Checking name and version of Linux OS (Operating System) as well as the kernel is the first step before installing any application on your system. Below are few commands and its output (on my environment) that we can use to check the Linux OS Version.
1. cat /proc/version
Linux version 2.6.18-348.12.1.el5 (email@example.com) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-54)) #1 SMP Wed Jul 10 05:31:48 EDT 2013
2. lsb_release -a
LSB Version: :core-4.0-ia32:core-4.0-noarch:graphics-4.0-ia32:graphics-4.0-noarch:printing-4.0-ia32:printing-4.0-noarch Distributor ID: CentOS Description: CentOS release 5.9 (Final) Release: 5.9 Codename: Final
3. cat /etc/issue
CentOS release 5.9 (Final) Kernel \r on an \m