The terms command and program are used almost interchangeably, probably because the program name is typed first on a command line (at a shell prompt). Shells have some built-in commands that don't start a separate program running; one of these is cd, which changes the shell's working directory.
Popular media often call these people hackers. But, to most computer programmers, a hacker is someone who enjoys computing and programming, and may be an expert at some area of it. (For instance, a Perl hacker is someone who's good at programming in the Perl language.)
Multiscreen X displays aren't common, though, and sentences like "the result is displayed on the display" are clumsy. To avoid confusion, we use the term screen for the visual output of your computer--whether it's an alphanumeric terminal or a graphical workstation. See also screen.
Some Unix desktop environments support "drag and drop," which means dragging one object and dropping it over another object. For example, to print a file, you could drag the file's icon and drop it onto a printer icon.
In the X Window System, the pointer is actually called a cursor. But we use the term "pointer" in this book to distinguish the cursor under control of the mouse from other cursors that you'll sometimes see (such as the "I-beam" cursor in an xterm window).
Unix has always been multitasking. MS-DOS (an early Microsoft OS) was not.
A Unix filesystem is like an upside-down tree with a branching structure of directories inside directories. The first directory, where the filesystem starts, is called the root directory. Figure 3-1 is a filesystem diagram showing the root directory and a small part of a filesystem.
When you start using a terminal (by logging in) or a terminal window (by starting a program such as xterm), a shell program begins to run and prints a shell prompt. When you terminate that shell (by typing exit or CTRL-D at a prompt), you're logged out from that terminal; a terminal window will close.
An alphanumeric terminal can only display text, can't run a window system, and usually doesn't have a mouse or other pointing device.[23] An alphanumeric terminal displays alphanumeric characters--and possibly simple graphics (lines, boxes and maybe a few special symbols). An alphanumeric terminal can't handle a window system and typically doesn't have a mouse or other pointing device; if the cursor can be moved around the screen, it's probably done with arrows or other keys on the keyboard. See also alphanumeric.
[23] Before the widespread use of glass terminals (when data transmission rates were slow) it was common to use a teletype as both the input and output hardware. This is why Unix terminals are often called ttys.
A graphical terminal can usually run a window system--with arbitrary-sized windows, images (photographs and other graphics), sound, etc. Graphical terminals are typically bitmapped, which means that each pixel (dot of color) can be individually controlled by the computer--as opposed to an alphanumeric terminal, where the terminal itself chooses which dots to turn on and off to make letters, numbers, and other characters that the computer has told it to create.
If a window manager program is running, a window usually will have a well-defined border, a title, and other characteristics. The window manager lets you move and resize a window as well.
If your working directory is /home/joe/food and you type the command less recipes/fish, Unix opens the file /home/joe/food/recipes/fish. (Your working directory is still /home/joe/food.)
If you type the command "ls .." from any working directory, you get a listing of the files in your parent directory. That command uses the relative pathname to the parent directory (..). So if your working directory is /home/joe/food, that command would list the parent directory /home/joe. Or, if your working directory is /home/joe, that same command would list the parent directory /home.
Each process running on a Unix system has its own working directory, which the program can change at any time. For instance, you can give the shell the command cd to change its working directory.
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