code: func CopyFile(src, dst string) error {
  	// replace with a pure Go implementation?
  	// Golang proposal was: https://go-review.googlesource.com/#/c/1591/5/src/io/ioutil/ioutil.go
  	isDir, err := pathutil.IsDirExists(src)
  	if err != nil {
  		return err
  	}
  	if isDir {
  		return errors.New("Source is a directory: " + src)
  	}
  	args := []string{src, dst}
  	return RunCommand("rsync", args...)
  }
docs: // CopyFile ...