Implementation structured logs with multiple outputs (console, file) in Go lang using zerolog

logging.go

// internal/logging.go

package internal

import (
	"fmt"
	"io"
	"os"
	"path"
	"path/filepath"
	"strconv"

	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
)

// initLogging creates a logger, sets the UNIX timestamp as the timestamp format
// and configures logger to output to file in ./data/my_file.log
// Usage: 
//    logger := initLogging(zerolog.InfoLevel)
func initLogging(logLevel zerolog.Level) zerolog.Logger {
	// set the timestamp format. this is more performant than other formats
	zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

	// write logs to file - TODO: Change this to your own file path
	f, err := os.OpenFile(path.Join(appDataPath(""), "my_file.log"), os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
	if err != nil {
		log.Fatal().Msg("error opening file for logging")
	}

	// Never close the file
	//defer f.Close()

	wrt := io.MultiWriter(zerolog.ConsoleWriter{Out: os.Stderr}, f)

	// add caller info to logs.
	// this custom [CallerMarshalFunc] logs only the file name instead of the default full absolute path.
	// ref: https://github.com/rs/zerolog#add-file-and-line-number-to-log
	zerolog.CallerMarshalFunc = func(pc uintptr, file string, line int) string {
		short := file
		for i := len(file) - 1; i > 0; i-- {
			if file[i] == '/' {
				short = file[i+1:]
				break
			}
		}
		file = short
		return file + ":" + strconv.Itoa(line)
	}
	log.Logger = log.With().Caller().Logger()

	fmt.Printf("Using log level: %s (%d)\n", logLevel.String(), logLevel)
	zerolog.SetGlobalLevel(logLevel)

	// create a logger copy from the global config
	logger := log.Output(wrt)

	return logger
}


// AppDataPath
// Returns the path relative to the executable, e.g. ./data
func appDataPath(pathPrefix string) string {
	workingDir, err := os.Getwd()
	if err != nil {
		log.Fatal().Msg("Cannot get working directory")
	}

	return filepath.Join(workingDir, pathPrefix, "data")
}