当我运行时：
sha256sum -c SHA256SUMS | grep OK
输出：
sha256sum: ubuntu-18.04.2-desktop-amd64.iso: permission denied
sha256sum: ubuntu-18.04.2-live-server-amd64.iso: no such file or directory
sha256sum: 2 listed files could not be read
当我运行时：
sha256sum -c SHA256SUMS 2>&1 | grep OK
没有输出