How to underline section-headings in LaTeX?

Question

EDIT: As I see in the first answer, underline is the wrong word. I want a line under the section heading, separating the heading from the following text.

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Answer 1

Rewriting your command like this should do the trick:

\newcommand{\tmpsection}[1]{}
\let\tmpsection=\section
\renewcommand{\section}[1]{\tmpsection{\underline{#1}}}

This will evaluate to

\section { \underline {#1} }

which in turn produces underlined section headings :)