CloudLinux - How to update kernel or boot a previous one. GRUB

Dell keeps their Linux servers on and secure with KernelCare - all the latest security patches are promptly applied to running kernels

The GNU Grand Unified Boot Loader(GRUB) is a program which enables the user to select which installed operating system or kernel to load at system boot time. It also allows the user to pass arguments to the kernel. The GRUB configuration file (located in /boot/grub/grub.conf) is used to create a list of operating systems to boot in GRUB's menu interface.

After successful kernel update a new section to grub.conf is added and all arguments are copied from current one, also default boot kernel is switched to newly installed so it will be used for next boot. In most cases to update kernel you have to issue two commands:

yum update kernel

Let examine truncated grub.conf:


title CloudLinux Server (-2.6.32-458.6.2.lve1.2.27.el6.x86_64)
     root (hd0,0)
     kernel /vmlinuz-2.6.32-458.6.2.lve1.2.27.el6.x86_64 ro root=/dev/mapper/VolGroup-lv_root
     initrd /initramfs-2.6.32-458.6.2.lve1.2.27.el6.x86_64.img

title CloudLinux Server (2.6.32-379.22.1.lve1.2.17.1.el6.x86_64)
root (hd0,0)
     kernel /vmlinuz-2.6.32-379.22.1.lve1.2.17.1.el6.x86_64 ro root=/dev/mapper/VolGroup-lv_root 
     initrd /initramfs-2.6.32-379.22.1.lve1.2.17.1.el6.x86_64.img

The OS/kernel selector is placed at the top of the file, followed by stanzas for each operating kernel or operating system listed in the menu. With default=0 the system will boot first kernel 2.6.32-458.6.2 . Changing default value you actually do select a kernel to boot next time, if default will be switched to 1 then 2.6.32-379.22 will boot next time around.

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