The kernel is the essential center of a computer operating system, the core that provides basic services for all other parts of the operating system.
A kernel can be contrasted (compared) with a shell, shell is the outermost part of an operating system that interacts with user commands.
Kernel and shell are terms used more frequently in Unix operating systems than in IBM mainframe or Microsoft Windows systems.
The most popular Linux shells are
• Bash shell (Bourne again Shell): Developed for the GNU Project. The actual standard Linux shell Bash shell is the default shell for Red Hat Linux.
• Borne shell (sh): Original Unix shell written by Bill Joy at UC Berkeley.
• C shell (csh): Added many features such as command, history and job control.