I want this to work, but it doesn't:
\documentclass{article}
\begin{document}
\newcommand\hello{Hello, \LaTeX!}
\newwrite\foo
\immediate\openout\foo foo.tex
\immediate\write\foo{\hello}
\immediate\closeout\foo
\end{document}
I expect to see this in the foo.tex file:
Hello, \LaTeX!
How to make it work?
\documentclass{article}
\begin{document}
\newcommand\hello{Hello, \LaTeX!}
\newwrite\foo
\immediate\openout\foo foo.tex
\immediate\write\foo{\unexpanded\expandafter{\hello}} % documentation of `\unexpanded` is in texdoc etex_man
%\immediate\write\foo{\detokenize\expandafter{\hello}} % this is an alternative, I believe it's slower than the above, although I didn't benchmark
\immediate\closeout\foo
\end{document}
although the actual issue here is: \LaTeX command is robust, but it's old-style robust. So an alternative fix (identical in this case but hopefully you can see what's the difference) is
\documentclass{article}
\begin{document}
\newcommand\hello{Hello, \LaTeX!}
\newwrite\foo
\immediate\openout\foo foo.tex
\makeatletter
\set@display@protect % see source2e...
\immediate\write\foo{\hello}
\restore@protect
\makeatother
\immediate\closeout\foo
\end{document}
there's \protected@write but no analogous \immediate@protected@write as far as I can see.
(or you can copy the \protected@iwrite from https://tex.stackexchange.com/a/110885/250119.
Similar question: Storing environment arguments by \immediate\write
You can use expl3 for the job, which also allows to have a sort of namespace for your text variables.
\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\vardef}{mm}
{% #1 = symbolic name, #2 = text
\tl_clear_new:c { l_yegor_var_#1_tl }
\tl_set:cn { l_yegor_var_#1_tl } { #2 }
}
\NewDocumentCommand{\varwrite}{mm}
{% #1 = symbolic name, #2 = file
\iow_open:Nn \g_yegor_var_outfile_iow { #2 }
\iow_now:Nv \g_yegor_var_outfile_iow { l_yegor_var_#1_tl }
\iow_close:N \g_yegor_var_outfile_iow
}
\NewExpandableDocumentCommand{\varuse}{m}
{
\tl_use:c { l_yegor_var_#1_tl }
}
\iow_new:N \g_yegor_var_outfile_iow
\cs_generate_variant:Nn \iow_now:Nn { Nv }
\ExplSyntaxOff
\begin{document}
\vardef{foo}{Hello, \LaTeX!}
\varwrite{foo}{\jobname.txt}
\end{document}
The file written out will contain
Hello, \LaTeX !
The space before the exclamation mark should be of no concern (it follows a control sequence name, so it will be ignored by TeX).
\newcommand\hello{Hello, \string\LaTeX!}works for \write, but also prevents expansion when used in text. – John Kormylo Feb 04 '23 at 16:50