bash-programming-from-scratch/manuscript/BashShell/command-line-interface.md

Command Interpreter

Interpreters work in two modes: non-interactive and interactive. In the first mode, the interpreter executes a program. The program is loaded from the disk into RAM. Then it is executed line by line.

In interactive mode, the user types a command in the terminal emulator window. After pressing the Enter key, the command is executed. The interactive mode of the interpreter is called command shell or shell.

A command shell is built into the operating system often. It provides access to the settings and functions of the OS. In Linux, the shell allows the following:

• Run programs and system services.
• Control peripherals and internal devices.
• Access the kernel features.