CoLog is a prefix-based leveled execution log for Go


Build Status godoc reference

What's CoLog?

CoLog is a prefix-based leveled execution log for Go. It's heavily inspired by Logrus and aims to offer similar features by parsing the output of the standard library log. If you don't understand what this means take a look at this picture.

CoLog showcase

But why?

An introduction and the rationale behind CoLog can be found in this blog post:


  • Supports hooks to receive log entries and send them to external systems via AddHook
  • Supports customs formatters (color/pain text and JSON built-in) via SetFormatter
  • Provides 6 built-in levels: trace, debug, info, warning, error, alert
  • Understands full, 3 letter, and 1 letter headers: error:, err:, e:
  • Supports custom prefixes (headers in CoLog terms) via SetHeaders and AddHeader
  • Control levels used via SetMinLevel and SetDefaultLevel
  • Supports optionally parsing key=value or key='some value' pairs
  • Supports custom key-value extractor via SetExtractor
  • Supports permanent context values via FixedValue and ClearFixedValues
  • Supports standalone loggers via NewCoLog and NewLogger
  • Compatible with existing Logrus hooks and formatters via cologrus
  • Supports Windows terminal colors via wincolog

API stability

CoLog's API is very unlikely to get breaking changes, but there are no promises. That being said, CoLog only needs to be imported by final applications and if you have one of those, you should be vendoring you dependencies in the first place. CoLog has no external dependencies, to vendor it you just need to clone this repo anywhere you want and start using it.

Usage examples

Basic usage

package main

import (


func main() {
	log.Print("info: that's all it takes!")

JSON output to a file with field parsing

package main

import (


func main() {
	file, err := os.OpenFile("temp_json.log", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0777)
	if err != nil {

		TimeFormat: time.RFC3339,
		Flag:       log.Lshortfile,

	log.Print("info: logging this to json")
	log.Print("warning: with fields foo=bar")

// cat tempjson.log
// {"level":"info","time":"2015-08-16T13:26:07+02:00","file":"json_example.go","line":24,"message":"logging this to json"}
// {"level":"warning","time":"2015-08-16T13:26:07+02:00","file":"json_example.go","line":25,"message":"with fields","fields":{"foo":"bar"}}

Standalone logger with level control and fixed values

package main

import (


func main() {
	cl := colog.NewCoLog(os.Stdout, "worker ", log.LstdFlags)
	cl.FixedValue("worker_id", 42)

	logger := cl.NewLogger()
	logger.Print("this gets warning level")
	logger.Print("debug: this won't be displayed")

// [  warn ] worker 2015/08/16 13:43:06 this gets warning level    worker_id=42

Adding custom hooks

package main

import (


type myHook struct {
	levels []colog.Level

func (h *myHook) Levels() []colog.Level {
	return h.levels

func (h *myHook) Fire(e *colog.Entry) error {
	fmt.Printf("We got an entry: \n%#v", e)
	return nil

func main() {

	hook := &myHook{
		levels: []colog.Level{
			colog.LInfo,    // the hook only receives
			colog.LWarning, // these levels


	colog.SetMinLevel(colog.LError) // this affects only the output
	log.Print("info: something foo=bar")

// We got an entry:
// &colog.Entry{Level:0x3, Time:time.Time{sec:63575323196, nsec:244349216, loc:(*time.Location)(0x23f8c0)}, Host:"",
// Prefix:"", File:"/data/workspace/comail/comail/src/", Line:37,
// Message:[]uint8{0x73, 0x6f, 0x6d, 0x65, 0x74, 0x68, 0x69, 0x6e, 0x67}, Fields:colog.Fields{"foo":"bar"}}%
  • question about best usage when using for testing

    question about best usage when using for testing

    Does it parse errors on Tests ?

    , i was hoping that doing a t.Error("alert: blah"), would display in Red. In the below example, i end up having to do a t.Error & a log.Println(alert), in order to get the RED visual warning.

    I know this is picky ! But when you have lots of tests, looking at your Bash Terminal can get really slow trying to hunt for it. The visual aids are great.

    Any advice welcome, as you can see what i can getting at.

    func init() {
    func TestGetProduct(t *testing.T) {
        t.Error("alert: thrown from t.Errror !!")
        log.Println("alert: thrown from log.Print !!")

    Bash Terminal has:

    [ alert ] 2016/04/15 18:58:35 thrown from log.Print !!                                          
    --- FAIL: TestGetProducts_CategoryOnly (0.01s)
        products_test.go:105: alert: thrown from t.Errro !!
        products_test.go:112: Passed
    opened by joeblew99 2
  • Using `SetFormatter` requires `SetOutput` to print color

    Using `SetFormatter` requires `SetOutput` to print color

    prints no color:

    log.Print("warn: foobar")

    prints with color:

    log.Print("warn: foobar")
    opened by kyoh86 1
  • fix


    I change example code import "" -> import "". And delete trailing whitespace.

    I want to change CoLog showcase image too. But I can not.

    opened by kmtr 1
  • support colors on windows

    support colors on windows

    Following up #1 color should be supported on windows. Since colog shouldn't bring any dependencies this needs to happen via a subpackage or more likely via an extra package like cologrus

    this package could do something like have a Register() function itself and call directly colog.Register() then set colorable.NewColorableStderr() as the output.

    The question is how the colorSupported() function should become smarter to handle all this scenarios:

    • windows/no-windows OS
    • colorable/no-colorable output
    • tty/no-tty output
    // figure if output supports color
    func (cl *CoLog) colorSupported() bool {
        if runtime.GOOS == "windows" {
            return false
        if cl.out == os.Stderr && isTerminal(int(os.Stderr.Fd())) {
            return true
        if cl.out == os.Stdout && isTerminal(int(os.Stdout.Fd())) {
            return true
        return false
    opened by bictorman 0
  • Consider merging with logutils, logfilter

    Consider merging with logutils, logfilter

    I found out about colog via Another related project is

    Would you be interested in joining forces? What would you need to see to deprecate colog in favor of something else? Feature parity, migration docs, perf tests, &c? Would you accept changes to enable something else to be deprecated in favor of colog?

    opened by josephholsten 0
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