include/input get relative paths wrong when doing out-of-source compilation

Question

I have a document which compiles fine. I am trying to refactor it into separate files but the input command seems to be basing its paths relative to the build directory rather than the source directory.

My directory structure:

project/src/main.tex
project/build/

main.tex looks initially like this, simplified

\documentclass[a4paper,12pt]{book}
\renewcommand{\thesection}{\thechapter.\arabic{section}}
\begin{document}
...
\end{document}

If I do this

$> cd project/build
$> pdflatex ../src/main.tex

then I get the desired output in project/build/ with no junk added to project/src, great.

Now move \renewcommand... into project/src/commands.tex and edit main.tex

\documentclass[a4paper,12pt]{book}
\input{commmands}
\begin{document}
...
\end{document}

Now when I run pdflatex it halts on the second line:

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

If I edit main.tex thus:

\documentclass[a4paper,12pt]{book}
\input{../src/commands}
\begin{document}
...
\end{document}

then it compiles fine.

This is clearly idiotic. The source code should not care about the working directory of the compiler invocation.

How to I write main.tex so that it includes all other files relative to its own path (and for this behaviour to apply to includes-of-includes and so on) so that I can compile the document out-of-source?

Answer 1

Just use

\documentclass[a4paper,12pt]{book}
\input{commands}
\begin{document}
...
\end{document}

or better call the file commands.sty not commands.tex and use

\documentclass[a4paper,12pt]{book}
\usepackage{commands}
\begin{document}
...
\end{document}

with a calling sequence of

cd project/build
TEXINPUTS=../src//:
pdflatex main.tex