ed, vi and EMACS are the most common UNIX editors. ed is available on virtually all systems, vi an the majority and EMACS on nearly all modern systems. There are situations where only ed can be used, which is the only reason to learn its basics. For fast editing vi is the best to use and the majority of system tools use vi, which makes a basic knowledge of it essential. EMACS is a luxury editor which is not essential, but is a lot more user-friendly than ed or vi.