How to redefine the behavior of dollar and then use dollar for \write18?

Question

I did what was suggested here (I just want to use $...$ instead of \texttt{...}, for the sake of experiment):

\documentclass{article}
\catcode`\$=\active
\gdef$#1${\texttt{#1}}
\begin{document}
Hello, $world$!
\immediate\write18{echo $HOME}
\end{document}

The Hello, $world$ line works fine. However, \write18 fails:

! Argument of $ has an extra }.
<inserted text>
                \par
l.6 \immediate\write18{echo $HOME}

What is the right way to use $ inside \write18 in this case? I tried \$, no luck.

Answer 1

echo \string$HOME

Or locally reset the catcode

Some other possiblities

1 xyz, /home/davidc
2 abc , string /home/davidc
3 abc , string /home/davidc
4 xyz, /home/davidc
5 xyz, /home/davidc

from

\documentclass{article}
\catcode`\$=\active
\gdef$#1${\texttt{#1}}
\def\abc{xyz}
\begin{document}
Hello, $world$!
\immediate\write18{echo 1  \abc, \string$HOME}
\immediate\write18{\detokenize{echo 2  \abc, $HOME}}
\immediate\write18{\unexpanded{echo 3  \abc, $HOME}}
{\catcode`$=12
\immediate\write18{echo 4  \abc, $HOME}}
{\edef${\string$}
\immediate\write18{echo 5  \abc, $HOME}}
Hello, $world$!
\end{document}