How to remove old Kernel form RHEL or CentOS 6-8 in proper manner.

The kernel is the core of a computer's operating system with complete control over everything in the system. If anything wrong with kernel then it may very difficult for you to recover the operating system. Hence i will discuss here the proper way to remove the old kernel.

Remove old kernel from RHEL or CentOS 6-7

Check current version of your kernel

# uname -r
3.10.0-1127.18.2.el7.x86_64

Check how many kernel are installed into your OS

# rpm -qa|grep kernel-[0-9]
kernel-3.10.0-1127.18.2.el7.x86_64
kernel-3.10.0-1062.12.1.el7.x86_64

Remove old kernel from RHEl/CentOS 6-7

package-cleanup --oldkernels --count=1
Loaded plugins: fastestmirror
--> Running transaction check
---> Package kernel.x86_64 0:3.10.0-1062.12.1.el7 will be erased
---> Package kernel-devel.x86_64 0:3.10.0-1062.12.1.el7 will be erased
--> Finished Dependency Resolution

Dependencies Resolved

======================================================================
 Package                Arch    Version              Repository  Size
======================================================================
Removing:                                            
 kernel                 x86_64  3.10.0-1062.12.1.el7 @updates    64 M
 kernel-devel           x86_64  3.10.0-1062.12.1.el7 @updates    38 M
													 
Transaction Summary                                  
======================================================================
Remove  2 Packages

Installed size: 102 M
Is this ok [y/N]: y
Downloading packages:
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Erasing    : kernel-devel-3.10.0-1062.12.1.el7.x86_64      1/2
  Erasing    : kernel-3.10.0-1062.12.1.el7.x86_64            2/2
  Verifying  : kernel-3.10.0-1062.12.1.el7.x86_64            1/2
  Verifying  : kernel-devel-3.10.0-1062.12.1.el7.x86_64      2/2

Removed:
  kernel.x86_64 0:3.10.0-1062.12.1.el7   kernel-devel.x86_64 0:3.10.0-1062.12.1.el7

Complete!

Check the final active kernel version

# rpm -qa|grep kernel-[0-9]
kernel-3.10.0-1127.18.2.el7.x86_64

Remove old kernel from RHEL or CentOS 8

Check current version of kernel

# uname -r
4.18.0-193.6.3.el8_2.x86_64

Check how many kernel are installed into your OS

# rpm -qa|grep kernel-[0-9]
kernel-4.18.0-80.11.2.el8_0.x86_64
kernel-4.18.0-193.6.3.el8_2.x86_64
kernel-4.18.0-147.5.1.el8_1.x86_64

Remove old kernel from RHEl or CentOS 8

# dnf remove --oldinstallonly --setopt installonly_limit=2
Updating Subscription Management repositories.
Dependencies resolved.
================================================================================================
 Package                Architecture  Version                Repository                    Size
================================================================================================
Removing:
 kernel                 x86_64        4.18.0-80.11.2.el8_0   @RHEL8-Base-OS-RHN             0
 kernel                 x86_64        4.18.0-147.5.1.el8_1   @ITCL_RHEL-8_RHEL-8-Baseos     0
 kernel-core            x86_64        4.18.0-80.11.2.el8_0   @RHEL8-Base-OS-RHN            57 M
 kernel-core            x86_64        4.18.0-147.5.1.el8_1   @ITCL_RHEL-8_RHEL-8-Baseos    58 M
 kernel-modules         x86_64        4.18.0-80.11.2.el8_0   @RHEL8-Base-OS-RHN            19 M
 kernel-modules         x86_64        4.18.0-147.5.1.el8_1   @ITCL_RHEL-8_RHEL-8-Baseos    20 M

Transaction Summary
================================================================================================
Remove  6 Packages

Freed space: 154 M
Is this ok [y/N]: y
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        :                                                1/1
  Erasing          : kernel-4.18.0-147.5.1.el8_1.x86_64             1/6
  Running scriptlet: kernel-4.18.0-147.5.1.el8_1.x86_64             1/6
  Erasing          : kernel-4.18.0-80.11.2.el8_0.x86_64             2/6
  Running scriptlet: kernel-4.18.0-80.11.2.el8_0.x86_64             2/6
  Erasing          : kernel-modules-4.18.0-80.11.2.el8_0.x86_64     3/6
  Running scriptlet: kernel-modules-4.18.0-80.11.2.el8_0.x86_64     3/6
  Erasing          : kernel-modules-4.18.0-147.5.1.el8_1.x86_64     4/6
  Running scriptlet: kernel-modules-4.18.0-147.5.1.el8_1.x86_64     4/6
  Running scriptlet: kernel-core-4.18.0-147.5.1.el8_1.x86_64        5/6
  Erasing          : kernel-core-4.18.0-147.5.1.el8_1.x86_64        5/6
  Running scriptlet: kernel-core-4.18.0-147.5.1.el8_1.x86_64        5/6
  Running scriptlet: kernel-core-4.18.0-80.11.2.el8_0.x86_64        6/6
  Erasing          : kernel-core-4.18.0-80.11.2.el8_0.x86_64        6/6
  Running scriptlet: kernel-core-4.18.0-80.11.2.el8_0.x86_64        6/6
  Verifying        : kernel-4.18.0-80.11.2.el8_0.x86_64             1/6
  Verifying        : kernel-4.18.0-147.5.1.el8_1.x86_64             2/6
  Verifying        : kernel-core-4.18.0-80.11.2.el8_0.x86_64        3/6
  Verifying        : kernel-core-4.18.0-147.5.1.el8_1.x86_64        4/6
  Verifying        : kernel-modules-4.18.0-80.11.2.el8_0.x86_64     5/6
  Verifying        : kernel-modules-4.18.0-147.5.1.el8_1.x86_64     6/6
Installed products updated.
Last metadata expiration check: 0:00:03 ago on Wed 19 Aug 2020 11:08:24 AM +06.
Uploading Tracer Profile

Removed:
  kernel-4.18.0-80.11.2.el8_0.x86_64       kernel-4.18.0-147.5.1.el8_1.x86_64          kernel-core-4.18.0-80.11.2.el8_0.x86_64
  kernel-core-4.18.0-147.5.1.el8_1.x86_64  kernel-modules-4.18.0-80.11.2.el8_0.x86_64  kernel-modules-4.18.0-147.5.1.el8_1.x86_64

Complete!

Check the final active kernel version

# rpm -qa|grep kernel-[0-9]
kernel-4.18.0-193.6.3.el8_2.x86_64

That's it.

Hope this will help.