57.3.4 Local Variables in Files
A file can specify local variable values for use when you edit the
file with Emacs. Visiting the file checks for local variable
specifications; it automatically makes these variables local to the
buffer, and sets them to the values specified in the file.
There are two ways to specify local variable values: in the first
line, or with a local variables list. Here's how to specify them in the
first line:
-*- mode: modename; var: value; ... -*-
You can specify any number of variables/value pairs in this way, each
pair with a colon and semicolon as shown above. mode:
modename; specifies the major mode; this should come first in the
line. The values are not evaluated; they are used literally.
Here is an example that specifies Lisp mode and sets two variables with
numeric values:
;; -*- mode: Lisp; fill-column: 75; comment-column: 50; -*-
You can also specify the coding system for a file in this way: just
specify a value for the “variable” named coding. The “value”
must be a coding system name that Emacs recognizes. See Coding Systems. ‘unibyte: t’ specifies unibyte loading for a
particular Lisp file. See Enabling Multibyte.
The eval pseudo-variable, described below, can be specified in
the first line as well.
In shell scripts, the first line is used to identify the script
interpreter, so you cannot put any local variables there. To accommodate
for this, when Emacs visits a shell script, it looks for local variable
specifications in the second line.
A local variables list goes near the end of the file, in the
last page. (It is often best to put it on a page by itself.) The local
variables list starts with a line containing the string ‘Local
Variables:’, and ends with a line containing the string ‘End:’. In
between come the variable names and values, one set per line, as
‘variable: value’. The values are not
evaluated; they are used literally. If a file has both a local
variables list and a ‘-*-’ line, Emacs processes everything
in the ‘-*-’ line first, and everything in the local
variables list afterward.
Here is an example of a local variables list:
;;; Local Variables: ***
;;; mode:lisp ***
;;; comment-column:0 ***
;;; comment-start: ";;; " ***
;;; comment-end:"***" ***
;;; End: ***
As you see, each line starts with the prefix ‘;;; ’ and each line
ends with the suffix ‘ ***’. Emacs recognizes these as the prefix
and suffix based on the first line of the list, by finding them
surrounding the magic string ‘Local Variables:’; then it
automatically discards them from the other lines of the list.
The usual reason for using a prefix and/or suffix is to embed the
local variables list in a comment, so it won't confuse other programs
that the file is intended as input for. The example above is for a
language where comment lines start with ‘;;; ’ and end with
‘***’; the local values for comment-start and
comment-end customize the rest of Emacs for this unusual
syntax. Don't use a prefix (or a suffix) if you don't need one.
If you write a multi-line string value, you should put the prefix
and suffix on each line, even lines that start or end within the
string. They will be stripped off for processing the list. If you
want to split a long string across multiple lines of the file, you can
use backslash-newline, which is ignored in Lisp string constants.
Here's an example of doing this:
# Local Variables:
# compile-command: "cc foo.c -Dfoo=bar -Dhack=whatever \
# -Dmumble=blaah"
# End:
Some “variable names” have special meanings in a local variables
list. Specifying the “variable” mode really sets the major
mode, while any value specified for the “variable” eval is
simply evaluated as an expression (its value is ignored). A value for
coding specifies the coding system for character code
conversion of this file, and a value of t for unibyte
says to visit the file in a unibyte buffer. These four “variables”
are not really variables; setting them in any other context has no
special meaning.
If mode is used to set a major mode, it should be the
first “variable” in the list. Otherwise, the entries that precede
it will usually be ignored, since most modes kill all local variables
as part of their initialization.
You can use the mode “variable” to set minor modes as well
as the major modes; in fact, you can use it more than once, first to
set the major mode and then to set minor modes which are specific to
particular buffers. But most minor modes should not be specified in
the file at all, regardless of how, because they represent user
preferences.
For example, you may be tempted to try to turn on Auto Fill mode with
a local variable list. That is a mistake. The choice of Auto Fill mode
or not is a matter of individual taste, not a matter of the contents of
particular files. If you want to use Auto Fill, set up major mode hooks
with your .emacs file to turn it on (when appropriate) for you
alone (see Init File). Don't use a local variable list to impose
your taste on everyone.
The start of the local variables list must be no more than 3000
characters from the end of the file, and must be in the last page if the
file is divided into pages. Otherwise, Emacs will not notice it is
there. The purpose of this rule is so that a stray ‘Local
Variables:’ not in the last page does not confuse Emacs, and so that
visiting a long file that is all one page and has no local variables
list need not take the time to search the whole file.
Use the command normal-mode to reset the local variables and
major mode of a buffer according to the file name and contents,
including the local variables list if any. See Choosing Modes.
The variable enable-local-variables controls whether to process
local variables in files, and thus gives you a chance to override them.
Its default value is t, which means do process local variables in
files. If you set the value to nil, Emacs simply ignores local
variables in files. Any other value says to query you about each file
that has local variables, showing you the local variable specifications
so you can judge.
The eval “variable,” and certain actual variables, create a
special risk; when you visit someone else's file, local variable
specifications for these could affect your Emacs in arbitrary ways.
Therefore, the variable enable-local-eval controls whether Emacs
processes eval variables, as well variables with names that end
in ‘-hook’, ‘-hooks’, ‘-function’ or ‘-functions’,
and certain other variables. The three possibilities for the variable's
value are t, nil, and anything else, just as for
enable-local-variables. The default is maybe, which is
neither t nor nil, so normally Emacs does ask for
confirmation about file settings for these variables.
The safe-local-eval-forms is a customizable list of eval
forms which are safe to eval, so Emacs should not ask for
confirmation to evaluate these forms, even if
enable-local-variables says to ask for confirmation in general.
