android的APP中用linux命令拷贝文件

public void clickUpdate(View view) {
        Process process = null;
        DataOutputStream os = null;
        String fileName = "SystemUI.apk";
        String srcFilename = Environment.getExternalStorageDirectory().getAbsolutePath() + "/" + fileName;
        String destFilename = "/system/priv-app/" + fileName;
        try {
            FileUtil.copyFileFromAssets(this, fileName, srcFilename);
            process = Runtime.getRuntime().exec("/system/xbin/su");
            os = new DataOutputStream(process.getOutputStream());
            os.writeBytes("mount -o remount /dev/block/by-name/system /system\n");
            os.writeBytes("rm -r " + destFilename + "\n");
            os.writeBytes("busybox cp -rf " + srcFilename + " " + destFilename + "\n");
            os.writeBytes("chmod 644 " + destFilename + "\n");
            os.writeBytes(" reboot \n");
            os.writeBytes(" exit \n");
            os.flush();
            process.waitFor();
            int ret = process.exitValue();
            Log.e(LOG_TAG, "exitValue = "+ret);
            if (ret == 0) {
                mTV_status.setText("更新成功！请重启系统!");
            } else {
                mTV_status.setText("更新失败！");
            }
        } catch (Exception e) {
            e.printStackTrace();
            mTV_status.setText("更新失败！");
        } finally {
            try {
                if (os != null) {
                    os.close();
                }
                if (process != null) {
                    process.destroy();
                }
            } catch (Exception e) {
            }
        }
    }