How to get the section title by section number?

Question

This is a follow-up question to How to print section titles like tab-list?.

I want to insert section titles somewhere in the background in every pages. (It could be chapter titles, well)

I've searched around but did'nt find anything other then the current section title.

MWE-1:

\documentclass{article}
\usepackage{lipsum}

\begin{document}

    \section{The first section}
    \lipsum[1-3]

    \section{The second section}
    \lipsum[1-3]

    \section{The third section}
    \lipsum[1-3]

    \section{The fourth section}
        Hi, I'm the 4th.
        The title of the 2nd section is... ?? Huh??

    \section{The fifth section}
    \lipsum[1-3]

    \section{The sixth section}
    \lipsum[1-3]

    \section{The seventh section}
    \lipsum[1-3]

\end{document}

MWE-2:

In practice, I need to typeset actual section titles in the following tab buttons:

\documentclass{article}
\usepackage[a5paper,vmargin=2cm]{geometry}
\usepackage{background}
\usepackage{etoolbox}
\usepackage{totcount}
\usepackage{lipsum}

% to have access to the total number of sections
\regtotcounter{section}

% every section starts on a new page
\pretocmd{\section}{\clearpage}{}{}

% auxiliary lengths for the height of the frame and the width of each tab
\newlength\frameheight
\newlength\avgtabwidth

% style for the section tabs
\tikzset{tab/.style={
    text width=\avgtabwidth,anchor=south,
    draw=gray,thick,rectangle,rounded corners=12pt,rotate=270,
    align=center,text height=20pt,
    text depth=25pt,inner sep=0pt,fill=gray!20,
    yshift=-18pt,font=\sffamily\LARGE}}
% style for the current section tab
\tikzset{selectedtab/.style={tab,color=white,fill=gray!90}}

% the page number is showed in the background material
\pagestyle{empty}

\AtBeginDocument{
    % calculation of the width for each tab
    \setlength\frameheight{\dimexpr\textheight+2cm\relax}
    \ifnum\totvalue{section}>0
        \setlength\avgtabwidth{\dimexpr\frameheight/\totvalue{section}\relax}
    \fi

    % the main part; as background material we place the border, 
    % the section (current and other) tabs and the page number 
    \backgroundsetup{scale=1, color=black, angle=0, opacity=1, contents={
        \begin{tikzpicture}[remember picture, overlay]
            \node[inner sep=0pt, text width=\the\dimexpr\textwidth+1.5cm] 
                at (current page.center) (border) 
                {\rule{0pt}{\dimexpr\textheight+2cm\relax}};
            \foreach \valsection in {0,...,\numexpr\totvalue{section}-1\relax} {
                \node[\ifnum\thesection<\numexpr\valsection+1 tab\relax
                        \else \ifnum\thesection>\numexpr\valsection+1 tab\relax
                        \else selectedtab\fi\fi, 
                      xshift=(0.5+\valsection)*\avgtabwidth]
                    at (border.north east) (tab-\valsection)
                    {Section \the\numexpr\valsection+1\relax};
            }
            \node[draw=gray, line width=2pt, rectangle, rounded corners=10pt,
                  inner sep=0pt, text width=\the\dimexpr\textwidth+1.5cm\relax, fill=white] 
                at (current page.center) {\rule{0pt}{\dimexpr\textheight+2cm\relax}};
            \node[font=\LARGE\sffamily,fill=white] 
                at (border.south) {\makebox[3em][c]{\thepage}};
        \end{tikzpicture}
    }}
}


\begin{document}

\section{Test Section One}
\lipsum[1-3]
\section{Test Section Two}
\lipsum[1-3]
\section{Test Section Three}
\lipsum[1-3]
\section{Test Section Four}
\lipsum[2]

\end{document}

Answer 1

Here is a slightly modified version of your MWE that shows how to insert the names of sections in an automated fashion:

\documentclass{article}
\usepackage[a5paper,vmargin=2cm]{geometry}  % http://ctan.org/pkg/geometry
\usepackage{background}                     % http://ctan.org/pkg/background
\usepackage{etoolbox}                       % http://ctan.org/pkg/etoolbox
\usepackage{totcount}                       % http://ctan.org/pkg/totcount
\usepackage{lipsum}                         % http://ctan.org/pkg/lipsum
\usepackage{nameref}                        % http://ctan.org/pkg/nameref

% Automatically insert `\label` with every `\section`
\let\oldsection\section
\renewcommand{\section}[2][\relax]{%
    \ifx#1\relax
      \oldsection{#2}%
    \else
      \oldsection[#1]{#2}%
    \fi%
    \label{sec:\thesection}%
}

% to have access to the total number of sections
\regtotcounter{section}

% every section starts on a new page
\pretocmd{\section}{\clearpage}{}{}

% auxiliary lengths for the height of the frame and the width of each tab
\newlength\frameheight
\newlength\avgtabwidth

% style for the section tabs
\tikzset{tab/.style={
    text width=\avgtabwidth,anchor=south,
    draw=gray,thick,rectangle,rounded corners=12pt,rotate=270,
    align=center,text height=20pt,
    text depth=25pt,inner sep=0pt,fill=gray!20,
    yshift=-18pt,font=\sffamily\LARGE}}
% style for the current section tab
\tikzset{selectedtab/.style={tab,color=white,fill=gray!90}}

% the page number is showed in the background material
\pagestyle{empty}

\AtBeginDocument{
    % calculation of the width for each tab
    \setlength\frameheight{\dimexpr\textheight+2cm\relax}
    \ifnum\totvalue{section}>0
        \setlength\avgtabwidth{\dimexpr\frameheight/\totvalue{section}\relax}
    \fi

    % the main part; as background material we place the border, 
    % the section (current and other) tabs and the page number 
    \backgroundsetup{scale=1, color=black, angle=0, opacity=1, contents={
        \begin{tikzpicture}[remember picture, overlay]
            \node[inner sep=0pt, text width=\the\dimexpr\textwidth+1.5cm] 
                at (current page.center) (border) 
                {\rule{0pt}{\dimexpr\textheight+2cm\relax}};
            \foreach \valsection in {0,...,\numexpr\totvalue{section}-1\relax} {
                \node[\ifnum\thesection<\numexpr\valsection+1 tab\relax
                        \else \ifnum\thesection>\numexpr\valsection+1 tab\relax
                        \else selectedtab\fi\fi, 
                      xshift=(0.5+\valsection)*\avgtabwidth]
                    at (border.north east) (tab-\valsection)
                    {\nameref{sec:\the\numexpr\valsection+1\relax}};
            }
            \node[draw=gray, line width=2pt, rectangle, rounded corners=10pt,
                  inner sep=0pt, text width=\the\dimexpr\textwidth+1.5cm\relax, fill=white] 
                at (current page.center) {\rule{0pt}{\dimexpr\textheight+2cm\relax}};
            \node[font=\LARGE\sffamily,fill=white] 
                at (border.south) {\makebox[3em][c]{\thepage}};
        \end{tikzpicture}
    }}
}


\begin{document}

\section[First]{Test Section One}
\lipsum[1-3]
\section[Second]{Test Section Two}
\lipsum[1-3]
\section[Third]{Test Section Three}
\lipsum[1-3]
\section[Fourth]{Test Section Four}
\lipsum[2]

\end{document}

The changes include:

1. Modified \section to automatically insert a label of the form sec:\thesection; and
2. Retrieve the name of the section labelled sec:<num> using nameref.

The \section modification is, of course, sub-optimal, since it doesn't allow for starred * variants. However, this may not be needed in your implementation, since you list the section titles in the form of tabs anyway. Regardless, this can be improved/changed, if needed.