import "fmt"

func main() {
   fmt.Println("line1")
   fmt.Print("line2")
   fmt.Printf("line%d \n", 3)

   str1 := fmt.Sprintln("hello", 3)
   str2 := fmt.Sprint("hello ", 1, " 2")
   str3 := fmt.Sprintf("hello %d", 1)
   fmt.Print(str1, str2, str3)
}

import “log"

func main() {
   log.Println("line1")
   log.Print("line2")
   log.Printf("line%d \n", 3)
}

// For example, flags Ldate | Ltime (or LstdFlags) produce,
// 2009/01/23 01:23:23 message
// while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,
// 2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message
const (
   Ldate         = 1 << iota     // the date in the local time zone: 2009/01/23
   Ltime                         // the time in the local time zone: 01:23:23
   Lmicroseconds                 // microsecond resolution: 01:23:23.123123.  assumes Ltime.
   Llongfile                     // full file name and line number: /a/b/c/d.go:23
   Lshortfile                    // final file name element and line number: d.go:23. overrides Llongfile
   LUTC                          // if Ldate or Ltime is set, use UTC rather than the local time zone
   Lmsgprefix                    // move the "prefix" from the beginning of the line to before the message
   LstdFlags     = Ldate | Ltime // initial values for the standard logger
)

import “log"

func main() {
   log.SetFlags(log.Ldate | log.Ltime | log.Lshortfile)

   log.Println("line1")
   log.Print("line2")
   log.Printf("line%d \n", 3)
}

// SetOutput sets the output destination for the standard logger.
func SetOutput(w io.Writer) {
    // ...
}

// 創建、追加、讀寫，777，所有權限
f, err := os.OpenFile("log.log", os.O_CREATE|os.O_APPEND|os.O_RDWR, os.ModePerm)
if err != nil {
   return
}
defer func() {
   f.Close()
}()

log.SetOutput(f)

// MultiWriter creates a writer that duplicates its writes to all the
// provided writers, similar to the Unix tee(1) command.
//
// Each write is written to each listed writer, one at a time.
// If a listed writer returns an error, that overall write operation
// stops and returns the error; it does not continue down the list.
func MultiWriter(writers ...Writer) Writer {
   allWriters := make([]Writer, 0, len(writers))
   for _, w := range writers {
      if mw, ok := w.(*multiWriter); ok {
         allWriters = append(allWriters, mw.writers...)
      } else {
         allWriters = append(allWriters, w)
      }
   }
   return &multiWriter{allWriters}
}

// 重寫io.Writer的Write函數函數，本質上就是遍歷數組，比較巧妙
func (t *multiWriter) Write(p []byte) (n int, err error) {
   for _, w := range t.writers {
      n, err = w.Write(p)
      if err != nil {
         return
      }
      if n != len(p) {
         err = ErrShortWrite
         return
      }
   }
   return len(p), nil
}

func main() {
   f, err := os.OpenFile("log.log", os.O_CREATE|os.O_APPEND|os.O_RDWR, os.ModePerm)
   if err != nil {
      return
   }
   defer func() {
      f.Close()
   }()

   // 組合一下即可，os.Stdout代表標準輸出流
   multiWriter := io.MultiWriter(os.Stdout, f)
   log.SetOutput(multiWriter)

   log.SetFlags(log.Ldate | log.Ltime | log.Lshortfile)

   log.Println("line1")
   log.Print("line2")
   log.Printf("line%d \n", 3)
}

    open, _:= os.Open("文件名")
    stat, _， := open.Stat()
    stat.Size()//拿到文件大小