Debra Cameron - Learning GNU Emacs, 3rd Edition

You don't really edit files with Emacs. Instead, Emacs copies the contents of a file into a temporary buffer and you edit that. The file on disk doesn't change until you save the buffer. Like files, Emacs buffers have names. The name of a buffer is usually the same as the name of the file that you're editing. There are a few exceptions. Some buffers don't have associated files—for example, *scratch*is just a temporary practice buffer, like a scratchpad; the help facility displays help messages in a buffer named *Help*, which also isn't connected to a file.

Emacs achieves some of its famed versatility by having various editing modes in which it behaves slightly differently. The word mode may sound technical, but what it really means is that Emacs becomes sensitive to the task at hand. When you're writing, you often want features like word wrap so that you don't have to press Enterat the end of every line. When you're programming, the code must be formatted correctly depending on the language. For writing, there's text mode; for programming, there are modes for different languages, including C, Java, and Perl. Modes, then, allow Emacs to be the kind of editor you want for different tasks.

Text mode and Java mode are major modes. A buffer can be in only one major mode at a time; to exit a major mode, you have to enter another one. Table 1-1lists some of the major modes, what they do, and where they're covered in this book.

Table 1-1. Major modes

Whenever you edit a file, Emacs attempts to put you into the correct major mode for what you're going to edit. If you edit a file that ends in .c , it puts you into cc mode. If you edit a file that ends in .el , it puts you in Lisp mode. Sometimes it looks at the contents of the file rather than just its name. If you edit a file formatted for TEX, Emacs puts you in LaTeX mode. If it cannot tell what mode you should be in, it puts you in fundamental mode, the most general of all. Because Emacs is extensible, add-in modes are also available; we talk about some in this book, though we do not list them in Table 1-1.

In addition to major modes there are also minor modes . These define a particular aspect of Emacs's behavior and can be turned on and off within a major mode. For example, auto-fill mode means that Emacs should do word wrap; when you type a long line, it should automatically make an appropriate line break. Table 1-2lists some minor modes, what they do, and where they're covered in this book.

Table 1-2. Minor modes

You may have noticed that several modes, including paragraph indent text mode, outline mode, and compilation mode, are both major and minor modes. Each can be used alone—as a major mode—or with another major mode as a minor mode.