The document discusses the three main layers of the Unix operating system: programs/applications/tools which use a language humans can understand, the kernel which interacts with hardware using machine language, and the shell which acts as an intermediary interpreting commands for the kernel. It then focuses on describing the different types of shells, including their origins and key features. The main shells discussed are the Bourne shell, C shell, and Korn shell.

3. What Is the Shell? The Shell provides you with an interface to the UNIX system. It gathers input from you and executes programs based on that input. When a program has finished executing , it displays that program’s output . The shell is sometimes called a command interpreter . The real power of UNIX shell is that is much more than a command interpreter. It is also a powerful programming language , complete with conditional statements , loops and functions .

5. Different types of Shells Drawbacks: 1 . Weak input and output controls | 2. Lack of functions | 3. Confusing syntax due to a “lazy” command interpreter | 2) The C Shell: 1980s, designed to make interactive use of the shell easier for users. Programming style to the C style. Advantages over Bourne Shell: 1. Command History 2. Aliases 3. File Name Completion 4. Job Controls William Nelson Joy (Bill Joy)