Debug/see individual steps in yaml-knitr-pandoc-lua-tex toolchain

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/1756

^{Originally posted by baptiste August 9, 2022}
Is there a way to get more information on the commands that are run by quarto? Specifically, with the previous Rmarkdown workflow I could see exactly what commands were passed to pandoc, which helped a bit debug individual steps in the toolchain (is it a Yaml option passed incorrectly? Is it a pandoc issue? Is it something that knitr produced? etc.)

As a specific example, I'm struggling to set the size of an image and keep the aspect ratio, for latex output. I suspect a combination of pandoc and quarto, but it's hard to tell what's happening.

![](penguins){width=100%}

gives me

\includegraphics[width=1\textwidth,height=\textheight]{penguins.pdf}

(and a stretched image)

while

![](penguins){width=100%,keepaspectratio}

produces

\includegraphics{penguins.pdf}

(this is mostly for the sake of concrete illustration, I'm also interested in the general toolchain debugging question)