run command and pip stdout to file

cmd-start.go

// Rnx2crx creates a copy of the file that is Hatanaka-compressed (compact RINEX).
// see http://terras.gsi.go.jp/ja/crx2rnx.html
func Rnx2crx(obsFil string) (string, error) {
	tool, err := exec.LookPath("RNX2CRX")
	if err != nil {
		return "", err
	}

	if !strings.HasSuffix(obsFil, ".rnx") {
		return "", fmt.Errorf("file %s with invalid extension", obsFil)
	}

	// compact RINEX file to create
	newPath := obsFil + ".xxx"
	newFil, err := os.Create(newPath)
	if err != nil {
		return "", fmt.Errorf("Could not create file %s: %v", newPath, err)
	}
	defer newFil.Close()

	writer := bufio.NewWriter(newFil)

	cmd := exec.Command(tool, obsFil, "-")
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		log.Fatal(err)
	}

	var stderr bytes.Buffer
	cmd.Stderr = &stderr

	err = cmd.Start() // as new OS process
	if err != nil {
		return "", fmt.Errorf("starting %s failed: %v", tool, err)
	}

	go io.Copy(writer, stdout)

	//see https://stackoverflow.com/questions/10385551/get-exit-code-go
	if err := cmd.Wait(); err != nil { // no timeout, see context
		return "", fmt.Errorf("cmd %s failed: %v: %s", tool, err, stderr.Bytes())
	}
	writer.Flush()

	log.Printf("file %s created", newPath)

	return newPath, nil
}