Relevant Node.js issue: https://github.com/nodejs/node/issues/8033

[ehmicky]

Re-opening because this feature still depends on #695.

[ehmicky]

PR at #699 implementing this.

The initial message's example would then written as:

const prefixLine = function * (line) {
  yield `[job ${i + 1} of jobs.length] ${line}`
}

await Promise.all(jobs.map((job, i) => execa(
  job.cmd, 
  job.args, 
  { stdout: ['inherit', prefixLine] },
)))