Emacs Init: Set Default Major Mode

By Xah Lee. Date: 2008-07-30. Last updated: 2023-08-26.

Set Default Major Mode for New Empty Buffer

This works for setting default mode for scratch buffer too.

;; example of setting a default major mode for new buffer
(setq initial-major-mode 'python-mode)

;; other sample values
;; 'text-mode (for plain text)
;; 'emacs-lisp-mode
;; 'lisp-interaction-mode  (the lisp mode used in scratch buffer)
;; 'python
;; 'js (JavaScript)

Remap a Default Major Mode to Another

major-mode-remap-alist

an Association List for changing a default major mode to another.

In each element, key is the old mode symbol, value is the new symbol.

new in emacs 29. [see Emacs 29 (Released 2023-07)]

(if (< emacs-major-version 29)
    (add-to-list 'auto-mode-alist '("\\.css\\'" . xah-css-mode))
  (add-to-list 'major-mode-remap-alist '(css-mode . xah-css-mode)))

(setq
 major-mode-remap-alist
 '((perl-mode . cperl-mode)
   (nxml-mode . xah-html-mode)))

Associate Major Mode by File Name Extension

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

auto-mode-alist is a built-in variable. Its value is a Association List . Each key is a Regular Expression string, and value is mode name symbol. [see Emacs: Show Variable Value]

;; setup files ending in .html to open in xah-html-mode
(add-to-list
 'auto-mode-alist
 '("\\.html?\\'" . xah-html-mode))

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.

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

[see Evaluate Emacs Lisp Code]

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

Associate Major Mode by First Line in File

The magic-mode-alist is variable with value a Association List, for associating first line of a file with a mode. [see Emacs: Show Variable Value] Use it like this:

;; example of associate a major mode by first line of file

;; case matters
(add-to-list 'magic-mode-alist '("<!doctype html>" . xah-html-mode))
(add-to-list 'magic-mode-alist '("<!DOCTYPE html>" . xah-html-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.

Look for a special emacs-specific syntax in the file. e.g. if first line in the file contains -*- mode: xx-*-, emacs will load xx-mode. This is from a general mechanism for emacs to load elisp variables. (See: File Variables (Emacs Manual) .) This has the top priority, but this mechanism is not the usual way for programing language files to associate with a major mode.
Check the first line in the file for unix “shebang” syntax (For example, #!/usr/bin/perl) and match it with interpreter-mode-alist.
Try to match first line text with magic-mode-alist.
Match the file name with auto-mode-alist.

Choosing Modes (Emacs Manual)

Note: 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.