Logging

Logging in Go is typically done using the built-in log package or other third-party logging libraries for more advanced features.

Normal Logging

package main

import "log"

func main() {
	// Basic logging
	log.Println("This is a basic log message")

	// Logging with formatting
	name := "John"
	age := 30
	log.Printf("Name: %s, Age: %d", name, age)

	// Setting log prefix
	log.SetPrefix("[MyApp] ")
	log.Println("Log message with custom prefix")

	// Setting log flags
	log.SetFlags(log.Ldate | log.Ltime)
	log.Println("Log message with custom date and time format")
}

Its output:

2013/10/07 09:42:00 This is a basic log message
2013/10/07 09:42:00 Name: John, Age: 30
[MyApp] 2013/10/07 09:42:00 Log message with custom prefix
[MyApp] 2013/10/07 09:42:00 Log message with custom date and time format

Structured Logging

Package slog provides structured logging, in which log records include a message, a severity level, and various other attributes expressed as key-value pairs.

package main

import (
	"log/slog"
	"os"
)

func main() {
	jsonHandler := slog.NewJSONHandler(os.Stdout, nil)
	myslog := slog.New(jsonHandler)
	myslog.Info("hi there")

	myslog.Info("hello again", "key", "val", "age", 25)
}

Its output:

{"time":"2009-11-10T23:00:00Z","level":"INFO","msg":"hi there"}
{"time":"2009-11-10T23:00:00Z","level":"INFO","msg":"hello again","key":"val","age":25}