io.TeeReader example

tee.go

package main

import (
	"os"
	"io"
	"log"
)

func main() {
	filename := os.Args[1]
	file, err := os.Create(filename)
	if err != nil {
		log.Fatalln(err)
	}
	defer file.Close()

	tee := io.TeeReader(os.Stdin, file)
	if _, err := io.Copy(os.Stdout, tee); err != nil {
		log.Fatalln(err)
	}
}