Emacs: Set File to Open in a Major Mode

By Xah Lee. Date: . Last updated: .

This page tells you how emacs determine what major mode to load when you open a file, and how you can change the setup.

Associate Major Mode by File Name Extension

Use auto-mode-alist to associate a major mode with file name extension.

;; setup files ending in “.js” to open in js2-mode
(add-to-list 'auto-mode-alist '("\\.js\\'" . js2-mode))

auto-mode-alist is a built-in variable. Its value is a association list. Each key is a regex string, and value is mode name symbol.

〔►see Elisp: Association List

Note: in the elisp code above, the double backslash in the string \\.js\\' is used to escape the backslash. So, the regex engine just got \.js\'. The \. is to match a period. The \' is one of emacs special regex syntax, to match end of a string (not including end of a line that's not end of string. End of line is $). (See also: emacs regex tutorial.) (info "(elisp) Regexp Backslash")

You can see the value of auto-mode-alist by Alt+x describe-variable.

Remove File Extension Association

You can remove file name association with a major mode. Example:

;; remove any file name suffix associated with js-mode
(setq auto-mode-alist (rassq-delete-all 'js-mode auto-mode-alist))

To check if a mode is in the list, eval the following and look at the output. (view-echo-area-messages)

;; check if js-mode is in the list.
(rassoc 'js-mode auto-mode-alist)

〔►see How to Evaluate Emacs Lisp Code

Normally, you should not need to remove items in auto-mode-alist. Simply add to the front using add-to-list.

Associate Major Mode by First Line in File

The magic-mode-alist is for associating first line of a file with a mode. (when the line otherwise isn't the unix shebang #!… or embedded elisp variable.) Use it like this:

;; if first line of file matches, activate nxml-mode
(add-to-list 'magic-mode-alist '("<!DOCTYPE html .+DTD XHTML .+>" . nxml-mode) )

The magic-mode-alist is variable. Its value is a list of pairs. The first element is a regex string, the second is a mode name (of type symbol). Emacs tries to match the first line of a file to values in magic-mode-alist. If there's a match, it sets the buffer to that mode.

How Emacs Determines Which Major Mode to Load

Emacs determines what mode to activate by the following mechanisms, in order. If a match is found, the process stops.

  1. Look for a special emacs-specific syntax in the file. For example: if first line in the file contains -*- mode: xyz-*-, emacs will load “xyz-mode”. This is from a general mechanism for emacs to load elisp variables. (See: (info "(emacs) File Variables").) This has the top priority, but this mechanism is not the usual way for programing language files to associate with a major mode.
  2. Check the first line in the file for unix “shebang” syntax (For example, #!/usr/bin/perl) and match it with interpreter-mode-alist.
  3. Trys to match first line text with magic-mode-alist. (As of emacs 24.1.1, by default this list is empty.)
  4. Match the file name with auto-mode-alist.

(info "(emacs) Choosing Modes")

Note that when you install a new package, some has the file association setting code within the package, while others ask you to put a few lines in your emacs init file instead.

Emacs Customization

  1. How to Set Emacs's User Interface
  2. Emacs: How to Install Packages Using ELPA, MELPA
  3. How to Install Emacs Package Manually
  4. Emacs: How to Define Keys
  5. Emacs: M-x customize Tutorial
  6. Emacs: Set File to Open in a Major Mode
  7. Emacs: Organize Init File
  8. Emacs: Byte Compile Elisp Files
  9. Emacs: What's Hook?
  10. Emacs: Set Environment Variables within Emacs
  11. Elisp: Determine OS, Emacs Version, Machine Host Name
  12. Elisp: Check If a {function, variable, feature} is Defined/Loaded
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