Identifying the kernel version and changing the default boot kernel in RedHat and Centos Linux.

Use the command uname to print system information in Linux.

# uname -r
2.6.18-194.32.1.el5

The –r option will print the kernel release.

# uname -s
Linux

-s prints the kernel name.

# uname -n
mycomputer.wiivil.com

Print the node name or hostname.

# uname -p
i686

Print the processor type, the i686 signifies a 32 bit processor. If it was a 64 processor it would display x86_64 instead.

# uname -a
Linux myserver.wiivil.com 2.6.18-194.32.1.el5 #1 SMP Wed Jan 5 17:53:09 EST 2011 i686 i686 i386 GNU/Linux

Print all the information except omit –o and –I if unknown.

# cat /etc/redhat-release
CentOS release 5.5 (Final)

This will give you the version of RedHat Linux you are running.

# cat /proc/version
Linux version 2.6.18-194.32.1.el5 (mockbuild@builder10.centos.org) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-48)) #1 SMP Wed Jan 5 17:53:09 EST 2011

This will give you who and when compiled the kernel and what gcc compiler was used to build it.

# rpm -q kernel
kernel-2.6.18-194.11.3.el5
kernel-2.6.18-194.26.1.el5
kernel-2.6.18-194.32.1.el5

This will list all the kernels installed on your system.

If the new kernel causes issues then you have the option to revert to the older version. You may have to boot the system with the boot media and re-configure the boot loader. The AMD and Intel architecture uses GRUB boot loader. You will need to make changes to /boot/grub/grub.conf to activate a different default kernel.

Please not that when the kernel is installed using rpm, the kernel package creates an entry in the boot loader configuration file for the new kernel. But the rpm does not configure the new kernel to boot as the default kernel, this must be done manually.

Below is an example of /boot/grub/grub.conf. The directive default=0 means that default boot kernel is 0, which is the first stanza that starts with the title entry.

So in this case title CentOS (2.6.18-194.32.1.el5) is stanza 0. When the system boots this kernel will be used. The title contains the kernel version number 2.6.18-194.32.1.el5, which must match with the version number in the kernel /vmlinuz-2.6.18-194.32.1.el5 (this is the line below root.

If a separate /boot/ partition was created, the paths to the kernel and the initramfs image are relative to /boot/. This is the case in Example 23.2, “/boot/grub/grub.conf”, above. Therefore the initrd /initramfs-2.6.32-22.el6.x86_64.img line in the first kernel stanza means that the initramfs image is actually located at /boot/initramfs-2.6.32-22.el6.x86_64.img when the root file system is mounted, and likewise for the kernel path (for example: kernel /vmlinuz-2.6.32-22.el6.x86_64) in each stanza of grub.conf

To clarify further, stanza 0 starts with title CentOS (2.6.18-194.32.1.el5)
Stanza 1 starts with title CentOS (2.6.18-194.26.1.el5)
Stanza 2 is title CentOS (2.6.18-194.11.3.el5)

To change the default boot kernel to stanza 2 simply change the line default=0 to default=2.

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/VolGroup00/LogVol00
# initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-194.32.1.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.32.1.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.18-194.32.1.el5.img
title CentOS (2.6.18-194.26.1.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.26.1.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.18-194.26.1.el5.img
title CentOS (2.6.18-194.11.3.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-194.11.3.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.18-194.11.3.el5.img