There are three main uses for the shell:
Customization of your UNIX session
When the shell is used interactively, the system waits for you to type a command at the UNIX prompt. Your commands can include special symbols that let you abbreviate filenames or redirect input and output.
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. Some variables are preset by the system; you can define others in start-up files that are read when you log in. Start-up files can also contain UNIX commands or special shell commands. These will be executed every time you log in.
UNIX shells provide a set of special (or built-in) commands that can be used to create programs called shell scripts. 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. This is similar to BATCH files in MS-DOS. Scripts can also execute commands repeatedly (in a loop) or conditionally (if-else), as in many high-level programming languages.