1# lumberjack [](https://godoc.org/gopkg.in/natefinch/lumberjack.v2) [](https://travis-ci.org/natefinch/lumberjack) [](https://ci.appveyor.com/project/natefinch/lumberjack) [](https://coveralls.io/r/natefinch/lumberjack?branch=v2.0)
2
3### Lumberjack is a Go package for writing logs to rolling files.
4
5Package lumberjack provides a rolling logger.
6
7Note that this is v2.0 of lumberjack, and should be imported using gopkg.in
8thusly:
9
10 import "gopkg.in/natefinch/lumberjack.v2"
11
12The package name remains simply lumberjack, and the code resides at
13https://github.com/natefinch/lumberjack under the v2.0 branch.
14
15Lumberjack is intended to be one part of a logging infrastructure.
16It is not an all-in-one solution, but instead is a pluggable
17component at the bottom of the logging stack that simply controls the files
18to which logs are written.
19
20Lumberjack plays well with any logging package that can write to an
21io.Writer, including the standard library's log package.
22
23Lumberjack assumes that only one process is writing to the output files.
24Using the same lumberjack configuration from multiple processes on the same
25machine will result in improper behavior.
26
27
28**Example**
29
30To use lumberjack with the standard library's log package, just pass it into the SetOutput function when your application starts.
31
32Code:
33
34```go
35log.SetOutput(&lumberjack.Logger{
36 Filename: "/var/log/myapp/foo.log",
37 MaxSize: 500, // megabytes
38 MaxBackups: 3,
39 MaxAge: 28, //days
40 Compress: true, // disabled by default
41})
42```
43
44
45
46## type Logger
47``` go
48type Logger struct {
49 // Filename is the file to write logs to. Backup log files will be retained
50 // in the same directory. It uses <processname>-lumberjack.log in
51 // os.TempDir() if empty.
52 Filename string `json:"filename" yaml:"filename"`
53
54 // MaxSize is the maximum size in megabytes of the log file before it gets
55 // rotated. It defaults to 100 megabytes.
56 MaxSize int `json:"maxsize" yaml:"maxsize"`
57
58 // MaxAge is the maximum number of days to retain old log files based on the
59 // timestamp encoded in their filename. Note that a day is defined as 24
60 // hours and may not exactly correspond to calendar days due to daylight
61 // savings, leap seconds, etc. The default is not to remove old log files
62 // based on age.
63 MaxAge int `json:"maxage" yaml:"maxage"`
64
65 // MaxBackups is the maximum number of old log files to retain. The default
66 // is to retain all old log files (though MaxAge may still cause them to get
67 // deleted.)
68 MaxBackups int `json:"maxbackups" yaml:"maxbackups"`
69
70 // LocalTime determines if the time used for formatting the timestamps in
71 // backup files is the computer's local time. The default is to use UTC
72 // time.
73 LocalTime bool `json:"localtime" yaml:"localtime"`
74
75 // Compress determines if the rotated log files should be compressed
76 // using gzip. The default is not to perform compression.
77 Compress bool `json:"compress" yaml:"compress"`
78 // contains filtered or unexported fields
79}
80```
81Logger is an io.WriteCloser that writes to the specified filename.
82
83Logger opens or creates the logfile on first Write. If the file exists and
84is less than MaxSize megabytes, lumberjack will open and append to that file.
85If the file exists and its size is >= MaxSize megabytes, the file is renamed
86by putting the current time in a timestamp in the name immediately before the
87file's extension (or the end of the filename if there's no extension). A new
88log file is then created using original filename.
89
90Whenever a write would cause the current log file exceed MaxSize megabytes,
91the current file is closed, renamed, and a new log file created with the
92original name. Thus, the filename you give Logger is always the "current" log
93file.
94
95Backups use the log file name given to Logger, in the form `name-timestamp.ext`
96where name is the filename without the extension, timestamp is the time at which
97the log was rotated formatted with the time.Time format of
98`2006-01-02T15-04-05.000` and the extension is the original extension. For
99example, if your Logger.Filename is `/var/log/foo/server.log`, a backup created
100at 6:30pm on Nov 11 2016 would use the filename
101`/var/log/foo/server-2016-11-04T18-30-00.000.log`
102
103### Cleaning Up Old Log Files
104Whenever a new logfile gets created, old log files may be deleted. The most
105recent files according to the encoded timestamp will be retained, up to a
106number equal to MaxBackups (or all of them if MaxBackups is 0). Any files
107with an encoded timestamp older than MaxAge days are deleted, regardless of
108MaxBackups. Note that the time encoded in the timestamp is the rotation
109time, which may differ from the last time that file was written to.
110
111If MaxBackups and MaxAge are both 0, no old log files will be deleted.
112
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121
122
123### func (\*Logger) Close
124``` go
125func (l *Logger) Close() error
126```
127Close implements io.Closer, and closes the current logfile.
128
129
130
131### func (\*Logger) Rotate
132``` go
133func (l *Logger) Rotate() error
134```
135Rotate causes Logger to close the existing log file and immediately create a
136new one. This is a helper function for applications that want to initiate
137rotations outside of the normal rotation rules, such as in response to
138SIGHUP. After rotating, this initiates a cleanup of old log files according
139to the normal rules.
140
141**Example**
142
143Example of how to rotate in response to SIGHUP.
144
145Code:
146
147```go
148l := &lumberjack.Logger{}
149log.SetOutput(l)
150c := make(chan os.Signal, 1)
151signal.Notify(c, syscall.SIGHUP)
152
153go func() {
154 for {
155 <-c
156 l.Rotate()
157 }
158}()
159```
160
161### func (\*Logger) Write
162``` go
163func (l *Logger) Write(p []byte) (n int, err error)
164```
165Write implements io.Writer. If a write would cause the log file to be larger
166than MaxSize, the file is closed, renamed to include a timestamp of the
167current time, and a new log file is created using the original log file name.
168If the length of the write is greater than MaxSize, an error is returned.
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