LaTeX: Which kind of “LaTeX” am I using?

hello.tex

%%
%% This is file 'hello.tex'.
%%
%% Copyright (c) 2018-2026 Takayuki YATO (aka. "ZR")
%%   GitHub:   https://github.com/zr-tex8r
%%   Twitter:  @zr_tex8r
%%
%% This file is distributed under the MIT License.
%%
\documentclass[a4paper]{article}
\ProvidesFile{hello.tex}[2026/05/01 v0.4 hello my workflow]
\makeatletter %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code BEGIN

%--------------------------------------- engine check

%% \my@if@primitive
\def\my@if@primitive#1{%
  \edef\my@tmpa{\string#1}\edef\my@tmpb{\meaning#1}%
  \ifx\my@tmpa\my@tmpb \expandafter\@firstoftwo
  \else \expandafter \@secondoftwo \fi}

%% \my@engine : TeX engine type
\let\my@engine=k
\my@if@primitive\pdftexversion{\let\my@engine=P}{}
\my@if@primitive\OmegaVersion{\let\my@engine=o}{}
\my@if@primitive\luatexversion{\let\my@engine=L}{}
\my@if@primitive\XeTeXversion{\let\my@engine=X}{}
\my@if@primitive\kanjiskip{\let\my@engine=p}{}
\my@if@primitive\kchardef{\let\my@engine=u}{}
\my@if@primitive\ngbanner{\let\my@engine=A}{}
\my@if@primitive\jintercharskip{\let\my@engine=j}{}
\my@if@primitive\HINTversion{\let\my@engine=H}{}
\ifx u\my@engine
  \ifnum\ucs"3000="3000 \else \let\my@engine=p \fi
\fi

%% \ifmy@pdf
\newif\ifmy@pdf
\@tempcnta\z@
\my@if@primitive\pdfoutput{\@tempcnta\pdfoutput}{}
\my@if@primitive\outputmode{\@tempcnta\outputmode}{}
\ifnum\@tempcnta>\z@ \my@pdftrue \fi
\if X\my@engine \my@pdftrue \fi
\if A\my@engine \my@pdftrue \fi

%% \my@variant
\let\my@variant\relax

%% LuaTeX variants
\if L\my@engine \ifx\luatexbanner\@undefined\else
  \ifx\luaescapestring\@undefined\else
    \ifcase\directlua{
      local s = "\luaescapestring{\luatexbanner}"
      if s:match("LuajitTeX") then tex.write("1")
      elseif s:match("HarfTeX") then tex.write("2")
      elseif s:match("LuaHBTeX") then tex.write("3")
      elseif s:match("LuajitHBTeX") then tex.write("4")
      else tex.write("0")
      end}\relax
    \or \let\my@engine=I % LuajitTeX
    \or \def\my@variant{on Harf\TeX} % HarfTeX
    \or \def\my@variant{on LuaHB\TeX} % LuaHBTeX
    \or \def\my@variant{on LuajitHB\TeX}\let\my@engine=I
    \fi
  \fi
\fi\fi

%% XeTeX variants
\if X\my@engine
  \my@if@primitive\TectonicCodaTokens{%
    \def\my@variant{on Tectonic}%
  }{}
\fi

%% \my@lmodern
\if X\my@engine
  \IfFileExists{tulmr.fd}{%
    \expandafter\ifx\csname T@TU\endcsname\relax
      \input{tuenc.def}%
    \fi
    \def\my@lmodern{\fontencoding{TU}}}%
  {%else
    \input{eu1enc.def}%
    \def\my@lmodern{\fontencoding{EU1}}}%
  \g@addto@macro\my@lmodern{\fontfamily{lmr}\selectfont}
\fi

%%<+> \myEngineName
% Prints the name of the *LaTeX* engine in use.
\newcommand*\myEngineName{%
  \ifx P\my@engine
    \ifmy@pdf pdf\/\fi \LaTeX
  \else\ifx o\my@engine Lamed% forget about Lambda
  \else\ifx L\my@engine Lua\LaTeX
  \else\ifx I\my@engine Luajit\LaTeX
  \else\ifx X\my@engine \begingroup
      \my@lmodern X\kern-.125em\lower.5ex\hbox{\char"018E}%
      \kern-.075em\LaTeX
    \endgroup
  \else\ifx p\my@engine p\LaTeX
  \else\ifx u\my@engine up\LaTeX
  \else\ifx A\my@engine Ap\LaTeX
  \else\ifx j\my@engine
    \leavevmode\lower.5ex\hbox{J}\kern-.1em \LaTeX
  \else\ifx H\my@engine Hi\LaTeX
  \else \LaTeX % good old...
  \fi\fi\fi\fi\fi\fi\fi\fi\fi\fi
  \ifx\my@variant\relax\else \ (\my@variant)\fi}

%--------------------------------------- dvips or dvipdfmx

%% \my@hedge
\def\my@hedge{2\paperwidth}

%% \my@ps
\def\my@ps#1{\special{ps:#1}}

%% \my@F
\def\my@F{gsave newpath F grestore 0 () gsave p grestore pop }

%% \my@dvips@or@pdf{<dvips>}{<dvipdfmx>}
\def\my@dvips@or@pdf#1#2{%
  \leavevmode
  \my@ps{currentpoint pop /F exch def}%
  \kern\my@hedge
  \my@ps{currentpoint pop \my@F sub /F exch def \my@F neg 0 translate}%
  \hb@xt@\z@{#1\hss}%
  \kern-\my@hedge \hb@xt@\z@{#2\hss}%
  \my@ps{\my@F 0 translate}}

%%<+> \myDviPsOrPdf{<trailer>}
% Prints either 'dvips' or 'dvipdfm' plus the trailer, according
% to the DVI driver actually in use. Note that the reference point
% does not advance.
\newcommand*\myDviPsOrPdf[1]{%
  \my@dvips@or@pdf{dvips#1}{dvipdfmx#1}}

%--------------------------------------- main

%%<+> \myWorkflowName{<trailer>}
% Print the name of current workflow, and then insert paragraph break.
\newcommand*\myWorkflowName[1]{%
  \myEngineName
  \ifmy@pdf #1%
  \else \space+\space \myDviPsOrPdf{#1}%
  \fi
  \par}% avoid the influence of zero-advance

%%<+> \myGreet
% Prints the greeting to the current workflow.
\newcommand*\myGreet{%
  \par Hello, \myWorkflowName{!}}

\makeatother  %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code END
\begin{document}
\myGreet
\end{document}