const getUpperCaseTransform = () =>
  new Transform({
    transform(chunk, encoding, callback) {
      callback(null, String(chunk).toUpperCase())
    },
  })
  
// $ > stream (promisified) > $ 
const o1 = await $`echo "hello"`
   .pipe(getUpperCaseTransform())
   .pipe($`cat`)
   
o1.stdout //  'HELLO\n'

// stream > $
const file = tempfile()
await fs.writeFile(file, 'test')
const o2 = await fs
  .createReadStream(file)
  .pipe(getUpperCaseTransform())
  .pipe($`cat`)
  
o2.stdout //  'TEST'