By default pdftex, or xelatex if fontspec is used.
\documentclass{article}\usepackage{iftex}\newcommand\test[1]{\par\expandafter#1\string#1: true\fi}\begin{document}\ttfamily\test\ifpdftex\test\ifluatex\test\ifxetex\test\ifptex\test\ifuptex\end{document}
lualatex
%!TEX lualatex\documentclass{article}\usepackage{iftex}\newcommand\test[1]{\par\expandafter#1\string#1: true\fi}\begin{document}\ttfamily\test\ifpdftex\test\ifluatex\test\ifxetex\test\ifptex\test\ifuptex\end{document}
xelatex
%!TEX xelatex\documentclass{article}\usepackage{iftex}\newcommand\test[1]{\par\expandafter#1\string#1: true\fi}\begin{document}\ttfamily\test\ifpdftex\test\ifluatex\test\ifxetex\test\ifptex\test\ifuptex\end{document}
Multiple engine comments may be used, all but the first are ignored. uplatex is used here.
%!TEX uplatex%!TEX xelatex\documentclass{article}\usepackage{iftex}\newcommand\test[1]{\par\expandafter#1\string#1: true\fi}\begin{document}\ttfamily\test\ifpdftex\test\ifluatex\test\ifxetex\test\ifptex\test\ifuptex\end{document}
By default using the PDF.js library to render the PDF.
\documentclass{article}\showoutput\begin{document}Example equation\[e=mc^2\]\end{document}
The browser's default action for PDF files.
%!TeX pdf\documentclass{article}\showoutput\begin{document}Example equation\[e=mc^2\]\end{document}
Returning the log file as plain text.
%!TeX log\documentclass{article}\showoutput\begin{document}Example equation\[e=mc^2\]\end{document}
Multiple !TEX makeindex may be used, each sets a makeindex[]
parameter in the form submission.
% !TEX makeindex document% !TEX makeindex -s nomencl.ist -o document.nls -t document.nlg document.nlo\documentclass[oneside,12pt]{article}\usepackage{makeidx}\makeindex\usepackage{nomencl}\makenomenclature\begin{document}Testing\index{testing} nomenclature\index{nomenclature}\[E = mc^2\]\nomenclature{$E$}{Energy}\nomenclature{$m$}{Mass}\nomenclature{$c$}{Speed of Light}\begin{center}\begin{minipage}[t]{.45\linewidth}\printnomenclature\end{minipage}\hfill\begin{minipage}[t]{.45\linewidth}\renewenvironment{theindex}{%\section*{\indexname}}{}\printindex\end{minipage}\end{center}\end{document}