I have some software packages customised for Ubuntu 10.04, 10.10 and 12.10, but now I have to install them on Debian Squeeze. I have been unable to Google for a mapping between specific Ubuntu and Debian versions. Can someone here help?
Specifically I want to know which Ubuntu version corresponds to Debian Squeeze.
You can find the Debian version on which your Ubuntu version is based in the file:
/etc/debian_version
Sid is the development distribution of Debian. That's how, for example, Ubuntu 20.04, released in April 2020, can be based on Debian 11 "Bullseye", which was released in August 2021.
You can find out the contents of the file without installing an entire system by view the sources for the package
basefiles
on Launchpad.Unfortunately there is no direct mapping /etc/debian_version as mentioned in Jacob's answer gives a rough idea but it only really reflects one package and said package is a package that is locally modified in Ubuntu so won't be automatically pulled in from Debian.
In particular Ubuntu is often ahead of Debian on core packages like libc6. Trying to install a package built on Ubuntu on a contemporary version of Debian is likely to end up with version errors on libc6.
According to https://superuser.com/questions/407745/wrong-libc6-version-in-debian-squeeze-can-i-go-back Debian squeeze had libc6 2.11 . According to http://web.archive.org/web/20110705212424/http://packages.ubuntu.com/search?keywords=libc6 ubuntu 10.04 lucid also had libc6 2.11 . So the packages for 10.4 lucid are IMO the most likely to work on Squeeze.
Depending on what other libraries your program needs you may well find that a specific build for Debian squeeze is needed.