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What Is the Shell

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This chapter is from the book

This chapter is from the book

In This Chapter

  • The Kernel and the Utilities

  • The Login Shell

  • Typing Commands to the Shell

  • The Shell's Responsibilities

In this chapter you'll learn what the shell is and what it does.

The Kernel and the Utilities

The Unix system is itself logically divided into two pieces: the kernel and the utilities (see Figure 3.1).

Figure 3.1Figure 3.1 The Unix system.

The kernel is the heart of the Unix system and resides in the computer's memory from the time the computer is turned on and booted until the time it is shut down.

The utilities, on the other hand, reside on the computer's disk and are only brought into memory as requested. Virtually every command you know under the Unix system is classified as a utility; therefore, the program resides on the disk and is brought into memory only when you request that the command be executed. So, for example, when you execute the date command, the Unix system loads the program called date from the computer's disk into memory and initiates its execution.

The shell, too, is a utility program. It is loaded into memory for execution whenever you log in to the system. In fact, it's worth learning the precise sequence of events that occurs when the first shell on a terminal or window starts up.

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