Emacs: Set Default Major Mode
Set Default Major Mode for New Empty Buffer
This works for setting default mode for scratch buffer too.
Put this in your Emacs Init File:
;; 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, List Variables]
;; 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, List Variables] 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.
For example: if first line in the file contains
-*- mode: xx-*-
, emacs will loadxx-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.
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.