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\ifPDFTeX 无法识别 pdflatex

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\ifPDFTeX 无法识别 pdflatex

我iftex从 CTAN 下载并安装了它。我想我是按照文档使用它的：

\documentclass{article}
\usepackage{iftex}
\ifPDFTeX
  \errmessage{Don't use pdflatex.  Use plain latex and dvipdf}
\else
  \errmessage{Good. You aren't using pdflatex}
\fi
\end

但是，此构造无法识别。无论我使用还是 ，pdflatex我都会收到消息“很好。您没有使用 pdflatex” 。latexpdflatex

$ latex iftex.tex
This is pdfeTeX, Version 3.141592-1.21a-2.2 (Web2C 7.5.4)
entering extended mode
(./iftex.tex
LaTeX2e <2003/12/01>
Babel <v3.8d> and hyphenation patterns for american, french, german, ngerman, b
ahasa, basque, bulgarian, catalan, croatian, czech, danish, dutch, esperanto, e
stonian, finnish, greek, icelandic, irish, italian, latin, magyar, norsk, polis
h, portuges, romanian, russian, serbian, slovak, slovene, spanish, swedish, tur
kish, ukrainian, nohyphenation, loaded.
(/usr/share/texmf/tex/latex/base/article.cls
Document Class: article 2004/02/16 v1.4f Standard LaTeX document class
(/usr/share/texmf/tex/latex/base/size10.clo))
(/usr/share/texmf/tex/latex/iftex/iftex.sty)
! Good. You aren't using pdflatex.
l.6   \errmessage{Good. You aren't using pdflatex}

? x
No pages of output.
Transcript written on iftex.log.
$
$ pdflatex iftex.tex
This is pdfeTeX, Version 3.141592-1.21a-2.2 (Web2C 7.5.4)
entering extended mode
(./iftex.tex
LaTeX2e <2003/12/01>
Babel <v3.8d> and hyphenation patterns for american, french, german, ngerman,
[...]
nohyphenation, loaded.
(/usr/share/texmf/tex/latex/base/article.cls
Document Class: article 2004/02/16 v1.4f Standard LaTeX document class
(/usr/share/texmf/tex/latex/base/size10.clo))
(/usr/share/texmf/tex/latex/iftex/iftex.sty)
! Good. You aren't using pdflatex.
l.6   \errmessage{Good. You aren't using pdflatex}

? x
No pages of output.
Transcript written on iftex.log.
$

难道我做错了什么？

答案1

这\ifPDFTEX正在检查引擎正在使用（TeX90 vs pdfTeX vs XeTeX vs LuaTeX），而不是输出格式. 为此，您需要以下ifpdf包

\documentclass{article}
\usepackage{ifpdf}
\ifpdf
  \errmessage{Don't use pdflatex.  Use plain latex and dvipdf}
\else
  \errmessage{Good. You aren't using pdflatex}
\fi
\end

答案2

在最近的 TeX 发行版中，latex 实际上是指向 pdftex（实际上是 pdfetex）的符号链接（或在不支持符号链接的系统中等效），它以 dvi 模式运行 latex 格式。

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