Some
operating systems include the command interpreter
in the kernel. But some other operating systems treat
the command interpreter as a special program that
is running when a job is initiated or when a user
first logs on. Some systems support multiple command
interpreters known as shells, allow users to choose
a shell based upon personal preferences. Most shells
provide similar functionality with minor differences.
For example, on Unix and Linux systems, there are
several different shells a user may choose from including
the Bourne Shell, C Shell, Bourne-Again Shell, the
Korn Shell, etc.
The main function of the command interpreter is to
get and execute the next user-specified command. Many
of the commands given at this level are used to manipulate
files, such as create, delete, list, print, copy,
execute, and so on. There are two general ways in
which these commands can be implemented. In one approach,
the command interpreter itself contains the code to
execute the command. In the second approach, the command
interpreter executes the users commands with the use
of system programs.
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A
second strategy for interfacing with the operating
system is through a user-friendly graphical user interface
or GUI. A GUI provides a mouse-based window-and-menu
system as an interface to enter user commands. A GUI
provides a desktop metaphor where the mouse is moved
to position its pointer on images, or icons, on the
screen that represent programs, files, directories,
and system functions. With the use of mouse pointer
position and clicks, users can invoke programs, select
files or directories and use pull down menus to execute
commands. The GUI became more widespread with the
advent of Apple Macintosh computers in 1980s. Different
versions of Windows operating system have GUI with
some cosmetic changes in their appearance. |
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