Prefix the output of a subprocess

prefix-output-of-subprocess.go

package main

import (
	"bufio"
	"context"
	"fmt"
	"io"
	"os"
	"os/exec"

	"golang.org/x/sync/errgroup"
)

func main() {
	g, gCtx := errgroup.WithContext(context.TODO())
	prefixes := []string{"foo", "bar", "baz"}
	for idx := range prefixes {
		prefix := prefixes[idx]
		g.Go(func() error {
			pr, pw := io.Pipe()
			cmd := exec.CommandContext(gCtx, "sh", "-c", `head -c16 /dev/urandom | base64`)
			cmd.Stdout = pw
			cmd.Stderr = pw

			go cmd.Run()

			scanner := bufio.NewScanner(pr)
			for scanner.Scan() {
				fmt.Fprintf(os.Stdout, "[%s] %s\n", prefix, scanner.Text())
			}

			return scanner.Err()
		})
	}
	g.Wait()
}

// https://play.golang.org/p/V1oE4Jh2fHB