Three Major Functions of the Login Shell

The interface between you and the UNIX "kernel"

The "kernel" is the portion of UNIX that is always in the computer's memory. It interfaces directly with the machine's hardware:

A Command Interpreter

The shell interprets the commands that you type at the keyboard. It essentially "listens" to the commands that you enter from the keyboard. Once you type those commands and enter them, the shell interprets the commands and takes action based on what you've typed.

The shell looks for flags and arguments to UNIX commands, looks for shell metacharacters, and other special symbols.

A Programming Language

UNIX shells allow you to create simple (or very complex) "shell scripts" to make your life easier when you are performing highly repetitive tasks, or when you want to do some programming. These scripts contain a sequence of UNIX commands, very similar to the way you would use the commands if you typed them from the keyboard interactively.

UNIX shell scripts are similar to DOS "batch" files (.bat extension), but the operating system tools within UNIX are much greater in number and more powerful than those available in DOS.