Let's suppose that the Unix operating system is a car.
When you drive, you issue a variety of “commands”:
you turn the steering wheel, press the accelerator, or press
the brake. But how does the car translate your commands
into the action you want? The car's drive mechanism,
which can be thought of as the car's user interface, is responsible.
Cars can be equipped with front-wheel drive, rear-wheel drive,
four-wheel drive, and sometimes combinations of these.
The shell is the user interface to Unix,
and by the same token, several shells are available in Unix.
Most systems provide more than
one for you to choose from. Each shell has different features,
but all of them affect how commands will be interpreted and
provide tools to create your Unix environment.