用户对问题“如何在golang中覆盖文件内容”的回答

a.txta.txt

    // open a file
    f, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()

    // another file
    af, err := os.OpenFile("a.txt", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
    if err != nil {
        log.Fatal(err)
    }
    defer af.Close()

    b := []byte{}

    scanner := bufio.NewScanner(f)
    for scanner.Scan() {
        b = append(b, scanner.Bytes()...)
        // how to output len(b) into a.txt?
    }