How to write log to file

I'm trying to write to a log file with Go.

I have tried several approaches, all of which have failed. This is what I have tried:

func TestLogging(t *testing.T) {
    if !FileExists("logfile") {
        CreateFile("logfile")
    }
    f, err := os.Open("logfile")
    if err != nil {
        t.Fatalf("error: %v", err)
    }

    // attempt #1
    log.SetOutput(io.MultiWriter(os.Stderr, f))
    log.Println("hello, logfile")

    // attempt #2
    log.SetOutput(io.Writer(f))
    log.Println("hello, logfile")

    // attempt #3
    log.SetOutput(f)
    log.Println("hello, logfile")
}

func FileExists(name string) bool {
    if _, err := os.Stat(name); err != nil {
       if os.IsNotExist(err) {
            return false
        }
    }
    return true
}

func CreateFile(name string) error {
    fo, err := os.Create(name)
    if err != nil {
        return err
    }
    defer func() {
        fo.Close()
    }()
    return nil
}

The log file gets created, but nothing ever gets printed or appended to it. Why?

os.Open() must have worked differently in the past, but this works for me:

f, err := os.OpenFile("testlogfile", os.O_RDWR | os.O_CREATE | os.O_APPEND, 0666)
if err != nil {
    log.Fatalf("error opening file: %v", err)
}
defer f.Close()

log.SetOutput(f)
log.Println("This is a test log entry")

Based on the Go docs, os.Open() can't work for log.SetOutput, because it opens the file "for reading:"

> func Open
>
> func Open(name string) (file *File, err error) Open opens the named
> file for reading. If successful, methods on the returned file can be
> used for reading; the associated file descriptor has mode O_RDONLY. If
> there is an error, it will be of type *PathError.

EDIT

Moved defer f.Close() to after if err != nil check

I prefer the simplicity and flexibility of the 12 factor app recommendation for logging. To append to a log file you can use shell redirection. The default logger in Go writes to stderr (2).

./app 2>> logfile

See also: http://12factor.net/logs

I usually print the logs on screen and write into a file as well. Hope this helps someone.

f, err := os.OpenFile("/tmp/orders.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
if err != nil {
	log.Fatalf("error opening file: %v", err)
}
defer f.Close()
wrt := io.MultiWriter(os.Stdout, f)
log.SetOutput(wrt)
log.Println(" Orders API Called")

This works for me

1. created a package called logger.go

   package logger
   
   import (
     "flag"
     "os"
     "log"
     "go/build"
   )
   
   var (
     Log      *log.Logger
   )
   
   
   func init() {
       // set location of log file
       var logpath = build.Default.GOPATH + "/src/chat/logger/info.log"
   
      flag.Parse()
      var file, err1 = os.Create(logpath)
   
      if err1 != nil {
         panic(err1)
      }
         Log = log.New(file, "", log.LstdFlags|log.Lshortfile)
         Log.Println("LogFile : " + logpath)
   }
   

2. import the package wherever you want to log e.g main.go

        package main
   
        import (
           "logger"
        )
   
        const (
           VERSION = "0.13"
         )
   
        func main() {
   
            // time to use our logger, print version, processID and number of running process
            logger.Log.Printf("Server v%s pid=%d started with processes: %d", VERSION, os.Getpid(),runtime.GOMAXPROCS(runtime.NumCPU()))
   
        }
   

If you run binary on linux machine you could use shell script.

overwrite into a file

./binaryapp > binaryapp.log

append into a file

./binaryapp >> binaryapp.log

overwrite stderr into a file

./binaryapp &> binaryapp.error.log

append stderr into a file

./binaryapp &>> binalyapp.error.log

it can be more dynamic using shell script file.

Declare up top in your global var so all your processes can access if needed.

package main

import (
	"log"
    "os"
)
var (
	outfile, _ = os.Create("path/to/my.log") // update path for your needs
	l      = log.New(outfile, "", 0)
)

func main() {
    l.Println("hello, log!!!")
}

The default logger in Go writes to stderr (2).
redirect to file

import ( 
    "syscall"
    "os" 
 )
func main(){
  fErr, err = os.OpenFile("Errfile", os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0600)
  syscall.Dup2(int(fErr.Fd()), 1) /* -- stdout */
  syscall.Dup2(int(fErr.Fd()), 2) /* -- stderr */

