UNIX® Shells by Example, Third Edition

1.1. Definition and Function

The shell is a special program used as an interface between the user and the heart of the UNIX operating system, a program called the kernel, as shown in Figure 1.1. The kernel is loaded into memory at boot-up time and manages the system until shutdown. It creates and controls processes, and manages memory, file systems, communications, and so forth. All other programs, including shell programs, reside out on the disk. The kernel loads those programs into memory, executes them, and cleans up the system when they terminate. The shell is a utility program that starts up when you log on. It allows users to interact with the kernel by interpreting commands that are typed either at the command line or in a script file. ...