To determine which Linux version / build / release / distribution you are running, look in the /proc/version entry as follows:
[root@racnode1 ~]# cat /proc/version
Linux version 2.6.18-194.el5 (mockbuild [at] builder10 [dot] centos.org”>mockbuild [at] builder10 [dot] centos.org) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-48)) #1 SMP Fri Apr 2 14:58:14 EDT 2010
On Red Hat based Linux distributions, look at the /etc/redhat-release file as follows:
[root@racnode1 ~]# cat /etc/redhat-release
CentOS release 5.5 (Final)
CentOS release 5.5 (Final)