Programming language: Go
License: MIT License
Tags: Logging    
Latest version: v2.1

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lumberjack GoDoc Build Status


This package, Luberjack (effectively v1) is deprecated in favor of v2 of Lumberjack, available from gopkg.in/natefinch/lumberjack.v2 (which redirects to the v2 branch of this repo).

Developerment will not continue on this branch, and it is highly recommended that you migrate to v2.

Lumberjack is a Go package for writing logs to rolling files.

Lumberjack is intended to be one part of a logging infrastructure. It is not an all-in-one solution, but instead is a pluggable component at the bottom of the logging stack that simply controls the files to which logs are written.

Lumberjack plays well with any logger that can write to an io.Writer, including the standard library's log package.

Lumberjack assumes that only one process is writing to the output files. Using the same lumberjack configuration from multiple processes on the same machine will result in improper behavior.


To use lumberjack with the standard library's log package, just pass it into the SetOutput function when your application starts.


    Dir:        "/var/log/myapp/",
    NameFormat: "2006-01-02T15-04-05.000.log",
    MaxSize:    lumberjack.Gigabyte,
    MaxBackups: 3,
    MaxAge:     28,


const (
    Megabyte = 1024 * 1024
    Gigabyte = 1024 * Megabyte

type Logger

type Logger struct {
    // Dir determines the directory in which to store log files.
    // It defaults to os.TempDir() if empty.
    Dir string `json:"dir" yaml:"dir"`

    // NameFormat is the time formatting layout used to generate filenames.
    // It defaults to "2006-01-02T15-04-05.000.log".
    NameFormat string `json:"nameformat" yaml:"nameformat"`

    // MaxSize is the maximum size in bytes of the log file before it gets
    // rolled. It defaults to 100 megabytes.
    MaxSize int64 `json:"maxsize" yaml:"maxsize"`

    // MaxAge is the maximum number of days to retain old log files based on
    // FileInfo.ModTime.  Note that a day is defined as 24 hours and may not
    // exactly correspond to calendar days due to daylight savings, leap
    // seconds, etc. The default is not to remove old log files based on age.
    MaxAge int `json:"maxage" yaml:"maxage"`

    // MaxBackups is the maximum number of old log files to retain.  The default
    // is to retain all old log files (though MaxAge may still cause them to get
    // deleted.)
    MaxBackups int `json:"maxbackups" yaml:"maxbackups"`

    // LocalTime determines if the time used for formatting the filename is the
    // computer's local time.  The default is to use UTC time.
    LocalTime bool `json:"localtime" yaml:"localtime"`
    // contains filtered or unexported fields

Logger is an io.WriteCloser that writes to a log file in the given directory with the given NameFormat. NameFormat should include a time formatting layout in it that produces a valid unique filename for the OS. For more about time formatting layouts, read a http://golang.org/pkg/time/#pkg-constants.

The date encoded in the filename by NameFormat is used to determine which log files are most recent in several situations.

Logger opens or creates a logfile on first Write. It looks for files in the directory that match its name format, and if the one with the most recent NameFormat date is less than MaxSize, it will open and append to that file. If no such file exists, or the file is >= MaxSize, a new file is created using the current time with NameFormat to generate the filename.

Whenever a write would cause the current log file exceed MaxSize, a new file is created using the current time.

Cleaning Up Old Log Files

Whenever a new file gets created, old log files may be deleted. The log file directory is scanned for files that match NameFormat. The most recent files according to their NameFormat date will be retained, up to a number equal to MaxBackups (or all of them if MaxBackups is 0). Any files with a last modified time (based on FileInfo.ModTime) older than MaxAge days are deleted, regardless of MaxBackups.

If MaxBackups and MaxAge are both 0, no old log files will be deleted.

func (*Logger) Close

func (l *Logger) Close() error

Close implements io.Closer, and closes the current logfile.

func (*Logger) Rotate

func (l *Logger) Rotate() error

Rotate causes Logger to close the existing log file and immediately create a new one. This is a helper function for applications that want to initiate rotations outside of the normal rotation rules, such as in response to SIGHUP. After rotating, this initiates a cleanup of old log files according to the normal rules.


Example of how to rotate in response to SIGHUP.


  l := &lumberjack.Logger{}
  c := make(chan os.Signal, 1)
  signal.Notify(c, syscall.SIGHUP)

  go func() {
      for {

func (*Logger) Write

func (l *Logger) Write(p []byte) (n int, err error)

Write implements io.Writer. If a write would cause the log file to be larger than MaxSize, a new log file is created using the current time formatted with PathFormat. If the length of the write is greater than MaxSize, an error is returned.

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