I've installed Debian bullseye on an older machine to play around with. I just updated all repos to testing with the guide from DebianTesting.
- Edit your apt sources, changing 'stable' (or bullseye, the current codename for stable) to 'testing' (or bookworm, the current code name for the next stable release).
- Remove, disable or comment out your stable security updates apt sources (anything with security.debian.org in it).
- Remove, disable or comment out any other stable-specific apt sources, like *-backports or *-updates.
- Verify that your installation is not fixed to a specific release in /etc/apt/apt.conf.d/00default-release
It states I should remove all stable-specific sources, but as I generally don't follow orders, I replaced bullseye-updates and bullseye-backports with the appropriate testing tag. And see:
apt update doesn't even complain. I double checked and there is indeed a testing-backports folder in remote repository.
But after running
apt upgrade -s -t testing-backports to see what packages have newer version in this repo, it failes with the above mentioned error message:
E: The value 'testing-backports' is invalid for APT::Default-Release as such a release is not available in the sources
I verified that the command works on another Debian system with
deb https://deb.debian.org/debian/ testing-backports main
So is it there indeed no repo for
testing-backports, because the release file exists.