Unfortunately, searching for "golang copy file" gives subtly wrong code snippets (e.g. https://groups.google.com/d/msg/golang-nuts/JNyQxQLyf5o/kbGnTUK32TkJ that don't check close error code). This is an attempt to copy file content from `src` to `dst`

cp.go

package cp

import (
	"io"
	"os"
)

func cp(dst, src string) error {
	s, err := os.Open(src)
	if err != nil {
		return err
	}
	// no need to check errors on read only file, we already got everything
	// we need from the filesystem, so nothing can go wrong now.
	defer s.Close()
	d, err := os.Create(dst)
	if err != nil {
		return err
	}
	if _, err := io.Copy(d, s); err != nil {
		d.Close()
		return err
	}
	return d.Close()
}

https://gist.github.com/elazarl/5507969#gistcomment-1458291 helped a lot!