mode-line-format is a variable defined in `C source code'.
Its value is

("%e" mode-line-front-space mode-line-mule-info mode-line-client mode-line-modified mode-line-remote mode-line-frame-identification mode-line-buffer-identification " " mode-line-position (vc-mode vc-mode) " " mode-line-modes mode-line-misc-info mode-line-end-spaces)

  • Automatically becomes buffer-local when set.
  • This variable may be risky if used as a file-local variable.

Template for displaying mode line for current buffer.

The value may be nil, a string, a symbol or a list.

A value of nil means don't display a mode line.

For any symbol other than t or nil, the symbol's value is processed as
a mode line construct. As a special exception, if that value is a
string, the string is processed verbatim, without handling any
%-constructs (see below). Also, unless the symbol has a non-nil
`risky-local-variable' property, all properties in any strings, as
well as all :eval and :propertize forms in the value, are ignored.

A list whose car is a string or list is processed by processing each
of the list elements recursively, as separate mode line constructs,
and concatenating the results.

A list of the form `(:eval FORM)' is processed by evaluating FORM and
using the result as a mode line construct. Be careful--FORM should
not load any files, because that can cause an infinite recursion.

A list of the form `(:propertize ELT PROPS...)' is processed by
processing ELT as the mode line construct, and adding the text
properties PROPS to the result.

A list whose car is a symbol is processed by examining the symbol's
value, and, if that value is non-nil, processing the cadr of the list
recursively; and if that value is nil, processing the caddr of the
list recursively.

A list whose car is an integer is processed by processing the cadr of
the list, and padding (if the number is positive) or truncating (if
negative) to the width specified by that number.

A string is printed verbatim in the mode line except for %-constructs:
%b -- print buffer name. %f -- print visited file name.
%F -- print frame name.
%* -- print %, * or hyphen. %+ -- print *, % or hyphen.
%& is like %*, but ignore read-only-ness.
% means buffer is read-only and * means it is modified.
For a modified read-only buffer, %* gives % and %+ gives *.
%s -- print process status. %l -- print the current line number.
%c -- print the current column number (this makes editing slower).
To make the column number update correctly in all cases,
`column-number-mode' must be non-nil.
%i -- print the size of the buffer.
%I -- like %i, but use k, M, G, etc., to abbreviate.
%p -- print percent of buffer above top of window, or Top, Bot or All.
%P -- print percent of buffer above bottom of window, perhaps plus Top,
or print Bottom or All.
%n -- print Narrow if appropriate.
%t -- visited file is text or binary (if OS supports this distinction).
%z -- print mnemonics of keyboard, terminal, and buffer coding systems.
%Z -- like %z, but including the end-of-line format.
%e -- print error message about full memory.
%@ -- print @ or hyphen. @ means that default-directory is on a
remote machine.
%[ -- print one [ for each recursive editing level. %] similar.
%% -- print %. %- -- print infinitely many dashes.
Decimal digits after the % specify field width to which to pad.

You can customize this variable.