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#### 22.4.3 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 (mode-line-mode-name))
       '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.)

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