how to write content of a command into a file

Question

This question is a possible solution to question Code fails with Emergency stop, when using \expandafter\csname and such in listings, though not really an answer.

I have defined a command which may contain up to hundred lines of code including paragraph lines breaks, but basically it is just a lot of LaTeX Code. Let's assume this command is called \printhis

Now I want to export the content of \printthis into a file, which can be loaded by listings afterwards.

I have not worked before with any tex code that exports something to a file. If anyone can provide and example or link to a tutorial that would be great.

EDIT: Approach using answer package.

\documentclass{scrbook}

\makeatletter
\providecommand{\packagename}{templatedemo}

\RequirePackage{etoolbox}
\RequirePackage{answers}

\providecommand\demo@filename{demofile}
\providecommand\AddDemo[2]{%
  % #1 : package/identifier
  % #2 : content
  \ifcsdef{demo@content@#1}{
    \expandafter\renewcommand\csname demo@content@#1\endcsname{#2}%
  }{
    \expandafter\newcommand\csname demo@content@#1\endcsname{#2}%
  }
}

\providecommand{\demo@printhis}{}
\newcommand{\outputtofile}[1]{
  \Opensolutionfile{\demo@filename}
  \scantokens{\begin{Filesave}{\demo@filename}#1\end{Filesave}}
  \Closesolutionfile{\demo@filename}
}

\providecommand\PrintDemo[1]{%
  \ifcsdef{demo@content@#1}%
  { % print 
    \renewcommand{\demo@printhis}{\csuse{demo@content@#1}}
    To file: \demo@printhis
    \outputtofile{\demo@printhis}
  }%
  { % error if not defined   
  }
}

\AddDemo{text}{Sample text in file}%
\makeatother
\listfiles

\begin{document}
\PrintDemo{text}\par
From File: \IfFileExists{demofile.tex}{\input{demofile.tex}}{}
\end{document}

Answer 1

With some corrections your file compiles:

\documentclass{scrbook}

\RequirePackage{etoolbox}
\RequirePackage{answers}

\makeatletter

\newcommand\demo@filename{demofile}
\newcommand\AddDemo[2]{%
  % #1 : package/identifier
  % #2 : content
  \@namedef{demo@content@#1}{#2}}

\newcommand{\demo@printhis}{}
\newcommand{\outputtofile}[1]{
  \Opensolutionfile{\demo@filename}
  \begingroup\makeatletter
  \scantokens{\begin{Filesave}{\demo@filename}#1\end{Filesave}}
  \endgroup
  \Closesolutionfile{\demo@filename}
}

\newcommand\PrintDemo[1]{%
  \ifcsdef{demo@content@#1}%
  { % print 
    \letcs\demo@printhis{demo@content@#1}
    To file: \demo@printhis
    \expandafter\outputtofile\expandafter{\demo@printhis}
  }%
  { % error if not defined   
  }
}

\makeatother

\AddDemo{text}{Sample text in file \today}%

\begin{document}
\PrintDemo{text}\par
From File: \IfFileExists{demofile.tex}{\input{demofile.tex}}{}
\end{document}

In \AddDemo the conditional is useless, as you want to redefine the token anyway. So \@namedef (LaTeX kernel) or \csdef (etoolbox) do what's needed.

When you do \scantokens, the current category codes are applied, so the use of \demo@filename would break because @ is not a letter any more in the document. Applying \makeatletter cures the problem (but pay attention to @ characters in your examples); it can be done in a group, since we are only to write a file and the \endgroup will undo it.

However this approach has a severe limitation: line breaks cannot be preserved in the output file. That is,

\AddDemo{text}{Sample text}

and

\AddDemo{text}{%
   Sample
   text}

will save the same thing on the output file.

Answer 2

I think the answers package might have what you're looking for. It has many commands for writing to files- in particular, you could use something like

\begin{Filesave}{myfile}
Any code you like, even paragraph skips!

\end{Filesave}

See the MWE below

\documentclass{article}
\usepackage{answers}    

% open myfile.tex
\Opensolutionfile{myfile}

% sounds like you won't need this line
%\Newassociation{shortsolution}{shortSoln}{myfile}

\begin{document}

\section{Main body}
Here we are in the body.
\begin{Filesave}{myfile}
      This will be written to the file. It can contain 
      linebreaks, tables, anything at all.

      \begin{table}[!h]
       \centering
       \begin{tabular}{cc}
        1   & 2
       \end{tabular}
       \caption{My table}
      \end{table}
     \end{Filesave}
\newpage

% close myfile.tex
\Closesolutionfile{myfile}

\section{Inputted file}
% input myfile.tex
\IfFileExists{myfile.tex}{\input{myfile.tex}}{}

\end{document}

EDIT

Following the comments, the printthis command can be put in the pre-amble

\newcommand{\printthis}[1]{\scantokens{\begin{Filesave}{myfile}#1\end{Filesave}}}

which can be used as follows:

\printthis{Goes to myfile

  \begin{table}[!h]
   \centering
   \begin{tabular}{cc}
    1   & 2
   \end{tabular}
   \caption{My other table}
  \end{table}
}