Emacs is undoubtedly the most powerful programmer's editor in existence. It's a big, feature-laden program with a great deal of flexibility and customizability. As we observed in the Chapter14 section on Emacs Lisp, Emacs has an entire programming language inside it that can be used to write arbitrarily powerful editor functions.

Unlike vi, Emacs doesn't have interface modes; instead, commands are normally control characters or prefixed with an ESC. However, in Emacs it is possible to bind just about any key sequence to any command, and commands can be stock or customized Lisp programs.

The designers of Emacs^[119] built a programmable editor that could have task-related intelligence customized into it for hundreds of different specialized editing jobs. They then gave it the ability to drive other tools. As a result, Emacs supports dealing with all things textual in one shared context — files, mail, news, debugger symbols. It can serve as a customizable front end to any command with an interactive textual interface.

It is a common joke, both among fans and detractors of Emacs, to describe it as an operating system masquerading as an editor. That overstates the case, but Emacs certainly does fulfill the role occupied by integrated development environments (IDEs) under non-Unix operating systems (a theme to which we shall return in Chapter15).

This power comes at a price in complexity. To use a customized Emacs you have to carry around the Lisp files that define your personal Emacs preferences. Learning how to customize Emacs is an entire art in itself. Emacs is correspondingly harder to learn than vi.