}

Building on Allison and Deepak's answer, I started using logrus and really like it:

var log = logrus.New()

func init() {

	// log to console and file
	f, err := os.OpenFile("crawler.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
	if err != nil {
		log.Fatalf("error opening file: %v", err)
	}
	wrt := io.MultiWriter(os.Stdout, f)

	log.SetOutput(wrt)
}

I have a defer f.Close() in the main function

I'm writing logs to the files, which are generate on daily basis (per day one log file is getting generated). This approach is working fine for me :

var (
serverLogger *log.Logger
)
func init() {
// set location of log file
date := time.Now().Format("2006-01-02")
var logpath = os.Getenv(constant.XDirectoryPath) + constant.LogFilePath + date + constant.LogFileExtension
os.MkdirAll(os.Getenv(constant.XDirectoryPath)+constant.LogFilePath, os.ModePerm)
flag.Parse()
var file, err1 = os.OpenFile(logpath, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
if err1 != nil {
panic(err1)
}
mw := io.MultiWriter(os.Stdout, file)
serverLogger = log.New(mw, constant.Empty, log.LstdFlags)
serverLogger.Println("LogFile : " + logpath)
}
// LogServer logs to server's log file
func LogServer(logLevel enum.LogLevel, message string) {
_, file, no, ok := runtime.Caller(1)
logLineData := "logger_server.go"
if ok {
file = shortenFilePath(file)
logLineData = fmt.Sprintf(file + constant.ColonWithSpace + strconv.Itoa(no) + constant.HyphenWithSpace)
}
serverLogger.Println(logLineData + logLevel.String() + constant.HyphenWithSpace + message)
}
// ShortenFilePath Shortens file path to a/b/c/d.go tp d.go
func shortenFilePath(file string) string {
short := file
for i := len(file) - 1; i > 0; i-- {
if file[i] == constant.ForwardSlash {
short = file[i+1:]
break
}
}
file = short
return file
}

"shortenFilePath()" method used to get the name of the file from full path of file. and "LogServer()" method is used to create a formatted log statement (contains : filename, line number, log level, error statement etc...)

To help others, I create a basic log function to handle the logging in both cases, if you want the output to stdout, then turn debug on, its straight forward to do a switch flag so you can choose your output.

func myLog(msg ...interface{}) {
defer func() { r := recover(); if r != nil { fmt.Print("Error detected logging:", r) } }()
if conf.DEBUG {
fmt.Println(msg)
} else {
logfile, err := os.OpenFile(conf.LOGDIR+"/"+conf.AppName+".log", os.O_RDWR | os.O_CREATE | os.O_APPEND,0666)
if !checkErr(err) {
log.SetOutput(logfile)
log.Println(msg)
}
defer logfile.Close()
}
}
</details>
# 答案11
**得分**: 0
也许这会对你有所帮助（如果日志文件存在，则使用它，如果不存在则创建）：
```go
package main
import (
"flag"
"log"
"os"
)
// 声明Log变量，用于记录事件。
var (
Log *log.Logger = Loggerx()
)
func Loggerx() *log.Logger {
LOG_FILE_LOCATION := os.Getenv("LOG_FILE_LOCATION")
// 如果环境变量存在，则使用Docker的配置。
if LOG_FILE_LOCATION == "" {
LOG_FILE_LOCATION = "../logs/" + APP_NAME + ".log"
} else {
LOG_FILE_LOCATION = LOG_FILE_LOCATION + APP_NAME + ".log"
}
flag.Parse()
// 如果文件不存在，则创建一个新的日志文件。
if _, err := os.Stat(LOG_FILE_LOCATION); os.IsNotExist(err) {
file, err1 := os.Create(LOG_FILE_LOCATION)
if err1 != nil {
panic(err1)
}
// 如果文件不存在，则创建一个新的日志文件。
return log.New(file, "", log.Ldate|log.Ltime|log.Lshortfile)
} else {
// 如果文件存在，则重用它。
file, err := os.OpenFile(LOG_FILE_LOCATION, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0666)
if err != nil {
panic(err)
}
return log.New(file, "", log.Ldate|log.Ltime|log.Lshortfile)
}
}

更多详细信息请参阅：https://su9.co/9BAE74B

package main
import (
"flag"
"log"
"os"
)
//Se declara la variable Log. Esta será usada para registrar los eventos.
var (
Log *log.Logger = Loggerx()
)
func Loggerx() *log.Logger {
LOG_FILE_LOCATION := os.Getenv("LOG_FILE_LOCATION")
//En el caso que la variable de entorno exista, el sistema usa la configuración del docker.
if LOG_FILE_LOCATION == "" {
LOG_FILE_LOCATION = "../logs/" + APP_NAME + ".log"
} else {
LOG_FILE_LOCATION = LOG_FILE_LOCATION + APP_NAME + ".log"
}
flag.Parse()
//Si el archivo existe se rehusa, es decir, no elimina el archivo log y crea uno nuevo.
if _, err := os.Stat(LOG_FILE_LOCATION); os.IsNotExist(err) {
file, err1 := os.Create(LOG_FILE_LOCATION)
if err1 != nil {
panic(err1)
}
//si no existe,se crea uno nuevo.
return log.New(file, "", log.Ldate|log.Ltime|log.Lshortfile)
} else {
//si existe se rehusa.
file, err := os.OpenFile(LOG_FILE_LOCATION, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0666)
if err != nil {
panic(err)
}
return log.New(file, "", log.Ldate|log.Ltime|log.Lshortfile)
}
}