To find version of Ubuntu/Debian, you can run

cat /etc/*release

Example

root@Debian-Server:~# cat /etc/*release
PRETTY_NAME="Debian GNU/Linux 8 (jessie)"
NAME="Debian GNU/Linux"
VERSION_ID="8"
VERSION="8 (jessie)"
ID=debian
HOME_URL="http://www.debian.org/"
SUPPORT_URL="http://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
root@Debian-Server:~# 

This way show exact point release, for example this is Debian 8.8. You can use lsb_release command to find more details, lets first install lsb_release if not already installed.

apt install lsb-release

Now run

lsb_release -a

Example

root@Debian-Server:~# lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 8.8 (jessie)
Release:	8.8
Codename:	jessie
root@Debian-Server:~# 

On Ubuntu 16.04

boby@hon-pc-01:~ $ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.2 LTS
Release:	16.04
Codename:	xenial
boby@hon-pc-01:~ $ 

lsb_release -cs

root@debian:~# lsb_release -cs
stretch
root@debian:~#