You can find the Debian version on which your Ubuntu version is based in the file: /etc/debian_version
Ubuntu |
Debian |
22.04 jammy |
bookworm/ sid - 12 |
21.10 impish |
bullseye/ sid - 11 |
21.04 hirsute |
bullseye/ sid |
20.10 groovy |
bullseye/ sid |
20.04 focal |
bullseye/ sid |
19.10 eoan |
buster / sid - 10 |
19.04 disco |
buster / sid |
18.10 cosmic |
buster / sid |
18.04 bionic |
buster / sid |
17.10 artful |
stretch / sid - 9 |
17.04 zesty |
stretch / sid |
16.10 yakkety |
stretch / sid |
16.04 xenial |
stretch / sid |
15.10 wily |
jessie / sid - 8 |
15.04 vivid |
jessie / sid |
14.10 utopic |
jessie / sid |
14.04 trusty |
jessie / sid |
13.10 saucy |
wheezy / sid - 7 |
13.04 raring |
wheezy / sid |
12.10 quantal |
wheezy / sid |
12.04 precise |
wheezy / sid |
11.10 oneiric |
wheezy / sid |
11.04 natty |
squeeze / sid - 6 |
10.10 maverick |
squeeze / sid |
10.04 lucid |
squeeze / sid |
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.