The Unix shell, also called the command line, is the traditional user interface for the Unix operating system. Users direct the operation of the computer by entering command input as text for the shell to execute. Within the Microsoft Windows suite of operating systems the equivalent program is command.com.
The most generic sense of the term shell means any program that users use to type commands; it is called a "shell" because it hides the details of the underlying operating system behind the shell's interface. Similarly, graphical user interfaces for Unix, such as GNOME and KDE, are sometimes called visual shells or graphical shells. By itself, the term shell is usually associated with the command line. In Unix, any program can be the user's shell; users who want to use a different syntax for typing commands can specify a different program as their shell.
The term shell also refers to a particular program, namely the Bourne shell. The Bourne shell was the shell used in early versions of Unix and became a de facto standard; every Unix-like system has the equivalent of the Bourne shell. The Bourne shell program is called sh and is located in the UNIX file hierarchy at /bin/sh. On some systems, such as BSD, /bin/sh is a Bourne shell or equivalent, but on other systems such as Linux, /bin/sh is likely to be a link to a compatible, but more feature-rich shell, such as bash.
The Unix shell is unusual since it is in both an interactive command language and the language used to script the system; it is a scripting programming language.
On systems using a windowing system, naive users may never use the shell directly. Many regular users of a UNIX system still find a modern shell much more convenient for many tasks than any GUI application.
See also: Shebang