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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.

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, cons cell, symbol, or string. If the value is a list, each element may be a list, a cons cell, 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 section %-Constructs in the Mode Line.

symbol

A symbol as a mode line construct stands for its value. The value of symbol is used in place of symbol unless symbol is t or nil, or is void, in which case symbol is ignored.

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

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

A list whose first element is a string or list, means to concatenate all the elements. 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 of the list (then) is processed recursively as a mode line element. But if the value of symbol is nil, the third element of the list (if there is one) is processed recursively.

(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 all the same variables that are used by the default value, rather than duplicating their contents or displaying the information in another fashion. This permits customizations made by the user, by libraries (such as display-time) or by 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 
   'minor-mode-alist 
   "%n" 
   'mode-line-process  
   ")%]----"
   '(-3 . "%p")
   "-%-"))

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