A UNIX shell defines variables to control the behavior of your UNIX session. Setting these variables will tell the system, for example, which directory to use as your home directory, or the file in which to store your mail. Start-up files can also contain UNIX commands or special shell commands. These will be executed every time you log in. In fact, many built-in commands can be used interactively like UNIX commands, and UNIX commands are frequently used in shell scripts.
Scripts are useful for executing a series of individual commands.