Sams Teach Yourself Shell Programming in 24 Hours, Second Edition

What Is a Command?

A command is a file containing a set of instructions that UNIX can run or execute. In operating systems such as Mac OS or Windows, commands are executed by clicking their icons. In UNIX, a command is executed by typing in its name and pressing Enter or Return. For example, in order to execute the date command, use the following:

$ date [ENTER]
Wed Dec  9 08:49:13 PST 1998
$

The purpose of the date command is to display the current day, date, time, and year. Notice that after the command finishes executing, the character $ is displayed. This character is the prompt. When a prompt is present, the name of a command can be given for execution. The shell reads the command name and tries to execute it. While the command executes, ...