How to Use io.MultiWriter in Go

Use io.MultiWriter to write the same data to multiple destinations simultaneously by passing them as arguments to the function. In the Go source code, this is used to write output to both a buffer and os.Stderr when verbose mode is enabled.

import "io"

var buf bytes.Buffer
var w io.Writer = &buf

if cfg.BuildV {
	w = io.MultiWriter(&buf, os.Stderr)
}

fmt.Fprintf(w, "output message")

io.MultiWriter acts like a splitter that sends the same stream of data to multiple places at once. You use it when you need to save output to a file or variable while also displaying it on the screen. It is similar to a network switch that duplicates a signal to several devices simultaneously.