Command to know the installed Debian version?

There are a few ways you can know the installed Debian version of your Linux System,

Using lsb_release command

The lsb_release package is not pre-installed, you will need to get it using apt-get package manager.

# lsb_release --all
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 22.04.1 LTS
Release:	22.04
Codename:	jammy

Using the debian_version config file

root@ubuntu cat /etc/debian_version
bookworm/sid

Debian 1.1 (Buzz)
Debian 1.2 (Rex)
Debian 1.3 (Bo)
Debian 2.0 (Hamm)
Debian 2.1 (Slink)
Debian 2.2 (Potato)
Debian 3.0 (Woody)
Debian 3.1 (Sarge)
Debian 4.0 (Etch)
Debian 5.0 (Lenny)
Debian 6.0 (Squeeze)
Debian 7 (Wheezy)
Debian 8 (Jessie)
Debian 9 (Stretch)
Debian 10 (Buster)
Debian 11 (Bullseye)
Debian 12 (Bookworm)

Reference: https://en.wikipedia.org/wiki/Debian_version_history

sid -> Debian Unstable -> rolling development version