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The Top Level of Mode Line Control

The variable in overall control of the mode line is mode-line-format.

User Option: mode-line-format

The value of this variable is a mode line construct that controls the contents of the mode-line. It is always buffer-local in all buffers.

If you set this variable to nil in a buffer, that buffer does not have a mode line. (A window that is just one line tall also does not display a mode line.)

The default value of mode-line-format is designed to use the values of other variables such as mode-line-position and mode-line-modes (which in turn incorporates the values of the variables mode-name and minor-mode-alist). Very few modes need to alter mode-line-format itself. For most purposes, it is sufficient to alter some of the variables that mode-line-format either directly or indirectly refers to.

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 a hypothetical example of a mode-line-format that might be useful for Shell mode (in reality, Shell mode does not set mode-line-format):

(setq mode-line-format
  (list "-"
   'mode-line-mule-info
   'mode-line-modified
   'mode-line-frame-identification
   "%b--"

   ;; Note that this is evaluated while making the list.
   ;; It makes a mode line construct which is just a string.
   (getenv "HOST")

   ":"
   'default-directory
   "   "
   'global-mode-string
   "   %[("
   '(:eval (format-time-string "%F"))
   'mode-line-process
   'minor-mode-alist
   "%n"
   ")%]--"

   '(which-func-mode ("" which-func-format "--"))
   '(line-number-mode "L%l--")
   '(column-number-mode "C%c--")
   '(-3 "%p")))

(The variables line-number-mode, column-number-mode and which-func-mode enable particular minor modes; as usual, these variable names are also the minor mode command names.)