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 of service hook 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
environment if your application has that.
#### Entries
Besides the fields added with `WithField` or `WithFields` some fields are
automatically added to all logging events:
1.`time`. The timestamp when the entry was created.
2.`msg`. The logging message passed to `{Info,Warn,Error,Fatal,Panic}` after
the `AddFields` call. E.g. `Failed to send event.`
3.`level`. The logging level. E.g. `info`.
#### Environments
Logrus has no notion of environment.
If you wish for hooks and formatters to only be used in specific environments,
you should handle that yourself. For example, if your application has a global
variable `Environment`, which is a string representation of the environment you
could do:
```go
import (
log "github.com/sirupsen/logrus"
)
init() {
// do something here to set environment depending on an environment variable
// or command-line flag
if Environment == "production" {
log.SetFormatter(&log.JSONFormatter{})
} else {
// The TextFormatter is default, you don't actually have to do this.
log.SetFormatter(&log.TextFormatter{})
}
}
```
This configuration is how `logrus` was intended to be used, but JSON in
production is mostly only useful if you do log aggregation with tools like
Splunk or Logstash.
#### Formatters
The built-in logging formatters are:
*`logrus.TextFormatter`. Logs the event in colors if stdout is a tty, otherwise
without colors.
* *Note:* to force colored output when there is no TTY, set the `ForceColors`
field to `true`. To force no colored output even if there is a TTY set the
* [`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).
// 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, %v", 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 generated with different config at different environment.|
|[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 add 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 a `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 a os.File handler opened with `O_APPEND` flag, and every write is smaller than 4k. (This allow multi-thread/multi-process writing)
(Refer to http://www.notthewizard.com/2014/06/17/are-files-appends-really-atomic/)
