Go by Example: Logging

The Go standard library provides straightforward tools for outputting logs from Go programs, with the log package for free-form output and the log/slog package for structured output.

package main
import (
func main() {

Simply invoking functions like Println from the log package uses the standard logger, which is already pre-configured for reasonable logging output to os.Stderr. Additional methods like Fatal* or Panic* will exit the program after logging.

    log.Println("standard logger")

Loggers can be configured with flags to set their output format. By default, the standard logger has the log.Ldate and log.Ltime flags set, and these are collected in log.LstdFlags. We can change its flags to emit time with microsecond accuracy, for example.

    log.SetFlags(log.LstdFlags | log.Lmicroseconds)
    log.Println("with micro")

It also supports emitting the file name and line from which the log function is called.

    log.SetFlags(log.LstdFlags | log.Lshortfile)
    log.Println("with file/line")

It may be useful to create a custom logger and pass it around. When creating a new logger, we can set a prefix to distinguish its output from other loggers.

    mylog := log.New(os.Stdout, "my:", log.LstdFlags)
    mylog.Println("from mylog")

We can set the prefix on existing loggers (including the standard one) with the SetPrefix method.

    mylog.Println("from mylog")

Loggers can have custom output targets; any io.Writer works.

    var buf bytes.Buffer
    buflog := log.New(&buf, "buf:", log.LstdFlags)

This call writes the log output into buf.


This will actually show it on standard output.

    fmt.Print("from buflog:", buf.String())

The slog package provides structured log output. For example, logging in JSON format is straightforward.

    jsonHandler := slog.NewJSONHandler(os.Stderr, nil)
    myslog := slog.New(jsonHandler)
    myslog.Info("hi there")

In addition to the message, slog output can contain an arbitrary number of key=value pairs.

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

Sample output; the date and time emitted will depend on when the example ran.

$ go run logging.go
2023/08/22 10:45:16 standard logger
2023/08/22 10:45:16.904141 with micro
2023/08/22 10:45:16 logging.go:40: with file/line
my:2023/08/22 10:45:16 from mylog
ohmy:2023/08/22 10:45:16 from mylog
from buflog:buf:2023/08/22 10:45:16 hello

These are wrapped for clarity of presentation on the website; in reality they are emitted on a single line.

 "level":"INFO","msg":"hi there"}
    "level":"INFO","msg":"hello again",

Next example: HTTP Client.