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Q. I know how to find out current running kernel version under CentOS Enterprise Linux 4.6 x86_64, but how do I find out latest available kernel version? How do I find out if it's the latest one? How to find the latest release version for package called xyz or kernel package?

A. You can simply use yum command to find out more about latest version available under your CentOS system. To list all packages with updates available in the yum repositories.
yum list updates syntax

Use the following syntax and you are allowed to use wild cards (the single quotes will keep your shell from expanding the globs):
yum list updates {package-name}
yum list updates 'package-name*'
yum list updates '*package-name*'

For example, find out if latest updated version of kernel or php package, enter:
# yum list updates 'php*'
OR
# yum list updates 'kernel*'
Sample output:

Loading "installonlyn" plugin
Loading "security" plugin
Setting up repositories
Reading repository metadata in from local files
Skipping security plugin, no data
Updated Packages
kernel.i686 2.6.18-53.1.21.el5 updates

You can list currently installed package version by typing any one of the following command:
# rpm -q kernel
OR
# yum list installed 'kernel*'
Or you can list multiple packages on separated by whitespace:
# yum list installed 'kernel*' 'php*'
Sample output:

Loading "installonlyn" plugin
Loading "security" plugin
Installed Packages
kernel.i686 2.6.18-53.1.4.el5 installed
kernel.i686 2.6.18-53.1.19.el5 installed
kernel.i686 2.6.18-53.1.21.el5 installed
kernel.i686 2.6.18-53.1.6.el5 installed
kernel.i686 2.6.18-8.1.15.el5 installed
kernel-devel.i686 2.6.18-8.1.15.el5 installed
kernel-devel.i686 2.6.18-53.1.4.el5 installed
kernel-devel.i686 2.6.18-53.1.6.el5 installed
kernel-devel.i686 2.6.18-53.1.19.el5 installed
kernel-devel.i686 2.6.18-53.1.21.el5 installed
kernel-headers.i386 2.6.18-53.1.21.el5 installed

Finally, you can apply updated version using yum command itself:
# yum update kernel.i686
Sample output:

Loading "installonlyn" plugin
Loading "security" plugin
Setting up Update Process
Setting up repositories
Reading repository metadata in from local files
Skipping security plugin, no data
Resolving Dependencies
Skipping security plugin, no data
--> Populating transaction set with selected packages. Please wait.
---> Package kernel.i686 0:2.6.18-53.1.21.el5 set to be installed
--> Running transaction check

Dependencies Resolved

=============================================================================
Package Arch Version Repository Size
=============================================================================
Installing:
kernel i686 2.6.18-53.1.21.el5 updates 13 M

Transaction Summary
=============================================================================
Install 1 Package(s)
Update 0 Package(s)
Remove 0 Package(s)

Total download size: 13 M
Is this ok [y/N]: y
Downloading Packages:
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing: kernel ######################### [1/1]

Installed: kernel.i686 0:2.6.18-53.1.21.el5
Complete!

Don't forget to reboot your system so that you can boot into latest kernel version:
# reboot


Reference: http://www.cyberciti.biz/faq/centos-rhel-linux-find-updated-software-version/