logging and debug

Felipe Kunzler — Wed, 11 Jun 2025 01:14:41 +0000

The most basic way to output an error message:

import "os"

fmt.Fprintf(os.Stderr, "error message")

A more sophisticated way to deal with logs would be with log/slog.
A simple usage of this package would be something like this:

import "log/slog"

func main() {
    slog.Debug("debug message, very detailed")
    slog.Info("info message, general")
    slog.Warn("warning message, potential issue")
    slog.Error("error message, serious issue")
}

Would produce an output similar to this:
2025/06/11 10:27:25 INFO info message, general 2025/06/11 10:27:25 WARN warning message, potential issue 2025/06/11 10:27:25 ERROR error message, serious issue

By default, slog doesn't print debug logs, but we can change that with the following instruction:
slog.SetLogLoggerLevel(slog.LevelX)

Also, the default logger always print to stderr and the output format is plain text with few details, but we can change this behavior if needed by creating our own logger, check the HandlerOptions.

We have the following levels of debug:

LevelDebug
LevelInfo
LevelWarn
LevelError

And we can define AddSource = true, to include details about the source code.

Putting everything together:

package main

import (
    "log/slog"
    "os"
)

func main() {
    logger := slog.New(slog.NewJSONHandler(os.Stdout,
        &slog.HandlerOptions{
            Level:     slog.LevelDebug,
            AddSource: true,
        }))

    slog.SetDefault(logger)
    slog.Debug("debug message, very detailed")
    slog.Info("info message, general")
    slog.Warn("warning message, potential issue")
    slog.Error("error message, serious issue")
}

Would produce an output similar to this:

{
  "time": "2025-06-11T11:05:49.077787203-03:00",
  "level": "DEBUG",
  "source": {
    "function": "main.main",
    "file": "/home/fk/dev/go/slog_tests/slog_tests.go",
    "line": 17
  },
  "msg": "debug message, very detailed"
}

{
  "time": "2025-06-11T11:05:49.077913712-03:00",
  "level": "INFO",
  "source": {
    "function": "main.main",
    "file": "/home/fk/dev/go/slog_tests/slog_tests.go",
    "line": 18
  },
  "msg": "info message, general"
}

{
  "time": "2025-06-11T11:05:49.077917689-03:00",
  "level": "WARN",
  "source": {
    "function": "main.main",
    "file": "/home/fk/dev/go/slog_tests/slog_tests.go",
    "line": 19
  },
  "msg": "warning message, potential issue"
}

{
  "time": "2025-06-11T11:05:49.077920535-03:00",
  "level": "ERROR",
  "source": {
    "function": "main.main",
    "file": "/home/fk/dev/go/slog_tests/slog_tests.go",
    "line": 20
  },
  "msg": "error message, serious issue"
}

Finally, we could have a logger to send data over the application, instead of printing to stderr.

import (
    "log/slog"
    "os"
)

func main() {
    f, err := os.OpenFile("/tmp/logs.txt", os.O_RDWR|os.O_CREATE, 0o644)
    if err != nil {
        slog.Error("operation failed", "error", err)
        os.Exit(-1)
    }
    defer f.Close()

    logger := slog.New(slog.NewJSONHandler(f,
        &slog.HandlerOptions{
            Level:     slog.LevelDebug,
            AddSource: true,
        }))

    slog.SetDefault(logger)
    slog.Debug("debug message, very detailed")
    slog.Info("info message, general")
    slog.Warn("warning message, potential issue")
    slog.Error("error message, serious issue")
}

DEV Community: Felipe Kunzler

logging and debug