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Logrus is a structured logger for Go (golang), completely API compatible withthe standard library logger.

Logrus is in maintenance-mode. We will not be introducing new features. It'ssimply too hard to do in a way that won't break many people's projects, which isthe last thing you want from your Logging library (again...).

This does not mean Logrus is dead. Logrus will continue to be maintained forsecurity, (backwards compatible) bug fixes, and performance (where we arelimited by the interface).

I believe Logrus' biggest contribution is to have played a part in today'swidespread use of structured logging in Golang. There doesn't seem to be areason to do a major, breaking iteration into Logrus V2, since the fantastic Gocommunity has built those independently. Many fantastic alternatives have sprungup. Logrus would look like those, had it been re-designed with what we knowabout structured logging in Go today. Check out, for example,Zerolog,Zap, andApex.

Seeing weird case-sensitive problems? It's in the past been possible toimport Logrus as both upper- and lower-case. Due to the Go package environment,this caused issues in the community and we needed a standard. Some environmentsexperienced problems with the upper-case variant, so the lower-case was decided.Everything usinglogrus will need to use the lower-case:github.com/sirupsen/logrus. Any package that isn't, should be changed.

To fix Glide, seethesecomments.For an in-depth explanation of the casing issue, seethiscomment.

Nicely color-coded in development (when a TTY is attached, otherwise justplain text):

Withlog.SetFormatter(&log.JSONFormatter{}), for easy parsing by logstashor Splunk:

{"animal":"walrus","level":"info","msg":"A group of walrus emerges from theocean","size":10,"time":"2014-03-10 19:57:38.562264131 -0400 EDT"}{"level":"warning","msg":"The group's number increased tremendously!","number":122,"omg":true,"time":"2014-03-10 19:57:38.562471297 -0400 EDT"}{"animal":"walrus","level":"info","msg":"A giant walrus appears!","size":10,"time":"2014-03-10 19:57:38.562500591 -0400 EDT"}{"animal":"walrus","level":"info","msg":"Tremendously sized cow enters the ocean.","size":9,"time":"2014-03-10 19:57:38.562527896 -0400 EDT"}{"level":"fatal","msg":"The ice breaks!","number":100,"omg":true,"time":"2014-03-10 19:57:38.562543128 -0400 EDT"}

With the defaultlog.SetFormatter(&log.TextFormatter{}) when a TTY is notattached, the output is compatible with thelogfmt format:

time="2015-03-26T01:27:38-04:00" level=debug msg="Started observing beach" animal=walrus number=8time="2015-03-26T01:27:38-04:00" level=info msg="A group of walrus emerges from the ocean" animal=walrus size=10time="2015-03-26T01:27:38-04:00" level=warning msg="The group's number increased tremendously!" number=122 omg=truetime="2015-03-26T01:27:38-04:00" level=debug msg="Temperature changes" temperature=-4time="2015-03-26T01:27:38-04:00" level=panic msg="It's over 9000!" animal=orca size=9009time="2015-03-26T01:27:38-04:00" level=fatal msg="The ice breaks!" err=&{0x2082280c0 map[animal:orca size:9009] 2015-03-26 01:27:38.441574009 -0400 EDT panic It's over 9000!} number=100 omg=true

To ensure this behaviour even if a TTY is attached, set your formatter as follows:

log.SetFormatter(&log.TextFormatter{DisableColors:true,FullTimestamp:true,})

If you wish to add the calling method as a field, instruct the logger via:

log.SetReportCaller(true)

This adds the caller as 'method' like so:

{"animal":"penguin","level":"fatal","method":"github.com/sirupsen/arcticcreatures.migrate","msg":"a penguin swims by","time":"2014-03-10 19:57:38.562543129 -0400 EDT"}

time="2015-03-26T01:27:38-04:00" level=fatal method=github.com/sirupsen/arcticcreatures.migrate msg="a penguin swims by" animal=penguin

Note that this does add measurable overhead - the cost will depend on the version of Go, but isbetween 20 and 40% in recent tests with 1.6 and 1.7. You can validate this in yourenvironment via benchmarks:

go test -bench=.*CallerTracing

The organization's name was changed to lower-case--and this will not be changedback. If you are getting import conflicts due to case sensitivity, please usethe lower-case import:github.com/sirupsen/logrus.

