Write special characters to (aux) file

Question

I would like to print a special character to an aux file. In particular, I would like to write the line

\@input{file.aux}

So far, I have achieved the best result with

\write\@auxout{\unexpanded{\@input{file.aux}}}

However, this approach produces an additional whitespace before the first bracket.

What is the appropriate way to print special characters like \ or @ to an (aux) file? The possibly related examples provided here and here appear unnecessarily complicated and I am not quite able to implement them for my purpose.

Answer 1

Use \string\@input{file.aux} to write the stuff literally into the .aux file. As long no further expansion is involved, this is (the best?) way to get TeX commands literally into file output.

The \immediate is not really necessary, I just use it in almost any case, that's the only reason.

\documentclass{article}

\begin{document}

\makeatletter
\immediate\write\@auxout{%
  \string\@input{file.aux}%
}
\makeatother
\end{document}