Is there a way to append io.multiwriter with timestamp in go?

I am dealing with logs generated in go. Things are printed to stdout and a log file using io.multiwriterlog := io.MultiWriter(os.Stdout, logfile). I would like to add timestamps to both os.Stdout and logfile.

My approach is to write a write function

type writer struct {
	io.Writer
	timeFormat string
}

func (w writer) Write(b []byte) (n int, err error) {
	return w.Writer.Write(append([]byte(time.Now().Format(w.timeFormat)), b...))
}
log := io.MultiWriter(&writer{os.Stdout, "2006/01/02 15:04:05"}, logFile)

However, it does not produce the timestamp both in Stdout and logfile. Does anyone know if there is another way to do this?

The reason why the timestamp doesn't show up in the log file is because only your writer adds the timestamp, not io.MultiWriter().

Change your use to set the multi-writer to writer.Writer, so the timestamp will be sent to both writers: os.Stdout and logFile.

logFile := &bytes.Buffer{}
log := &writer{io.MultiWriter(os.Stdout, logFile), "2006/01/02 15:04:05"}
log.Write([]byte(" hi"))

fmt.Println("\nlogFile:", logFile)

This will output (try it on the Go Playground):

2009/11/10 23:00:00 hi
logFile: 2009/11/10 23:00:00 hi

Also note that the log package (combined with io.MultiWriter) gives you this functionality out of the box, you can set the output writer using log.SetOutput():

logFile := &bytes.Buffer{}
log.SetOutput(io.MultiWriter(os.Stdout, logFile))
log.Print("hi")

fmt.Println("logFile:", logFile)

Output (try it on the Go Playground):

2009/11/10 23:00:00 hi
logFile: 2009/11/10 23:00:00 hi

Moreover, the log package also provides formatting capabilities and configurable output.