15.5 ExercisesThe primary interface to the operating system is called a shell. The shell is the outermost layer of the operating system. Shells incorporate a programming language to control processes and files, as well as to start and control other programs. The shell manages the interaction between you and the operating system by prompting you for input, interpreting that input for the operating system, and then handling any resulting output from the operating system.
Shells provide a way for you to communicate with the operating system. This communication is carried out either interactively (input from the keyboard is acted upon immediately) or as a shell script. A shell script is a sequence of shell and operating system commands that are stored in a file and may be reused.
In essence, a shell script is simply a combination of command line commands put into a single file. Variables and environments can also be set up in a shell script.
16.1 The Korn Shell or
POSIX Shell