Raspberry pi download file not verifiable from sha256 files supplied

Question

I was unable to verify the Ubuntu Server image for a pi 3 from the accompanying sha256sums.txt file on https://ubuntu.com/download/iot/raspberry-pi-2-3.

Output was

sha256sum -c SHA256SUMS.txt ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz | grep ok
sha256sum: ubuntu-18.04.2-preinstalled-server-armhf+raspi2.img.xz: No such file or directory
sha256sum: ubuntu-18.04.2-preinstalled-server-armhf+raspi3.img.xz: No such file or directory
sha256sum: ubuntu-18.04.2-server-amd64.iso: No such file or directory
sha256sum: ubuntu-18.04.2-server-arm64.iso: No such file or directory
sha256sum: ubuntu-18.04.2-server-ppc64el.iso: No such file or directory
sha256sum: ubuntu-18.04.2-server-s390x.iso: No such file or directory
sha256sum: WARNING: 6 listed files could not be read
sha256sum: ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz: no properly formatted SHA256 checksum lines found

Answer 1

The input files to sha256sum -c are signature files so when you do

sha256sum -c SHA256SUMS.txt ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz

... it expects ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz to be a file containing signatures and this is why it says "ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz: no properly formatted SHA256 checksum lines found"

Unless you specify --ignore-missing, sha256sum checks all the files in the signature file, so if you didn't load all the files it will complain for the missing ones.

Last, files with the correct signature are displayed with an uppercase OK and you are grep-ping for a lowercase one (grep ok) so you miss the one important message.

So I think you just want:

sha256sum -c --ignore-missing SHA256SUMS.txt