verificar a versão do CentOS

he CentOS version consists of three release versions as illustrated below:

Command	Description
$ rpm -q centos-release	CentOS version valid for CentOS 6 and higher. Causes to reveal major, minor and asynchronous CentOS version.
$ lsb_release -d	Requires redhat-lsb package to be installed before execution.
$ cat /etc/centos-release	Linux cat command to output content of the /etc/centos-release to query CentOS version. Works with CentOS 6 and higher.