OS: kernel and Shell

An operating system can be divided into two parts: kernel and shell ).
A shell program encapsulates the kernel that communicates directly with the hardware: hardware <-> kernel <-> shell <-> Application

In some operating systems, the kernel and shell are completely separated (such as UNIX and Linux), so that users can use different shells on a single kernel; other kernels are closely related to the shell (such as Microsoft Windows), and the kernel and shell are only different in the operation level.