In computers, a command is a specific order from a user to the computer's operating system or to an application to perform a service, such as "Show me all my files" or "Run this program for me." Operating systems such as DOS that do not have a graphical user interface (GUI) offer a simple command line interface in which you type the command on a designated line in a display panel. In a system such as Windows, a graphical user interface lets you implicitly enter commands by selecting objects or word selections and clicking your mouse button on them. (Windows does offer a command line interface for certain facilities if you select the "MS-DOS Prompt" under "Programs.")
The part of the operating system that handles commands is usually called a command interpreter or a shell. Some application programs, especially in DOS, provide a command mode for interfacing with the application from the display. In some operating systems, commands can be included in a list and presented to the system as a package to be executed in sequence, either immediately or at some specified time. Such a list is usually known as a script. One in which a time of execution is specified for each command is called a CRON script. In some systems, commands can be initiated from application programs as well as from a user interface.