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Colourful Logging in Go

··446 words·3 mins·

Why? #

Whenever you debug a program that produces a large amount of logs, sifting through them when a bug arises is only a matter of time. This tasks becomes more annoying when there is no consistent pattern you focus on with grep, especially when you want to find a pattern between related log lines that aren’t immediately next to each other in the log file. When I resort to manually looking through logs of programs I’m debugging, having colour-coded logs tends to make the experience more fun.

An r/ProgrammerHumor post describing the lack of colour in logs
An r/ProgrammerHumor post describing the lack of colour in logs

How? #

I recently found a logging library by Charm, a start-up dedicated to making pleasing-to-use libraries, utilities and application that are centered around the command-line experience. Since I never really thought of making my programs’ command-line output easy to read, I thought of giving it a try.

Custom Logger #

For my personal project, I override the default package settings with the following:

  • Each log line reports what source code line printed it.
  • RFC 3339 timestamp format
  • In order to show debugging logs by default, the default logging level is be set to log.DebugLevel.
import (


func init() {

Result #

In the following section, I’ll show some code snippets and the output they produce (captured with vhs).

Information/Success #

Regular logs are shown with a green-coloured INFO prefix.

log.Info("Hi there!", "version", runtime.Version())
log.Infof("%d + 0x%x = 0b%b", 22, 0x56, 22+0x56)

Debugging #

Debug logs are shown with a blue-coloured DEBU prefix.

log.Debug("Hi there!", "version", runtime.Version())
log.Debugf("%d + 0x%x = 0b%b", 22, 0x56, 22+0x56)

Warning #

Warning logs are shown with a yellow-coloured WARN prefix.

log.Warn("Hi there!", "version", runtime.Version())
log.Warnf("%d + 0x%x = 0b%b", 22, 0x56, 22+0x56)

Error & Fatal #

  • Error logs are shown with a red-coloured ERRO prefix.
  • Fatal logs are shown with a purple-coloured FATA prefix.
    • Any call to log.Fatal or to log.Fatalf will result in the program exiting immediately with code 1.
log.Error("Hi there!", "version", runtime.Version())
log.Errorf("%d + 0x%x = 0b%b", 22, 0x56, 22+0x56)

Using in Other Libraries #

Temporal #

The Go SDK for Temporal’s durable execution framework allows the developer to substitute a custom logger in order to suit the logging format of any project. Using the SDK’s structured logging support, the Charm logger can be used within the Temporal logging stack.

import (

	// ...
	tlog ""

func main() {
	options := client.Options{
		Logger: tlog.NewStructuredLogger(slog.New(log.Default())),
	c, err := client.Dial(options)
	if err != nil {
		log.Fatal("Unable to create client", err)
	defer c.Close()

	w := worker.New(c, "task-queue", worker.Options{})
	// ...

	if err := w.Run(worker.InterruptCh()); err != nil {
		log.Fatal("Unable to start worker", err)