Custom writer working code

customWriterFinal.go

package main

import (
	"bytes"
	"encoding/json"
	"io"
	"os"
	"os/exec"
	"time"
)

func main() {
	w1 := &customWriter{w: os.Stdout, severity: "info"}
	w2 := &customWriter{w: os.Stdout, severity: "error"}
	args := []string{"-c", `echo "stdout message"; echo "stdout message"; >&2 echo "stderr message";`}
	cmd := exec.Command("sh", args...)
	cmd.Stdout = w1
	cmd.Stderr = w2

	err := cmd.Run()
	if err != nil {
		panic(err)
	}
}

type line struct {
	Message   string
	Timestamp time.Time
	Severity  string
}

type customWriter struct {
	w        io.Writer
	severity string
}

func (e customWriter) Write(p []byte) (int, error) {
	l := &line{
		Message:   string(p),
		Timestamp: time.Now().UTC(),
		Severity:  e.severity,
	}
	buf := new(bytes.Buffer)
	json.NewEncoder(buf).Encode(l)

	data := buf.Bytes()
	n, err := e.w.Write(data)
	if err != nil {
		return n, err
	}
	if n != len(data) {
		return n, io.ErrShortWrite
	}
	return len(p), nil
}