log.Error("Unable to connect to local syslog daemon")
} else {
log.AddHook(hook)
}
}
```
Note: Syslog hook also support connecting to local syslog (Ex. "/dev/log" or "/var/run/syslog" or "/var/run/log"). For the detail, please check the [syslog hook README](hooks/syslog/README.md).
A list of currently known service hooks can be found in this wiki [page](https://github.com/sirupsen/logrus/wiki/Hooks)
#### Level logging
Logrus has seven logging levels: Trace, Debug, Info, Warning, Error, Fatal and Panic.
```go
log.Trace("Something very low level.")
log.Debug("Useful debugging information.")
log.Info("Something noteworthy happened!")
log.Warn("You should probably take a look at this.")
log.Error("Something failed but I'm not quitting.")
// Calls os.Exit(1) after logging
log.Fatal("Bye.")
// Calls panic() after logging
log.Panic("I'm bailing.")
```
You can set the logging level on a `Logger`, then it will only log entries with
that severity or anything above it:
```go
// Will log anything that is info or above (warn, error, fatal, panic). Default.
log.SetLevel(log.InfoLevel)
```
It may be useful to set `log.Level = logrus.DebugLevel` in a debug or verbose
Note: If you want different log levels for global (`log.SetLevel(...)`) and syslog logging, please check the [syslog hook README](hooks/syslog/README.md#different-log-levels-for-local-and-remote-logging).
* When colors are enabled, levels are truncated to 4 characters by default. To disable
truncation set the `DisableLevelTruncation` field to `true`.
* When outputting to a TTY, it's often helpful to visually scan down a column where all the levels are the same width. Setting the `PadLevelText` field to `true` enables this behavior, by adding padding to the level text.
* All options are listed in the [generated docs](https://godoc.org/github.com/sirupsen/logrus#TextFormatter).
*`logrus.JSONFormatter`. Logs fields as JSON.
* All options are listed in the [generated docs](https://godoc.org/github.com/sirupsen/logrus#JSONFormatter).
Third party logging formatters:
* [`FluentdFormatter`](https://github.com/joonix/log). Formats entries that can be parsed by Kubernetes and Google Container Engine.
* [`GELF`](https://github.com/fabienm/go-logrus-formatters). Formats entries so they comply to Graylog's [GELF 1.1 specification](http://docs.graylog.org/en/2.4/pages/gelf.html).
* [`logstash`](https://github.com/bshuster-repo/logrus-logstash-hook). Logs fields as [Logstash](http://logstash.net) Events.
* [`prefixed`](https://github.com/x-cray/logrus-prefixed-formatter). Displays log entry source along with alternative layout.
* [`zalgo`](https://github.com/aybabtme/logzalgo). Invoking the Power of Zalgo.
* [`nested-logrus-formatter`](https://github.com/antonfisher/nested-logrus-formatter). Converts logrus fields to a nested structure.
* [`powerful-logrus-formatter`](https://github.com/zput/zxcTool). get fileName, log's line number and the latest function's name when print log; Sava log to files.
* [`caption-json-formatter`](https://github.com/nolleh/caption_json_formatter). logrus's message json formatter with human-readable caption added.
You can define your formatter by implementing the `Formatter` interface,
requiring a `Format` method. `Format` takes an `*Entry`. `entry.Data` is a
`Fields` type (`map[string]interface{}`) with all your fields as well as the
// Note this doesn't include Time, Level and Message which are available on
// the Entry. Consult `godoc` on information about those fields or read the
// source of the official loggers.
serialized, err := json.Marshal(entry.Data)
if err != nil {
return nil, fmt.Errorf("Failed to marshal fields to JSON, %w", err)
}
return append(serialized, '\n'), nil
}
```
#### Logger as an `io.Writer`
Logrus can be transformed into an `io.Writer`. That writer is the end of an `io.Pipe` and it is your responsibility to close it.
```go
w := logger.Writer()
defer w.Close()
srv := http.Server{
// create a stdlib log.Logger that writes to
// logrus.Logger.
ErrorLog: log.New(w, "", 0),
}
```
Each line written to that writer will be printed the usual way, using formatters
and hooks. The level for those entries is `info`.
This means that we can override the standard library logger easily:
```go
logger := logrus.New()
logger.Formatter = &logrus.JSONFormatter{}
// Use logrus for standard log output
// Note that `log` here references stdlib's log
// Not logrus imported under the name `log`.
log.SetOutput(logger.Writer())
```
#### Rotation
Log rotation is not provided with Logrus. Log rotation should be done by an
external program (like `logrotate(8)`) that can compress and delete old log
entries. It should not be a feature of the application-level logger.
#### Tools
| Tool | Description |
| ---- | ----------- |
|[Logrus Mate](https://github.com/gogap/logrus_mate)|Logrus mate is a tool for Logrus to manage loggers, you can initial logger's level, hook and formatter by config file, the logger will be generated with different configs in different environments.|
|[Logrus Viper Helper](https://github.com/heirko/go-contrib/tree/master/logrusHelper)|An Helper around Logrus to wrap with spf13/Viper to load configuration with fangs! And to simplify Logrus configuration use some behavior of [Logrus Mate](https://github.com/gogap/logrus_mate). [sample](https://github.com/heirko/iris-contrib/blob/master/middleware/logrus-logger/example) |
#### Testing
Logrus has a built in facility for asserting the presence of log messages. This is implemented through the `test` hook and provides:
* decorators for existing logger (`test.NewLocal` and `test.NewGlobal`) which basically just adds the `test` hook
* a test logger (`test.NewNullLogger`) that just records log messages (and does not output any):
Logrus can register one or more functions that will be called when any `fatal`
level message is logged. The registered handlers will be executed before
logrus performs an `os.Exit(1)`. This behavior may be helpful if callers need
to gracefully shutdown. Unlike a `panic("Something went wrong...")` call which can be intercepted with a deferred `recover` a call to `os.Exit(1)` can not be intercepted.
```
...
handler := func() {
// gracefully shutdown something...
}
logrus.RegisterExitHandler(handler)
...
```
#### Thread safety
By default, Logger is protected by a mutex for concurrent writes. The mutex is held when calling hooks and writing logs.
If you are sure such locking is not needed, you can call logger.SetNoLock() to disable the locking.
Situation when locking is not needed includes:
* You have no hooks registered, or hooks calling is already thread-safe.
* Writing to logger.Out is already thread-safe, for example:
1) logger.Out is protected by locks.
2) logger.Out is an os.File handler opened with `O_APPEND` flag, and every write is smaller than 4k. (This allows multi-thread/multi-process writing)
(Refer to http://www.notthewizard.com/2014/06/17/are-files-appends-really-atomic/)
