Logrus - Writes logs with level greater than error to stderr and all others (errors included) to a file in json format

main.go

package main

import (
	"fmt"
	"os"

	log "github.com/sirupsen/logrus"
)

type MyHook struct{}

func (h *MyHook) Levels() []log.Level {
	return []log.Level{
		log.WarnLevel,
		log.ErrorLevel,
		log.FatalLevel,
		log.PanicLevel,
	}
}

var fmter = new(log.TextFormatter)

func (h *MyHook) Fire(entry *log.Entry) (err error) {
	line, err := fmter.Format(entry)
	if err == nil {
		fmt.Fprintf(os.Stderr, string(line))
	}
	return
}

func init() {
	log.SetFormatter(&log.JSONFormatter{})
	f, err := os.OpenFile(
		"./log",
		os.O_APPEND|os.O_WRONLY|os.O_CREATE,
		0600)
	if err != nil {
		panic(err)
	}

	log.SetOutput(f)
	log.AddHook(&MyHook{})
}

func main() {
	log.WithFields(log.Fields{
		"animal": "walrus",
		"size":   10,
	}).Debug("A group of walrus emerges from the ocean")

	log.WithFields(log.Fields{
		"animal": "walrus",
		"size":   10,
	}).Info("A group of walrus emerges from the ocean")

	log.WithFields(log.Fields{
		"animal": "walrus",
		"size":   10,
	}).Info("A group of walrus emerges from the ocean")

	log.WithFields(log.Fields{
		"omg":    true,
		"number": 122,
	}).Warn("The group's number increased tremendously!")

	log.WithFields(log.Fields{
		"omg":    true,
		"number": 122,
	}).Warn("The group's number increased tremendously!")

	log.WithFields(log.Fields{
		"omg":    true,
		"number": 122,
	}).Warn("The group's number increased tremendously!")

	log.WithFields(log.Fields{
		"omg":    true,
		"number": 100,
	}).Fatal("The ice breaks!")
}

cat log 
{"animal":"walrus","level":"info","msg":"A group of walrus emerges from the ocean","size":10,"time":"2017-05-25T12:05:41-03:00"}
{"animal":"walrus","level":"info","msg":"A group of walrus emerges from the ocean","size":10,"time":"2017-05-25T12:05:41-03:00"}
{"level":"warning","msg":"The group's number increased tremendously!","number":122,"omg":true,"time":"2017-05-25T12:05:41-03:00"}
{"level":"warning","msg":"The group's number increased tremendously!","number":122,"omg":true,"time":"2017-05-25T12:05:41-03:00"}
{"level":"warning","msg":"The group's number increased tremendously!","number":122,"omg":true,"time":"2017-05-25T12:05:41-03:00"}
{"level":"fatal","msg":"The ice breaks!","number":100,"omg":true,"time":"2017-05-25T12:05:41-03:00"}