GNU Emacs Lisp Reference Manual

20.3.1: The Data Structure of the Mode Line

The mode line contents are controlled by a data structure of lists, strings, symbols, and numbers kept in the buffer-local variable mode-line-format. The data structure is called a mode line construct, and it is built in recursive fashion out of simpler mode line constructs. The same data structure is used for constructing frame titles (see Frame Titles).

Variable: mode-line-format

The value of this variable is a mode line construct with overall responsibility for the mode line format. The value of this variable controls which other variables are used to form the mode line text, and where they appear.

A mode line construct may be as simple as a fixed string of text, but it usually specifies how to use other variables to construct the text. Many of these variables are themselves defined to have mode line constructs as their values.

The default value of mode-line-format incorporates the values of variables such as mode-name and minor-mode-alist. Because of this, very few modes need to alter mode-line-format. For most purposes, it is sufficient to alter the variables referenced by mode-line-format.

A mode line construct may be a list, a symbol, or a string. If the value is a list, each element may be a list, a symbol, or a string.

string

A string as a mode line construct is displayed verbatim in the mode line except for %-constructs. Decimal digits after the `%' specify the field width for space filling on the right (i.e., the data is left justified). See %-Constructs.

symbol

A symbol as a mode line construct stands for its value. The value of symbol is used as a mode line construct, in place of symbol. However, the symbols t and nil are ignored; so is any symbol whose value is void.

There is one exception: if the value of symbol is a string, it is displayed verbatim: the %-constructs are not recognized.

(string rest...) or (list rest...)

A list whose first element is a string or list means to process all the elements recursively and concatenate the results. This is the most common form of mode line construct.

(symbol then else)

A list whose first element is a symbol is a conditional. Its meaning depends on the value of symbol. If the value is non-nil, the second element, then, is processed recursively as a mode line element. But if the value of symbol is nil, the third element, else, is processed recursively. You may omit else; then the mode line element displays nothing if the value of symbol is nil.

(width rest...)

A list whose first element is an integer specifies truncation or padding of the results of rest. The remaining elements rest are processed recursively as mode line constructs and concatenated together. Then the result is space filled (if width is positive) or truncated (to -width columns, if width is negative) on the right.

For example, the usual way to show what percentage of a buffer is above the top of the window is to use a list like this: (-3 "%p").

If you do alter mode-line-format itself, the new value should use the same variables that appear in the default value (see Mode Line Variables), rather than duplicating their contents or displaying the information in another fashion. This way, customizations made by the user or by Lisp programs (such as display-time and major modes) via changes to those variables remain effective.

Here is an example of a mode-line-format that might be useful for shell-mode, since it contains the hostname and default directory.

(setq mode-line-format
  (list ""
   'mode-line-modified
   "%b--" 

   (getenv "HOST")      ; One element is not constant.
   ":" 
   'default-directory
   "   "
   'global-mode-string
   "   %[("
   'mode-name 
   'mode-line-process  
   'minor-mode-alist 
   "%n" 
   ")%]----"

   '(line-number-mode "L%l--")
   '(-3 . "%p")
   "-%-"))