Define global commands in aux file

Question

I'm writing a long document and, to make my life easier, I like to define quantites I use as macros so that I just have to update them in one place when they change. For me, it makes sense to define these as I go along, in the section of my document which is most relevent to them. However, sometimes I'd like to use these commands in other chapters, sometimes even in previous chapters.

I'd therefore like to add a macro which spits a definition of a command into the latex .aux files, so that a command can be used everywhere once it's been defined. So far I've got this:

% Make a command which defines a macro with \providecommand but in the aux file,
% so it's accessible to all other chapters
\makeatletter
\newcommand{\doccommand}[2]{%
\protected@write\@auxout{}{\protect\providecommand\protect#1{#2}}%
}
\makeatother

This puts the following into the aux file

\providecommand \tester {123}

for a chapter like this:

\chapter{Test chapter}

This is a test chapter!

Tester is \tester{}.

\doccommand{\tester}{123}

Unfortunately, it doesn't work. Even using the command within the test chapter gives me "Undefined control sequence", and the same in other chapters.

Originally I thought this might have something to do with the spaces that ended up in the .aux file, but I manually removed them and compiled with an \includeonly{a_different_chapter} which still gives the same error.

What gives?

Update:

Thanks to all the help here, I got this working and wrote it into a little package. If you're interested, you can find it on CTAN at https://ctan.org/pkg/globalvals

Answer 1

I'd use a different strategy: use a wrapper command, rather than directly defining macros.

\documentclass{article}

\makeatletter
\newcommand{\usevalue}[1]{%
  \ifcsname usevalue@#1\endcsname
    \csname usevalue@#1\endcsname
  \else
    ??%
  \fi
}
\newcommand{\definevalue}[2]{%
  \write\@auxout{%
    \unexpanded{\global\@namedef{usevalue@#1}{#2}}%
  }
}
\makeatother

\begin{document}

Something with \usevalue{tester}.

Something else.

Now we can define \texttt{tester} and use again it: \usevalue{tester}.

\definevalue{tester}{42}

\end{document}

Output of first run

Output of second run

Answer 2

You could write

\makeatletter
\newcommand{\doccommand}[2]{%
  \immediate\write\@auxout{\gdef\noexpand#1{\unexpanded{#2}}}%
  \gdef#1{#2}%
}
\makeatother

This will do a global define without expanding the replacement text. I added an additional direct \gdef so that the command can be used without rerunning TeX.

But this is not a good idea: If you never use the command before the point where it is defined, defining it in the aux-file is useless. If you use the command before defining it, LaTeX never reaches the point where you write the aux-file entry.

So you can only use the command after running TeX once with the command defined but not used. If you ever delete the aux-file, your document is broken. If you ever only include a different chapter, the aux-file entry will not be written, so your document is broken.

Instead you could create a separate file with the definitions which you include in the preample. It's more work, but it results in a much more stable document.

Answer 3

Thanks @egreg for a clever approach. I've altered his code very slightly to add an error message if two definitions are made in the same document:

\documentclass{article}    

\usepackage{siunitx}

\makeatletter
\newcommand{\useVal}[1]{%
  \ifcsname useVal@#1\endcsname
    \csname useVal@#1\endcsname
  \else
    ??%
  \fi
}
\newcommand{\defVal}[2]{%
\ifcsname useVal@#1@defined\endcsname
    \PackageError{useVal}{Value "#1" already defined}{}
\else
  \write\@auxout{%
    \unexpanded{\global\@namedef{useVal@#1}{#2}}%
  }
  \global\@namedef{useVal@#1@defined}{}
\fi
}
\makeatother

\begin{document}

Testval is \useVal{testVal}.

Now defining testval...

\defVal{testVal}{\SI{123}{\meter}}

Now it's \useVal{testVal}. 

% This would throw an error:
% \defVal{testVal}{Not this please!}

\end{document}