The simplest way to use Logrus is simply the package-level exported logger:

package mainimport (  log"github.com/sirupsen/logrus")funcmain() {log.WithFields(log.Fields{"animal":"walrus",  }).Info("A walrus appears")}

Note that it's completely api-compatible with the stdlib logger, so you canreplace yourlog imports everywhere withlog "github.com/sirupsen/logrus"and you'll now have the flexibility of Logrus. You can customize it all youwant:

package mainimport ("os"  log"github.com/sirupsen/logrus")funcinit() {// Log as JSON instead of the default ASCII formatter.log.SetFormatter(&log.JSONFormatter{})// Output to stdout instead of the default stderr// Can be any io.Writer, see below for File examplelog.SetOutput(os.Stdout)// Only log the warning severity or above.log.SetLevel(log.WarnLevel)}funcmain() {log.WithFields(log.Fields{"animal":"walrus","size":10,  }).Info("A group of walrus emerges from the ocean")log.WithFields(log.Fields{"omg":true,"number":122,  }).Warn("The group's number increased tremendously!")log.WithFields(log.Fields{"omg":true,"number":100,  }).Fatal("The ice breaks!")// A common pattern is to re-use fields between logging statements by re-using// the logrus.Entry returned from WithFields()contextLogger:=log.WithFields(log.Fields{"common":"this is a common field","other":"I also should be logged always",  })contextLogger.Info("I'll be logged with common and other field")contextLogger.Info("Me too")}

For more advanced usage such as logging to multiple locations from the sameapplication, you can also create an instance of thelogrus Logger:

package mainimport ("os""github.com/sirupsen/logrus")// Create a new instance of the logger. You can have any number of instances.varlog=logrus.New()funcmain() {// The API for setting attributes is a little different than the package level// exported logger. See Godoc.log.Out=os.Stdout// You could set this to any `io.Writer` such as a file// file, err := os.OpenFile("logrus.log", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)// if err == nil {//  log.Out = file// } else {//  log.Info("Failed to log to file, using default stderr")// }log.WithFields(logrus.Fields{"animal":"walrus","size":10,  }).Info("A group of walrus emerges from the ocean")}

Logrus encourages careful, structured logging through logging fields instead oflong, unparseable error messages. For example, instead of:log.Fatalf("Failed to send event %s to topic %s with key %d"), you should log the much morediscoverable:

log.WithFields(log.Fields{"event":event,"topic":topic,"key":key,}).Fatal("Failed to send event")

We've found this API forces you to think about logging in a way that producesmuch more useful logging messages. We've been in countless situations where justa single added field to a log statement that was already there would've saved ushours. TheWithFields call is optional.

In general, with Logrus using any of theprintf-family functions should beseen as a hint you should add a field, however, you can still use theprintf-family functions with Logrus.

Often it's helpful to have fieldsalways attached to log statements in anapplication or parts of one. For example, you may want to always log therequest_id anduser_ip in the context of a request. Instead of writinglog.WithFields(log.Fields{"request_id": request_id, "user_ip": user_ip}) onevery line, you can create alogrus.Entry to pass around instead:

requestLogger:=log.WithFields(log.Fields{"request_id":request_id,"user_ip":user_ip})requestLogger.Info("something happened on that request") #willlogrequest_idanduser_iprequestLogger.Warn("something not great happened")

You can add hooks for logging levels. For example to send errors to an exceptiontracking service onError,Fatal andPanic, info to StatsD or log tomultiple places simultaneously, e.g. syslog.

Logrus comes withbuilt-in hooks. Add those, or your custom hook, ininit:

import (  log"github.com/sirupsen/logrus""gopkg.in/gemnasium/logrus-airbrake-hook.v2"// the package is named "airbrake"  logrus_syslog"github.com/sirupsen/logrus/hooks/syslog""log/syslog")funcinit() {// Use the Airbrake hook to report errors that have Error severity or above to// an exception tracker. You can create custom hooks, see the Hooks section.log.AddHook(airbrake.NewHook(123,"xyz","production"))hook,err:=logrus_syslog.NewSyslogHook("udp","localhost:514",syslog.LOG_INFO,"")iferr!=nil {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 thesyslog hook README.

A list of currently known service hooks can be found in this wikipage

Logrus has seven logging levels: Trace, Debug, Info, Warning, Error, Fatal and Panic.

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 logginglog.Fatal("Bye.")// Calls panic() after logginglog.Panic("I'm bailing.")

You can set the logging level on aLogger, then it will only log entries withthat severity or anything above it:

// Will log anything that is info or above (warn, error, fatal, panic). Default.log.SetLevel(log.InfoLevel)

It may be useful to setlog.Level = logrus.DebugLevel in a debug or verboseenvironment if your application has that.

