tee.go

import (
  "fmt"
  "io/ioutil"
  "os"
  "sync"
)

var wg sync.WaitGroup

func main() {
  b, err := ioutil.ReadAll(os.Stdin)
  if err != nil {
    panic(err)
  }

  wg.Add(1)
  go func(b []byte) {
    fmt.Fprintf(os.Stdout, string(b))
    wg.Done()
  }(b)

  if os.Args[1] != "" {
    f, err := os.OpenFile(os.Args[1], os.O_RDWR|os.O_CREATE, 0755)
    if err != nil {
      panic(err)
    }

    wg.Add(1)
    go func(b []byte) {
      f.Write(b)
      wg.Done()
    }(b)
  }

  wg.Wait()
}