colorlog

A colored slog.Handler for Go, plus a CLI tool for pretty-printing JSON log streams. Produces output styled after humanlog.

Output format

Jan 02 15:04:05.000 |INFO| server started addr=:8080
Jan 02 15:04:05.123 |DEBU| incoming request method=GET path=/health
Jan 02 15:04:05.124 |WARN| slow query duration=1.2s table=users
Jan 02 15:04:05.200 |ERRO| connection failed err="dial tcp: refused" retries=3

Timestamp and level are colored by severity. Keys are green, values are muted, messages are white.

Library

Use colorlog.Handler as a drop-in slog.Handler:

import (
    "log/slog"
    "os"

    "github.com/lfsgroup/colorlog"
)

func main() {
    handler := colorlog.NewHandler(os.Stderr, nil)
    slog.SetDefault(slog.New(handler))

    slog.Info("server started", "addr", ":8080")
    slog.Warn("slow query", "duration", "1.2s", "table", "users")
    slog.Error("connection failed", "err", err, "retries", 3)
}

Handler options

Pass slog.HandlerOptions to configure the minimum log level:

handler := colorlog.NewHandler(os.Stderr, &slog.HandlerOptions{
    Level: slog.LevelDebug,
})

WithAttrs and WithGroup

Handler implements the full slog.Handler interface, including WithAttrs and WithGroup:

logger := slog.New(colorlog.NewHandler(os.Stderr, nil))

// Attach persistent fields
reqLogger := logger.With("request_id", "abc123", "user_id", 42)
reqLogger.Info("handled request", "status", 200)

// Group fields under a namespace
dbLogger := logger.WithGroup("db")
dbLogger.Info("query executed", "table", "users", "rows", 10)
// output: ... db.table=users db.rows=10

CLI tool

The colorlog CLI reads JSON log lines from stdin and renders them with color. Use it to pretty-print the output of any program that logs structured JSON.

Install

go install github.com/lfsgroup/colorlog/cmd/colorlog@latest

Usage

Pipe any JSON log stream into colorlog:

./myapp | colorlog

kubectl logs my-pod | colorlog

tail -f /var/log/app.log | colorlog

Non-JSON lines are passed through unchanged, so mixed output is handled gracefully.

Supported JSON fields

Field	Description
time	RFC3339/RFC3339Nano timestamp
level	Log level (DEBUG, INFO, WARN, ERROR)
msg or message	Log message
anything else	Rendered as key=value attributes

Install

go get github.com/lfsgroup/colorlog

Requires Go 1.21 or later (uses log/slog from the standard library).