Execute command stored in aux file

Question

My script needs to read a command from one aux file (testFileWriteReadExecute.yposIN.txt), then execute that same command which is a actually a call to a write macro which writes another command to a second file (testFileWriteReadExecute.yposOUT.txt).

Currently it produces the command in the PDF output instead of executing it. At some stage I had something similar working halfway, but have messed it up since, unfortunately.

testFileWriteReadExecute.yposin.txt looks like this:

\writeVerse{A}{1}{F}\comment{30580738}\endcomment
\writeVerse{A}{1}{L}\comment{29138946}\endcomment
\writeVerse{A}{2}{F}\comment{29138946}\endcomment
\writeVerse{A}{2}{L}\comment{27566082}\endcomment
\writeVerse{A}{3}{F}\comment{27566082}\endcomment
\writeVerse{A}{3}{L}\comment{25993218}\endcomment

MWE:

\documentclass[pagesize=pdftex, fontsize=10]{scrbook}
\usepackage[paperwidth=11.2cm, paperheight=17.4cm, top=1cm,bottom=1cm, left=1cm, right=1cm]{geometry}
\usepackage{fontspec}

\AtBeginDocument{%
    \newwrite\yposoutputfile%
    \openout\yposoutputfile=\jobname.yposout.txt%
}
\AtEndDocument{%
    \closeout\yposoutputfile%
}

\newcommand\writeVerse[3][]{%
  \writelines{#1}{#2}{#3}%
}

% Write the position to file.
\def\writelines#1#2#3{%
  \pdfsavepos%
    \write\yposoutputfile{%
    \string\writeVerse%
    \string{#1\string}%
    \string{#2\string}%
        \string{#3\string}%
        \string\comment\string{\the\pdflastypos\string}\string\endcomment%
    }%
}

% Open the file to read the commands.
\def\fromFile{
  \newread\yposinputfile%
  \openin\yposinputfile=\jobname.yposin.txt%
    \readline\yposinputfile to \fromFileLine%
  \closein\yposinputfile%
  \fromFileLine
}

\begin{document}
\fromFile %\writeVerse{A}{1}{F}
 LIBRO de la generación de Jesucristo, hijo de David, hijo de Abraham. \fromFile %\writeVerse{A}{1}{L}
\writeVerse{A}{2}{F} \textbf{2}~Abraham engendró á Isaac: é Isaac engendró á Jacob: y Jacob engendró á Judas y á sus hermanos:\writeVerse{A}{2}{L}
\writeVerse{A}{3}{F} \textbf{3}~Y Judas engendró de Thamar á Phares y á Zara: y Phares engendró á Esrom: y Esrom engendró á Aram:\writeVerse{A}{3}{L}
\end{document}

EDIT

The basic concept of what I want is simple.

1. The XeLaTeX script grabs a line from the one text file which happens to be a TeX command, i.e. \writeVerse{A}{1}{F}
2. It places in a particular place of the document this command, where this command gets executed/expanded instead of printed.
3. The command is a call to a macro which in turn writes to another file.

EDIT 2:

Since the closest answer is this one by egreg I'll accept it, although it does not answer the fundamentals of my question. It does seem that this is a limitation in TeX.

Answer 1

I don't really understand what \fromFile should do. However, you can avoid juggling the files by doing the write in the standard .aux file, and then overwriting the old "ypos" file at begin document.

\documentclass[pagesize=pdftex, fontsize=10]{scrbook}
\usepackage[
  paperwidth=11.2cm,
  paperheight=17.4cm,
  top=1cm,bottom=1cm,
  left=1cm,
  right=1cm]{geometry}
%\usepackage{fontspec}

% Allocate an output and an input stream
\newwrite\yposfileout
\newread\yposfilein

% If the file has been written, open it for reading
\AtBeginDocument{%
  \immediate\closeout\yposfileout
  \IfFileExists{\jobname.ypos.txt}
    {\openin\yposfilein=\jobname.ypos.txt}
    {\let\fromFile\relax}
}

