How to check the Debian version

Command cat /etc/issue
The easiest way to check the Debian version is to look at the contents of the /etc/issue file with the cat command
root@debian-host:~# cat /etc/issue
Debian GNU/Linux 8 \n \l

Command cat /etc/debian_version
However, this file contains only information about the version of the distribution, in order to check the version and release number of Debian, you need to use another command:
root@debian-host:~# cat /etc/debian_version

Program hostnamectl
Starting with version 8, it became possible to use the hostnamectl command, which will display even more information about the installed system:
root@debian-host:~# hostnamectl
Static hostname: debian-host
Icon name: computer-desktop
Chassis: desktop
Machine ID: 21ea4bf892e94d309288ef6284bff8e0
Boot ID: b8d0d1cdb10348f2bbbb59d07da5713f
Operating System: Debian GNU/Linux 8 (jessie)
Kernel: Linux 3.16.0-4-amd64
Architecture: x86-64

Program lsb-release
In addition, you can install the lsb-release utility, which is not available by default, and use it to find out the version of the operating system. To install, just use the command apt-get install lsb-release, and then run this command:
root@debian-host:~# lsb_release -da
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 8.7 (jessie)
Release: 8.7
Codename: jessie