Minimalistic logging library for Go.

$ go get github.com/azer/logger

First create an instance with a preferred name;

import "github.com/azer/logger"

var log = logger.New("example-app")

It gives only three methods; Info, Timer and Error.

log.Info("Running at %d", 8080)

err := DoSomething()

if err != nil {
  log.Error("Failed: %s", err.Error())
}

Check out example/simple.go for a more detailed example.

By default, it won't output. Enable it with LOG environment variable:

$ LOG=* go run example-app.go

This will enable all logs. You can choose the packages that you'd like to display:

$ LOG=images,users go run example-app.go

In the above example, you'll only see logs from images and users packages.

You can filter logs by their level, too. If INFO level is not useful for your case, pass LOG_LEVEL:

$ LOG=images,users LOG_LEVEL=error go run example-app.go

You can use timer logs for measuring your program. For example;

timer := log.Timer()

image, err := PullImage("http://foo.com/bar.jpg")

timer.End("Fetched foo.com/bar.jpg")

Timer log lines will be outputting the elapsed time in time.Duration in a normal terminal, or in int64 format when your program is running on a non-terminal environment. See below documentation for more info.

When your app isn't running on a terminal, it'll change the output in JSON:

{ "time":"2014-10-04 11:44:22.418595705 -0700 PDT", "package":"database", "level":"INFO", "msg":"Connecting to mysql://azer@localhost:9900/foobar" }
{ "time":"2014-10-04 11:44:22.418600851 -0700 PDT", "package":"images", "level":"INFO", "msg":"Requesting an image at foo/bar.jpg" }
{ "time":"2014-10-04 11:44:22.668645527 -0700 PDT", "package":"images", "level":"TIMER", "elapsed":"250032416", "msg":"Fetched foo/bar.jpg" }
{ "time":"2014-10-04 11:44:22.668665527 -0700 PDT", "package":"database", "level":"ERROR", "msg":"Fatal connection error." }
{ "time":"2014-10-04 11:44:22.668673037 -0700 PDT", "package":"users", "level":"INFO", "msg":"John just logged  from Istanbul" }
{ "time":"2014-10-04 11:44:22.668676732 -0700 PDT", "package":"websocket", "level":"INFO", "msg":"Connecting..." }
{ "time":"2014-10-04 11:44:22.668681092 -0700 PDT", "package":"websocket", "level":"ERROR", "msg":"Unable to connect." }
{ "time":"2014-10-04 11:44:22.919726985 -0700 PDT", "package":"app", "level":"ERROR", "msg":"Failed to start, shutting down..." }

So you can parse & process the output easily. Here is a command that lets you see the JSON output in your terminal;

LOG=* go run examples/simple.go 2>&1 | less

To add custom attributes to the structured output;

log.Info("Sending an e-mail...", logger.Attrs{
  "from": "foo@bar.com",
  "to": "qux@corge.com",
})

The above log will appear in the structured output as:

{ "time":"2014-10-04 11:44:22.919726985 -0700 PDT", "package":"mail", "level":"INFO", "msg":"Sending an e-mail", "from": "foo@foobar.com", "to": "qux@corge.com" }

In your command-line as:

By default, it outputs to stderr. You can change it by calling SetOutput with an *os.File parameter.