Is it possible to add prefix to pipe function?

[pie6k]

I'd like to run a few, long running tasks at once.

Each of them outputs lots of logs I'd like to see in the console.

I can do

$`foo`.pipe(process.stdout)

and this works fine, but then all logs from various processes are mixed together.

I'd like to do something like

$`foo`.pipe(process.stdout, { prefix: `[A] ` })

that would result in logs from foo eg Hello world to be displayed as [A] Hello world.

[antonmedv]

Something like this?

async function prefix(proc, prefix) {
  proc.stdout.setEncoding('utf8')
  for await (let chunk of proc.stdout) {
    process.stdout.write(prefix + chunk)
  }
}

$.verbose = false
let a = $`while true; do echo Aaa; sleep 1; done`
let b = $`while true; do echo Bbb; sleep 5; done`

await Promise.all([
  prefix(a, '[a] '),
  prefix(b, '[b] '),
])

Outputs:

zx .anton/index.mjs
[a] Aaa
[b] Bbb
[a] Aaa
[a] Aaa
[a] Aaa
[a] Aaa
[b] Bbb
[a] Aaa
[a] Aaa
[a] Aaa
[a] Aaa
[a] Aaa
[b] Bbb
[a] Aaa
[a] Aaa
[a] Aaa
[a] Aaa
[a] Aaa
[b] Bbb
[a] Aaa
^C

[antongolub]

Or you can apply a middleware transformer:

const addOutputPrefix = (prefix) =>
  new Transform({
    transform(chunk, encoding, callback) {
      callback(null, prefix + String(chunk))
    },
  })
  
const p = $`cmd`
  .pipe(addOutputPrefix('[A] '))
  .pipe(process.stdout)