Of course, since my computer only has one monitor screen, I can only see one command line, and so you might think that I could only interact with one program at a time. But if I hold down the Alt key and then hit the F2 function button at the top of my keyboard, I am presented with a fresh, blank, black screen with a login prompt at the top of it. I can log in here and start some other program, then hit Alt-F1 and go back to the first screen, which is still doing whatever it was when I left it. Or I can do Alt-F3 and log in to a third screen, or a fourth, or a fifth. On one of these screens I might be logged in as myself, on another as root (the system administrator), on yet another I might be logged on to some other computer over the Internet.
Each of these screens is called, in Unix-speak, a tty, which is an abbreviation for teletype. So when I use my Linux system in this way I am going right back to that small room at Ames High School where I first wrote code twenty-five years ago, except that a tty is quieter and faster than a teletype, and capable of running vastly superior software, such as emacs or the GNU development tools.
It is easy (easy by Unix, not Apple/Microsoft standards) to configure a Linux machine so that it will go directly into a GUI when you boot it up. This way, you never see a tty screen at all. I still have mine boot into the white-on-black teletype screen however, as a computational memento mori. It used to be fashionable for a writer to keep a human skull on his desk as a reminder that he was mortal, that all about him was vanity. The tty screen reminds me that the same thing is true of slick user interfaces.
The X Windows System, which is the GUI of Unix, has to be capable of running on hundreds of different video cards with different chipsets, amounts of onboard memory, and motherboard buses. Likewise, there are hundreds of different types of monitors on the new and used market, each with different specifications, and so there are probably upwards of a million different possible combinations of card and monitor. The only thing they all have in common is that they all work in VGA mode, which is the old command-line screen that you see for a few seconds when you launch Windows. So Linux always starts in VGA, with a teletype interface, because at first it has no idea what sort of hardware is attached to your computer. In order to get beyond the glass teletype and into the GUI, you have to tell Linux exactly what kinds of hardware you have. If you get it wrong, you'll get a blank screen at best, and at worst you might actually destroy your monitor by feeding it signals it can't handle.
When I started using Linux this had to be done by hand. I once spent the better part of a month trying to get an oddball monitor to work for me, and filled the better part of a composition book with increasingly desperate scrawled notes. Nowadays, most Linux distributions ship with a program that automatically scans the video card and self-configures the system, so getting X Windows up and running is nearly as easy as installing an Apple/Microsoft GUI. The crucial information goes into a file (an ASCII text file, naturally) called XF86Config, which is worth looking at even if your distribution creates it for you automatically. For most people it looks like meaningless cryptic incantations, which is the whole point of looking at it. An Apple/Microsoft system needs to have the same information in order to launch its GUI, but it's apt to be deeply hidden somewhere, and it's probably in a file that can't even be opened and read by a text editor. All of the important files that make Linux systems work are right out in the open. They are always ASCII text files, so you don't need special tools to read them. You can look at them any time you want, which is good, and you can mess them up and render your system totally dysfunctional, which is not so good.
At any rate, assuming that my XF86Config file is just so, I enter the command "startx" to launch the X Windows System. The screen blanks out for a minute, the monitor makes strange twitching noises, then reconstitutes itself as a blank gray desktop with a mouse cursor in the middle. At the same time it is launching a window manager. X Windows is pretty low-level software; it provides the infrastructure for a GUI, and it's a heavy industrial infrastructure. But it doesn't do windows. That's handled by another category of application that sits atop X Windows, called a window manager. Several of these are available, all free of course. The classic is twm (Tom's Window Manager) but there is a smaller and supposedly more efficient variant of it called fvwm, which is what I use. I have my eye on a completely different window manager called Enlightenment, which may be the hippest single technology product I have ever seen, in that (a) it is for Linux, (b) it is freeware, (c) it is being developed by a very small number of obsessed hackers, and (d) it looks amazingly cool; it is the sort of window manager that might show up in the backdrop of an Aliens movie.
Anyway, the window manager acts as an intermediary between X Windows and whatever software you want to use. It draws the window frames, menus, and so on, while the applications themselves draw the actual content in the windows. The applications might be of any sort: text editors, Web browsers, graphics packages, or utility programs, such as a clock or calculator. In other words, from this point on, you feel as if you have been shunted into a parallel universe that is quite similar to the familiar Apple or Microsoft one, but slightly and pervasively different. The premier graphics program under Apple/Microsoft is Adobe Photoshop, but under Linux it's something called The GIMP. Instead of the Microsoft Office Suite, you can buy something called ApplixWare. Many commercial software packages, such as Mathematica, Netscape Communicator, and Adobe Acrobat, are available in Linux versions, and depending on how you set up your window manager you can make them look and behave just as they would under MacOS or Windows.
But there is one type of window you'll see on Linux GUI that is rare or nonexistent under other OSes. These windows are called "xterm" and contain nothing but lines of text--this time, black text on a white background, though you can make them be different colors if you choose. Each xterm window is a separate command line interface--a tty in a window. So even when you are in full GUI mode, you can still talk to your Linux machine through a command-line interface.
There are many good pieces of Unix software that do not have GUIs at all. This might be because they were developed before X Windows was available, or because the people who wrote them did not want to suffer through all the hassle of creating a GUI, or because they simply do not need one. In any event, those programs can be invoked by typing their names into the command line of an xterm window. The whoami command, mentioned earlier, is a good example. There is another called wc ("word count") which simply returns the number of lines, words, and characters in a text file.
The ability to run these little utility programs on the command line is a great virtue of Unix, and one that is unlikely to be duplicated by pure GUI operating systems. The wc command, for example, is the sort of thing that is easy to write with a command line interface. It probably does not consist of more than a few lines of code, and a clever programmer could probably write it in a single line. In compiled form it takes up just a few bytes of disk space. But the code required to give the same program a graphical user interface would probably run into hundreds or even thousands of lines, depending on how fancy the programmer wanted to make it. Compiled into a runnable piece of software, it would have a large overhead of GUI code. It would be slow to launch and it would use up a lot of memory. This would simply not be worth the effort, and so "wc" would never be written as an independent program at all. Instead users would have to wait for a word count feature to appear in a commercial software package.