Think back to your first computer. Was it a Macintosh? A PC running some version of Windows? If your computing life started in the world of graphical interfaces, you may find Unix to be somewhat bewildering. However, if you can remember writing tiny BASIC programs on a Commodore 64 or an Amiga, you might think that Unix is a welcome throwback to the days when the monitor showed nothing but a command prompt.
Long before there were any such things as computer desktops, windows, icons, or mice, there were text-based terminals and command lines. In fact, Unix was the first operating system that was machine-independent—before Unix, each machine needed its own personalized operating system that handled its particular mechanical quirks. Imagine how hard it must have been to develop a common operating system in the days before mass-produced computers! Since its earliest days, Unix has primarily used a command-line interface: a simple prompt, followed by a cursor, using only text. In this kind of environment, there is only one mode of interacting with the machine, and that's via commands.
Of course, Unix machines now offer graphical interfaces. Under some Unix variations, the graphical interface is the default user environment. However, you can always work in a text-only mode on a Unix machine. In fact, should there be a malfunction or a crash, the text-only environment may be the only one available to you.
Commands are executable programs. Sometimes they are ...