Your 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 is stored in a file.
When you log in to the system, the system locates the name of a shell program to execute. Once executed, the shell displays a command prompt. This prompt is usually a $ (dollar sign). When you type a command at the prompt and press the Enter key, the shell evaluates the command and attempts to carry it out. Depending on your command instructions, the shell writes the command output to the screen or redirects the output. It then returns the command prompt and waits for you to type another command.
A command line is the line on which you type. It contains the shell prompt. The basic format for each line is:
$ Command Argument(s)
The shell considers the first word of a command line (up to the first blank space) as the command, and all words after that as arguments.