% Write the position to file.
\makeatletter
\def\writelines#1#2#3{%
  \pdfsavepos%
  \write\@auxout{%
    \string\rewriteVerse{%
      \string\writeVerse{#1}{#2}{#3}%
      \string\comment\string{\the\pdflastypos\string}\string\endcomment
    }}%
}
\newif\if@writeverseopened
% when the `.aux` file is being read in at begin document, write out a new ypos file
\def\rewriteVerse#1{%
  \if@writeverseopened\else
    \immediate\openout\yposfileout=\jobname.ypos.txt
    \@writeverseopenedtrue
  \fi
  \immediate\write\yposfileout{\unexpanded{#1}}%
}
% Do nothing when the aux file is read in at end document
\AtEndDocument{\let\rewriteVerse\@gobble}

% Read the commands from the ypos file
\def\fromFile{%
  \readline\yposfilein to \fromFileLine
  \texttt{\fromFileLine}
}
\makeatother

% \writeVerse has an optional argument and two mandatory ones
\newcommand\writeVerse[3][]{%
  \writelines{#1}{#2}{#3}%
}


\begin{document}
\fromFile \writeVerse[A]{1}{F} LIBRO de la generación de Jesucristo,
hijo de David, hijo de Abraham. \fromFile \writeVerse[A]{1}{L}
\writeVerse[A]{2}{F} \textbf{2}~Abraham engendró á Isaac: é Isaac
engendró á Jacob: y Jacob engendró á Judas y á sus
hermanos:\writeVerse[A]{2}{L} \writeVerse[A]{3}{F} \textbf{3}~Y Judas
engendró de Thamar á Phares y á Zara: y Phares engendró á Esrom: y
Esrom engendró á Aram:\writeVerse[A]{3}{L}
\end{document}

---

You can use two files, provided your script changes the name of the former output file to the input file.

\documentclass[pagesize=pdftex, fontsize=10]{scrbook}
\usepackage[
  paperwidth=11.2cm,
  paperheight=17.4cm,
  top=1cm,bottom=1cm,
  left=1cm,
  right=1cm]{geometry}
\usepackage{atveryend,pdftexcmds}

\newwrite\yposfileout
\newread\yposfilein
\AtBeginDocument{%
  \IfFileExists{\jobname.yposin.txt}
    {\openin\yposfilein=\jobname.yposin.txt}
    {\let\fromFile\relax}
  \immediate\openout\yposfileout=\jobname.yposout.txt
}

% Write the position to file.
\def\writelines#1#2#3{%
  \pdfsavepos%
  \write\yposfileout{%
    \string\writeVerse{#1}{#2}{#3}%
    \string\comment\string{\the\pdflastypos\string}\string\endcomment
    }%
}
% Open the file to read the commands.
\def\fromFile{%
  \readline\yposfilein to \fromFileLine
  \texttt{\fromFileLine}
}

\makeatletter
\AtVeryEndDocument{%
  \closein\yposfilein
  \immediate\closeout\yposfileout
  \ifnum\pdf@shellescape>\z@
    \immediate\write18{mv \jobname.yposout.txt \jobname.yposin.txt}
  \fi
}
\makeatother

\newcommand\writeVerse[3][]{%
  \writelines{#1}{#2}{#3}%
}


\begin{document}
\fromFile \writeVerse[A]{1}{F} LIBRO de la generación de Jesucristo,
hijo de David, hijo de Abraham. \fromFile \writeVerse[A]{1}{L}
\writeVerse[A]{2}{F} \textbf{2}~Abraham engendró á Isaac: é Isaac
engendró á Jacob: y Jacob engendró á Judas y á sus
hermanos:\writeVerse[A]{2}{L} \writeVerse[A]{3}{F} \textbf{3}~Y Judas
engendró de Thamar á Phares y á Zara: y Phares engendró á Esrom: y
Esrom engendró á Aram:\writeVerse[A]{3}{L}
\end{document}

The \fromFile macro reads one line of the input file at each call.

If I run the file with -shell-escape, the \write18 istruction takes care of the name switching for the next run. You can do it with your script, of course.