How to check Debian version

On Debian, type cat /etc/debian_version to display the Debian version.

$ cat /etc/debian_version
9.5

Alternatively, cat /etc/issue

$ cat /etc/issue
Debian GNU/Linux 9 \n \l

The last one, lsb-release -a, it will display the Debian detail.

$ lsb-release -a
-bash: lsb-release: command not found
$ sudo apt-get install lsb-release
$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 9.5 (stretch)
Release:        9.5
Codename:       stretch

References

1. Wikipedia – Debian
2. lsb_release – Linux man page