Reading from both stdout and stderr in one func

main.go

package main

import (
	"bytes"
	"fmt"
	"io"
	"os/exec"
)

// RunAndListen intercepts cmd.Stdout and cmd.Stderr and runs cmd.
func RunAndListen(cmd *exec.Cmd, listener func(line string, stderr bool) error) error {
	out := func(data []byte, stderr bool) (n int, err error) {
		buf := bytes.NewBuffer(data)
		for {
			line, err := buf.ReadString('\n')
			if err == io.EOF {
				break
			}
			if err == nil {
				err = listener(line, stderr)
			}
			if err != nil {
				return len(data) - buf.Len(), err
			}
		}
		return len(data), nil
	}
	cmd.Stdout = WriterFunc(func(data []byte) (int, error) {
		return out(data, false)
	})
	cmd.Stderr = WriterFunc(func(data []byte) (int, error) {
		return out(data, true)
	})
	return cmd.Run()
}

func main() {
	cmd := exec.Command("cat", "main.go", "nonexistent.go")
	RunAndListen(cmd, func(line string, stderr bool) error {
		if stderr {
			fmt.Print("err: ")
		}
		fmt.Print(line)
		return nil
	})
}

type WriterFunc func([]byte) (n int, err error)

func (f WriterFunc) Write(data []byte) (n int, err error) {
	return f(data)
}