Emacs is the extensible, customizable, self-documenting, real-time display editor, known on many UNIX and other systems. The text being edited is visible on the screen and is updated automatically as you type your commands. It is a real-time editor because the display is updated very frequently, usually after each character or pair of characters you type. This minimizes the amount of information you must keep in your head as you edit. Emacs is called advanced because it provides facilities that go beyond simple insertion and deletion: controlling subprocesses; automatic indentation of programs; viewing two or more files at once; editing formatted text; and dealing in terms of characters, words, lines, sentences, paragraphs, and pages, as well as expressions and comments in several different programming languages.

Self-documenting means that at any time you can type a special character, Ctrl+H, to find out what your options are. You can also use it to find out what any command does, or to find all the commands that pertain to a topic. Customizable means that you can change the definitions of Emacs commands in little ways. For example, if you use a programming language in which comments start with “<**” and end with “**>”, you can tell the Emacs comment manipulation commands to use those strings. Another sort of customization is rearrangement of the command set. For example, if you prefer the four basic cursor motion commands (up, down, left and right) on keys in a diamond pattern on the keyboard, you can rebind the keys that way.

Extensible means that you can go beyond simple customization and write entirely new commands, programs in the Lisp language that are run by Emacs's own Lisp interpreter. Emacs is an online extensible system, which means that it is divided into many functions that call each other, any of which can be redefined in the middle of an editing session. Almost any part of Emacs can be replaced without making a separate copy of all of Emacs. Most of the editing commands of Emacs are written in Lisp already; the few exceptions could have been written in Lisp but are written in C for efficiency. Although only a programmer can write an extension, anybody can use it afterward.

When run under the X Window System (started as xemacs) Emacs provides its own menus and convenient bindings to mouse buttons. But Emacs can provide many of the benefits of a window system on a text-only terminal. For instance, you can look at or edit several files at once, move text between files, and edit files while running shell commands.

Vim stands for “Vi IMproved”. It used to be “Vi IMitation”, but there are so many improvements that a name change was appropriate. Vim is a text editor which includes almost all the commands from the UNIX program vi and a lot of new ones.

Commands in the vi editor are entered using only the keyboard, which has the advantage that you can keep your fingers on the keyboard and your eyes on the screen, rather than moving your arm repeatedly to the mouse. For those who want it, mouse support and a GUI version with scrollbars and menus can be activated.

We will refer to vi or vim throughout this book for editing files, while you are of course free to use the editor of your choice. However, we recommend to at least get the vi basics in the fingers, because it is the standard text editor on almost all UNIX systems, while emacs can be an optional package. There may be small differences between different computers and terminals, but the main point is that if you can work with vi, you can survive on any UNIX system.

Apart from the vim command, the vIm packages may also provide gvim, the Gnome version of vim. Beginning users might find this easier to use, because the menus offer help when you forgot or don't know how to perform a particular editing task using the standard vim commands.