The Unix operating system has many versions of the original and many clones (e.g. Linux, Android, MacOS, IOS…). All are recognisable as Unix based systems. The Unix operating system is made up of three parts: the kernel; the shell; and the programs.
Kernel
Kernel is the central component of Unix OS, it manages operations of memory and allocates CPU time to programs. Kernel acts as a bridge between applications and data processing performed at hardware level using inter-process communication and system calls (e.g. print characters on the screen, write data to disk, etc.).
Shell
The user is interacting with kernel via Shell which is a Command-Line Interpreter (CLI) that provides a command line user interface. The shell is both an interactive command language and a scripting language. A shell script is a set of shell commands that in a form of a executable file commonly with an extension of .sh.
One of the commonly used shells in Unix based systems, including Linux is the Bash. It is a specific type of Unix shell and comes with improved features.
Program
A program is stored on the persistent storage (eg disk) in a file. When a program is started, the operating system reads the file and generates a memory image that contains code and data and the associated metadata structures are created (e.g. standard input/out/error file descriptors etc). (Find more on Processes and Memory Management)
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