log is open source under the MIT licence. There is no licence fee and no seat count — you can self-host it or, where the project offers one, pay a vendor for a managed version instead.

A minimal, colorful Go logging library 🪵

What is log written in?

log is primarily written in Go. Its source is publicly available at https://github.com/charmbracelet/log, and it has 3,393 GitHub stars.

log: Open Source Monitoring & Observability

log is a free, open source monitoring & observability project written in Go and released under MIT. It has 3,393 GitHub stars, 102 forks and 41 open issues, and was last pushed 15 days ago. On this registry it ranks #118 of 191 tracked projects in Monitoring & Observability, with 5 head-to-head comparisons available.

Log

A minimal and colorful Go logging library. 🪵

It provides a leveled structured human readable logger with a small API. Unlike [standard log][stdlog], the Charm logger provides customizable colorful human readable logging with batteries included.

Uses [Lip Gloss][lipgloss] to style and colorize the output.
Colorful, human readable format.
Ability to customize the time stamp format.
Skips caller frames and marks function as helpers.
Leveled logging.
Text, JSON, and Logfmt formatters.
Store and retrieve logger in and from context.
Slog handler.
Standard log adapter.

Usage

Use go get to download the dependency.

go get charm.land/log/v2@latest

Then, import it in your Go files:

import "charm.land/log/v2"

The Charm logger comes with a global package-wise logger with timestamps turned on, and the logging level set to info.

log.Debug("Cookie 🍪") // won't print anything
log.Info("Hello World!")

All logging levels accept optional key/value pairs to be printed along with a message.

err := fmt.Errorf("too much sugar")
log.Error("failed to bake cookies", "err", err)

You can use log.Print() to print messages without a level prefix.

log.Print("Baking 101")
// 2023/01/04 10:04:06 Baking 101

New loggers

Use New() to create new loggers.

logger := log.New(os.Stderr)
if butter {
    logger.Warn("chewy!", "butter", true)
}

Levels

Log offers multiple levels to filter your logs on. Available levels are:

log.DebugLevel
log.InfoLevel
log.WarnLevel
log.ErrorLevel
log.FatalLevel

Use log.SetLevel() to set the log level. You can also create a new logger with a specific log level using log.Options{Level: }.

Use the corresponding function to log a message:

err := errors.New("Baking error 101")
log.Debug(err)
log.Info(err)
log.Warn(err)
log.Error(err)
log.Fatal(err) // this calls os.Exit(1)
log.Print(err) // prints regardless of log level

Or use the formatter variant:

format := "%s %d"
log.Debugf(format, "chocolate", 10)
log.Warnf(format, "adding more", 5)
log.Errorf(format, "increasing temp", 420)
log.Fatalf(format, "too hot!", 500) // this calls os.Exit(1)
log.Printf(format, "baking cookies") // prints regardless of log level

// Use these in conjunction with `With(...)` to add more context
log.With("err", err).Errorf("unable to start %s", "oven")

Structured

All the functions above take a message and key-value pairs of anything. The message can also be of type any.

ingredients := []string{"flour", "butter", "sugar", "chocolate"}
log.Debug("Available ingredients", "ingredients", ingredients)
// DEBUG Available ingredients ingredients="[flour butter sugar chocolate]"

Options

You can customize the logger with options. Use log.NewWithOptions() and log.Options{} to customize your new logger.

logger := log.NewWithOptions(os.Stderr, log.Options{
    ReportCaller: true,
    ReportTimestamp: true,
    TimeFormat: time.Kitchen,
    Prefix: "Baking 🍪 ",
})
logger.Info("Starting oven!", "degree", 375)
time.Sleep(10 * time.Minute)
logger.Info("Finished baking")

You can also use logger setters to customize the logger.

logger := log.New(os.Stderr)
logger.SetReportTimestamp(false)
logger.SetReportCaller(false)
logger.SetLevel(log.DebugLevel)

Use log.SetFormatter() or log.Options{Formatter: } to change the output format. Available options are:

log.TextFormatter (default)
log.JSONFormatter
log.LogfmtFormatter

Note styling only affects the TextFormatter. Styling is disabled if the output is not a TTY.

For a list of available options, refer to options.go.

Styles

You can customize the logger styles using [Lipgloss][lipgloss]. The styles are defined at a global level in styles.go.

// Override the default error level style.
styles := log.DefaultStyles()
styles.Levels[log.ErrorLevel] = lipgloss.NewStyle().
	SetString("ERROR!!").
	Padding(0, 1, 0, 1).
	Background(lipgloss.Color("204")).
	Foreground(lipgloss.Color("0"))
// Add a custom style for key `err`
styles.Keys["err"] = lipgloss.NewStyle().Foreground(lipgloss.Color("204"))
styles.Values["err"] = lipgloss.NewStyle().Bold(true)
logger := log.New(os.Stderr)
logger.SetStyles(styles)
logger.Error("Whoops!", "err", "kitchen on fire")

Sub-logger

Create sub-loggers with their specific fields.

logger := log.NewWithOptions(os.Stderr, log.Options{
    Prefix: "Baking 🍪 "
})
batch2 := logger.With("batch", 2, "chocolateChips", true)
batch2.Debug("Preparing batch 2...")
batch2.Debug("Adding chocolate chips")

<source media="(prefers-color-scheme: dark)"  srcset

Log