\documentclass{article}

\usepackage{lipsum}

\begin{document}

test

\makeatletter
\newwrite\lipsumoutput
\immediate\openout\lipsumoutput=\jobname.txt
\def\par{}% Remove \par from output
\immediate\write\lipsumoutput{\lipsum@x}% Write \lipsum[10]
\immediate\closeout\lipsumoutput
\makeatother

\end{document}