Wie kann ich Echtzeitausgaben von Shell-Befehlen in Go abrufen und verarbeiten?

cmdName := "ffmpeg -i t.webm  -acodec aac -vcodec libx264  cmd1.mp4"
cmdArgs := strings.Fields(cmdName)

cmd := exec.Command(cmdArgs[0], cmdArgs[1:len(cmdArgs)]...)
stderr, _ := cmd.StderrPipe()
cmd.Start()
go printProgress(stderr)
cmd.Wait()

r := bufio.NewReader(stderr)
for {
    line, _, err := r.ReadLine()
    if err != nil {
        fmt.Println("error in reading from stderr:", err)
        break
    }

    // Process the line (e.g., extract the progress percentage)
    percentage, err := extractPercentage(string(line))
    if err != nil {
        fmt.Println("error in extracting percentage:", err)
        continue
    }

    // Print the updated progress ratio
    fmt.Printf("%s%%", percentage)
}

parts := strings.Split(line, " ")
if len(parts) < 4 {
    return "", fmt.Errorf("invalid line format: %s", line)
}

size := parts[2]
sizeUnits := []string{"B", "kB", "MB", "GB", "TB", "PB"}
for i, unit := range sizeUnits {
    if strings.HasSuffix(size, unit) {
        factor := float64(1 << (10 * i))
        sizeBytes, err := strconv.ParseFloat(strings.TrimSuffix(size, unit), 64)
        if err != nil {
            return "", err
        }

        percentage := (sizeBytes / factor) / 100
        return strconv.FormatFloat(percentage, 'f', 2, 64), nil
    }
}

return "", fmt.Errorf("unknown size unit: %s", size)