Expansion: argument of \input handled differently on Windows and Linux

Question

Consider the following files main.tex and xyz.tex:

% main.tex
\documentclass{article}
\begin{document}
\newcommand\abc{XYZ.tex}
\input{\MakeLowercase{\abc}}
\end{document}
% xyz.tex (all lowercase letters)
Hey, it works!

LaTeXing main.tex works under Windows (would even work without trying to lowercase the filename, as filenames are case-insensitive), while under Linux, it gives the error

! LaTeX Error: File `MakeLowercase {XYZ.tex}' not found.

Question: Why does it work under Windows, even though \MakeLowercase is not expandable? Shouldn't we expect identical behavior across OSes? What would it take to treat macros the same in both cases?

(I know how to achieve the desired effect of lowercasing the filename, by using e.g. \explower from the post Expandably change letter case and use inside \csname, without a package.)

are you sure it is the OS rather than latex release? MakeUppercase was re-implemented recently. I am highly surprised either implementation worked — David Carlisle, Oct 14 '22 at 15:14
@DavidCarlisle on windows it fails in texlive 2021, but works in texlive 2022. — Ulrike Fischer, Oct 14 '22 at 15:18
where "works" means some accidental sequence of expansions does something approximating the behaviour intended? — David Carlisle, Oct 14 '22 at 15:20
@UlrikeFischer Thanks for your comments. I'm aware that MakeLowercase is not appropriate, but I'm still surprised that Windows does not issue an error. — gernot, Oct 14 '22 at 15:34
Under Windows, it is a recently updated MiKTeX, under Linux it is TeXLive 2022. — gernot, Oct 14 '22 at 15:36
is your tl 2022 up to date, as egreg shows it should work with a June release I think — David Carlisle, Oct 14 '22 at 16:18
@DavidCarlisle That's it. After updating all package of my TL2022, it now also seems to work under Linux. Thanks — gernot, Oct 14 '22 at 17:28

score 7 · Answer 1 · answered Oct 14 '22 at 17:20

7

We can see that LaTeX macros are changed from year to year. But TeX primitives keep unchanged. You can solve your problem by TeX primitive \lowercase:

\lowercase \expandafter{\expandafter\input \expandafter{\abc}}

If we have \def\abc{XYX.tex} then the command above is transformed to

\input {xyz.tex}

Win systems use case insensitive access to the file names(?), so it works without \lowercase too.

answered Oct 14 '22 at 17:20

wipet

74,238

try that with a filename with non-ascii characters, \MakeLowercase has access to the full Unicode mapping, even with pdflatex – David Carlisle Oct 14 '22 at 18:23
3

pdftex is obsolete engine because it doesn't support Unicode. – wipet Oct 14 '22 at 19:03

egreg · Accepted Answer · 2022-10-14T17:36:22.587

Here's what happens on my machine (with a case-sensitive, sort of, file system).

With TL 2022 it seems to work, but I wouldn't rely on it.

TeX Live 2022

! LaTeX Error: File `xyz.tex' not found.

TeX Live 2021

! LaTeX Error: File `MakeLowercase {XYZ.tex}' not found.

TeX Live 2020

Hangs

TeX Live 2019

! Missing endcsname inserted.
<to be read again>
                   protect
l.4 \input{\MakeLowercase{\abc}}

TeX Live 2018 and earlier

! Illegal parameter number in definition of \@filef@und.
<to be read again>
l.4 \input{\MakeLowercase{\abc}}

With a recent LaTeX:

\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\lowercaseinput}{m}
 {
  \file_input:e { \str_lowercase:e { #1 } }
 }
\cs_generate_variant:Nn \file_input:n { e }
\cs_generate_variant:Nn \str_lowercase:n { e }
\ExplSyntaxOff
\begin{document}
\newcommand\abc{XYZ.tex}
\lowercaseinput{\abc}
\renewcommand\abc{XÝŽ.tex}
\lowercaseinput{\abc}
\end{document}

Note that this should also work with (most) UTF-8 characters. The console will show

! LaTeX Error: File 'xyz.tex' not found.
! LaTeX Error: File 'xýž.tex' not found.