How to purge the old, not-used-anymore versions of the Linux kernel on Fedora, CentOS and RedHat (RHEL), in one line.

package-cleanup --oldkernels --count=2

Where 2 is the number of version you want to keep, including the lastest version. In other word a value of 2 means 1 active kernel and 1 former version.

It requires the yum-utils package to be installed.

To make this behavior permanent, add the following line under the [main] section in /etc/yum.conf:

installonly_limit=2

Now on, old kernels are purged whenever new ones are installed by YUM.