Change section name into subsection names in header

Question

I want to insert a header in the book class such that:

• Odd pages show only the book title on the left header:
• Even pages show either the section name, or the subsection (if this exists) on the right header

MWE:

\documentclass[12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage{fancyhdr}
\usepackage{hyperref}
\usepackage{lipsum}

\fancyhf{}
\fancyhead[LO]{Book title}\fancyfoot[CO]{\thepage}
\fancyhead[RE]{\nouppercase{\rightmark}}\fancyfoot[CE]{\thepage} %this should be defined in some other way
\pagestyle{fancy}

\author{Author}
\title{Test book}

\begin{document}
\maketitle
\tableofcontents

\chapter[Test chapter]{Test chapter}
\section{First section}
\lipsum[4-11]

\markedsection{Short section title}{Long section title}

\lipsum[1-6]
\end{document}

However, even pages do not show the section name, while they show the subsection name when that is already "initialized". How can I define a command that changes the section name into the subsection name of the header?

Answer 1

For the book class there is no real \subsectionmark command -- it uses the \let\subsectionmark\@gobble statement from latex.ltx.

Within all section levels commands defined with \@startsection and finally using \@sect, there is the call \csname #1mark\endcsname{#7}, which will set the mark if #1mark, i.e. if \subsectionmark would be really usefully defined, but it is used as \@gobble, i.e. throwing away the argument for book.cls.

So one possible solution is to define \subsectionmark, I used the definition of \sectionmark within \ps@headings style from book.cls and changed it slightly.

Since all \....mark commands use \markboth which is setting internal variables, the last call to \markboth will set the real \@leftmark and \@rightmark settings, so the last call of a mark on the page is the winner!

\documentclass[12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage{fancyhdr}
\usepackage[headheight=15pt]{geometry}
\usepackage{lipsum}
\usepackage{hyperref}

\makeatletter
\def\subsectionmark#1{%
  \markright {\MakeUppercase{%
      \ifnum \c@secnumdepth >\z@
      \thesubsection. \ %
      \fi
      #1}}}
\makeatother

\fancyhf{}
\fancyhead[LO]{Book title}\fancyfoot[CO]{\thepage}
\fancyhead[RE]{\nouppercase{\rightmark}}\fancyfoot[CE]{\thepage} %this should be defined in some other way
\pagestyle{plain}

\author{Author}
\title{Test book}

\begin{document}
\maketitle
\tableofcontents
\clearpage
\pagestyle{fancy}

\chapter[Test chapter]{Test chapter}
\section{First section}
\lipsum[4-11]

\subsection{Foo subsection}
%\markedsection{Short section title}{Long section title}

\lipsum[1-6]
\end{document}