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Info Node: (emacs)Choosing Modes

(emacs)Choosing Modes


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How Major Modes are Chosen
==========================

   You can select a major mode explicitly for the current buffer, but
most of the time Emacs determines which mode to use based on the file
name or on special text in the file.

   Explicit selection of a new major mode is done with a `M-x' command.
From the name of a major mode, add `-mode' to get the name of a command
to select that mode.  Thus, you can enter Lisp mode by executing `M-x
lisp-mode'.

   When you visit a file, Emacs usually chooses the right major mode
based on the file's name.  For example, files whose names end in `.c'
are edited in C mode.  The correspondence between file names and major
modes is controlled by the variable `auto-mode-alist'.  Its value is a
list in which each element has this form,

     (REGEXP . MODE-FUNCTION)

or this form,

     (REGEXP MODE-FUNCTION FLAG)

For example, one element normally found in the list has the form
`("\\.c\\'" . c-mode)', and it is responsible for selecting C mode for
files whose names end in `.c'.  (Note that `\\' is needed in Lisp
syntax to include a `\' in the string, which must be used to suppress
the special meaning of `.' in regexps.)  If the element has the form
`(REGEXP MODE-FUNCTION FLAG)' and FLAG is non-`nil', then after calling
MODE-FUNCTION, the suffix that matched REGEXP is discarded and the list
is searched again for another match.

   You can specify which major mode should be used for editing a certain
file by a special sort of text in the first nonblank line of the file.
The mode name should appear in this line both preceded and followed by
`-*-'.  Other text may appear on the line as well.  For example,

     ;-*-Lisp-*-

tells Emacs to use Lisp mode.  Such an explicit specification overrides
any defaults based on the file name.  Note how the semicolon is used to
make Lisp treat this line as a comment.

   Another format of mode specification is

     -*- mode: MODENAME;-*-

which allows you to specify local variables as well, like this:

     -*- mode: MODENAME; VAR: VALUE; ... -*-

Note: File Variables, for more information about this.

   When a file's contents begin with `#!', it can serve as an
executable shell command, which works by running an interpreter named on
the file's first line.  The rest of the file is used as input to the
interpreter.

   When you visit such a file in Emacs, if the file's name does not
specify a major mode, Emacs uses the interpreter name on the first line
to choose a mode.  If the first line is the name of a recognized
interpreter program, such as `perl' or `tcl', Emacs uses a mode
appropriate for programs for that interpreter.  The variable
`interpreter-mode-alist' specifies the correspondence between
interpreter program names and major modes.

   When the first line starts with `#!', you cannot (on many systems)
use the `-*-' feature on the first line, because the system would get
confused when running the interpreter.  So Emacs looks for `-*-' on the
second line in such files as well as on the first line.

   When you visit a file that does not specify a major mode to use, or
when you create a new buffer with `C-x b', the variable
`default-major-mode' specifies which major mode to use.  Normally its
value is the symbol `fundamental-mode', which specifies Fundamental
mode.  If `default-major-mode' is `nil', the major mode is taken from
the previously current buffer.

   If you change the major mode of a buffer, you can go back to the
major mode Emacs would choose automatically: use the command `M-x
normal-mode' to do this.  This is the same function that `find-file'
calls to choose the major mode.  It also processes the file's local
variables list (if any).

   The commands `C-x C-w' and `set-visited-file-name' change to a new
major mode if the new file name implies a mode (Note: Saving).
However, this does not happen if the buffer contents specify a major
mode, and certain "special" major modes do not allow the mode to
change.  You can turn off this mode-changing feature by setting
`change-major-mode-with-file-name' to `nil'.


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