OS Kernel
The kernel takes care of recognizing hardware, networking perifericals and setting up the CPU registers, memory and file system.
The kernel is loaded into memory during the boot process (GRUB direct your CPU at the address at which the kernel is stored) and initializes it.
After the kernel is loaded, it start the first process of the machine namely the init service, responsible for starting and managing other processes in the system.
To check what the kernel is loading after booting, follow a simlar command to parse the first of the process:
ps -ef > process_list && head -n 2 process_list
Which will return a similar result if you are using systemd as your init system.
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 11:46 ? 00:00:02 /usr/lib/systemd/systemd rhgb --switched-root --system --deserialize=35
Find the current kernel version
The uname utility print information about the machine and operating system is running:
uname -r # -r stands for --kernel-release
List all installed kernels
Use the rpm command or dpkg command on the terminal:
rpm -q kernel
Remove old kernels
Do not remove the kernel the system is currently running. These examples may result into unstable system if not executed with care. Choose which kernel you want to uninstall from the list of those installed. Type the following command to remove the kernel package under RHEL/CentOS/Fedora Linux:
rpm -e kernel-2.6.12-1.el5
Please note that on newer system all obsolete kernels and headers should automatically be flagged as no more needed, and thus can be purged with the following single command:
sudo dnf autoremove
Change the default kernel
If you experience problems with the latest kernel you can rollback to a previous one (usually your system store more than
one in the disks at /boot). Telling grub to point into a different location in memory, i.g.
sudo grubby --info=ALL | grep -E "^kernel|^index"
From the output above select one index and the use it as follows:
sudo grubby --set-default-index=1