/usr/bin/htmex is in texlive-htmlxml 2016.20170123-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../share/texlive/texmf-dist/scripts/tex4ht/htmex.sh
/usr/bin/../share/texlive/texmf-dist/scripts/tex4ht/htmex.sh