Note: If you want different log levels for global (log.SetLevel(...)) and syslog logging, please check thesyslog hook README.

Besides the fields added withWithField orWithFields some fields areautomatically 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} aftertheAddFields call. E.g.Failed to send event.
3. level. The logging level. E.g.info.

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 globalvariableEnvironment, which is a string representation of the environment youcould do:

import (  log"github.com/sirupsen/logrus")funcinit() {// do something here to set environment depending on an environment variable// or command-line flagifEnvironment=="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 howlogrus was intended to be used, but JSON inproduction is mostly only useful if you do log aggregation with tools likeSplunk or Logstash.

The built-in logging formatters are:

• logrus.TextFormatter. Logs the event in colors if stdout is a tty, otherwisewithout colors.
  • Note: to force colored output when there is no TTY, set theForceColorsfield totrue. To force no colored output even if there is a TTY set theDisableColors field totrue. For Windows, seegithub.com/mattn/go-colorable.
  • When colors are enabled, levels are truncated to 4 characters by default. To disabletruncation set theDisableLevelTruncation field totrue.
  • When outputting to a TTY, it's often helpful to visually scan down a column where all the levels are the same width. Setting thePadLevelText field totrue enables this behavior, by adding padding to the level text.
  • All options are listed in thegenerated docs.
• logrus.JSONFormatter. Logs fields as JSON.
  • All options are listed in thegenerated docs.

Third party logging formatters:

• FluentdFormatter. Formats entries that can be parsed by Kubernetes and Google Container Engine.
• GELF. Formats entries so they comply to Graylog'sGELF 1.1 specification.
• logstash. Logs fields asLogstash Events.
• prefixed. Displays log entry source along with alternative layout.
• zalgo. Invoking the Power of Zalgo.
• nested-logrus-formatter. Converts logrus fields to a nested structure.
• powerful-logrus-formatter. get fileName, log's line number and the latest function's name when print log; Save log to files.
• caption-json-formatter. logrus's message json formatter with human-readable caption added.

You can define your formatter by implementing theFormatter interface,requiring aFormat method.Format takes an*Entry.entry.Data is aFields type (map[string]interface{}) with all your fields as well as thedefault ones (see Entries section above):

typeMyJSONFormatterstruct {}log.SetFormatter(new(MyJSONFormatter))func (f*MyJSONFormatter)Format(entry*Entry) ([]byte,error) {// 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)iferr!=nil {returnnil,fmt.Errorf("Failed to marshal fields to JSON, %w",err)    }returnappend(serialized,'\n'),nil}

Logrus can be transformed into anio.Writer. That writer is the end of anio.Pipe and it is your responsibility to close it.

w:=logger.Writer()deferw.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 formattersand hooks. The level for those entries isinfo.

This means that we can override the standard library logger easily:

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())

Log rotation is not provided with Logrus. Log rotation should be done by anexternal program (likelogrotate(8)) that can compress and delete old logentries. It should not be a feature of the application-level logger.

Logrus has a built in facility for asserting the presence of log messages. This is implemented through thetest hook and provides:

• decorators for existing logger (test.NewLocal andtest.NewGlobal) which basically just adds thetest hook
• a test logger (test.NewNullLogger) that just records log messages (and does not output any):

import("github.com/sirupsen/logrus""github.com/sirupsen/logrus/hooks/test""github.com/stretchr/testify/assert""testing")funcTestSomething(t*testing.T){logger,hook:=test.NewNullLogger()logger.Error("Helloerror")assert.Equal(t,1,len(hook.Entries))assert.Equal(t,logrus.ErrorLevel,hook.LastEntry().Level)assert.Equal(t,"Helloerror",hook.LastEntry().Message)hook.Reset()assert.Nil(t,hook.LastEntry())}

Logrus can register one or more functions that will be called when anyfatallevel message is logged. The registered handlers will be executed beforelogrus performs anos.Exit(1). This behavior may be helpful if callers needto gracefully shutdown. Unlike apanic("Something went wrong...") call which can be intercepted with a deferredrecover a call toos.Exit(1) can not be intercepted.

...handler := func() {  // gracefully shutdown something...}logrus.RegisterExitHandler(handler)...

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 withO_APPEND flag, and every write is smaller than 4k. (This allows multi-thread/multi-process writing)

     (Refer tohttp://www.notthewizard.com/2014/06/17/are-files-appends-really-atomic/)