You can use the function
format-mode-line to compute the text that would appear in a mode line or header line based on a certain mode line construct.
function format-mode-line format \&optional face window buffer
This function formats a line of text according to
format as if it were generating the mode line for
window, but it also returns the text as a string. The argument
window defaults to the selected window. If
buffer is non-
nil, all the information used is taken from
buffer; by default, it comes from
The value string normally has text properties that correspond to the faces, keymaps, etc., that the mode line would have. Any character for which no
face property is specified by
format gets a default value determined by
t, that stands for either
window is selected, otherwise
nil or omitted, that stands for the default face. If
face is an integer, the value returned by this function will have no text properties.
You can also specify other valid faces as the value of
face. If specified, that face provides the
face property for characters whose face is not specified by
Note that using
face will actually redisplay the mode line or the header line, respectively, using the current definitions of the corresponding face, in addition to returning the formatted string. (Other faces do not cause redisplay.)
(format-mode-line header-line-format) returns the text that would appear in the selected window’s header line (
"" if it has no header line).
(format-mode-line header-line-format 'header-line) returns the same text, with each character carrying the face that it will have in the header line itself, and also redraws the header line.