2024-09-18-pybelfast-workshop

While I know USB flash drives are unreliable, I still use them as installation media. Depending on the circumstances I use different software to write a disk image to a physical drive. Even if the software includes a check on the written data, I remove the drive from the system and later double check.

I use a separate two step process to double check that data read from the drive matches the disk image:

1. Count the number of bytes in the image
2. Read that number of bytes from the drive and generate a checksum

The two step process is necessary because the image file and physical drive are practically never the same size. It is straightforward to use stat, head and sha256sum from GNU coreutils to implement this process.

This example uses ~/Downloads/Fedora-Workstation-Live-42-1.1.x86_64.iso as left behind after creating a bootable Fedora 42 Workstation USB with Fedora Media Writer.

Command to display the size of the ISO in bytes:

stat --format=%s ~/Downloads/Fedora-Workstation-Live-42-1.1.x86_64.iso

Output:

2398523392

Command to read 2,398,523,392 bytes from the drive and generate checksums:

sudo head --bytes=2398523392 /dev/sda \
| sha256sum - ~/Downloads/Fedora-Workstation-Live-42-1.1.x86_64.iso

Output:

98958d80e8a80eabe61275337f969c8e2212adc3a223d9bbdab9411bb1c95cba  -
98958d80e8a80eabe61275337f969c8e2212adc3a223d9bbdab9411bb1c95cba  /home/maxwell-k/Downloads/Fedora-Workstation-Live-42-1.1.x86_64.iso

This matches the values in the corresponding checksum file:

# Fedora-Workstation-Live-42-1.1.x86_64.iso: 2398523392 bytes
SHA256 (Fedora-Workstation-Live-42-1.1.x86_64.iso) = 98958d80e8a80eabe61275337f969c8e2212adc3a223d9bbdab9411bb1c95cba