There are several ways on how to check what version of CentOS is running on your system. Below you can find multiple examples on how to determine CentOS version. However, depending on your system not all examples may be applicable. For a Redhat specific guide visit How to check Redhat version guide.

First, example uses hostnamectl command:
# hostnamectl 
   Static hostname: localhost.localdomain
         Icon name: computer-laptop
           Chassis: laptop
        Machine ID: 6892999b8d8c43fab29fca961dbff24b
           Boot ID: b0a95bf56f61417ca8073b3847a73ab1
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0-229.7.2.el7.x86_64
      Architecture: x86_64
The next example uses rpm command to do query for centos-release package to reveal CentOS version:
# rpm --query centos-release
centos-release-7-1.1503.el7.centos.2.8.x86_64
Yet, another alternative is to use lsb_release command. However, this command is part of redhat-lsb package and is not installed by default so the actual lsb_release command may be unavailable on your system. If this is the case it is recommended to use some other alternative approach on how to check CentOS version on your system.
# lsb_release -d
Description:    CentOS Linux release 7.1.1503 (Core)
If none of the above does not reveal CentOS version we can try to query release numbers directly. Attempt to read the content of the following files to reveal CentOS version number:
  • /etc/centos-release
  • /etc/os-release
  • /etc/redhat-release
  • /etc/system-release
  • Use cat command to read their content in order to show CentOS version:
    # cat /etc/centos-release
    CentOS Linux release 7.1.1503 (Core)
    # cat /etc/os-release
    NAME="CentOS Linux"
    VERSION="7 (Core)"
    ID="centos"
    ID_LIKE="rhel fedora"
    VERSION_ID="7"
    PRETTY_NAME="CentOS Linux 7 (Core)"
    ANSI_COLOR="0;31"
    CPE_NAME="cpe:/o:centos:centos:7"
    HOME_URL="https://www.centos.org/"
    BUG_REPORT_URL="https://bugs.centos.org/"
    CENTOS_MANTISBT_PROJECT="CentOS-7"
    CENTOS_MANTISBT_PROJECT_VERSION="7"
    REDHAT_SUPPORT_PRODUCT="centos"
    REDHAT_SUPPORT_PRODUCT_VERSION="7"
    # cat /etc/redhat-release
    CentOS Linux release 7.1.1503 (Core)
    # cat /etc/system-release
    CentOS Linux release 7.1.1503 (Core)