How to write log to file

Question

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.