How to know the number of pages in a PDF file?

Question

I use graphicx to include pages of a PDF file. I want to know in LaTeX how many pages are in the file. How to do this?

Answer 1

pdfpages is obviously somehow able to get the number of pages of an included pdf. It does it with the pdftex primitive \pdflastximagepages. E.g.

\documentclass{article}
\begin{document}
  \pdfximage{test.pdf}%
  \the\pdflastximagepages
\end{document}

Update 2022

There is also an expl3 function which works with all backends:

\documentclass{article}
\usepackage{l3graphics}
\begin{document}
\ExplSyntaxOn
\graphics_get_pagecount:nN{example-image-duck.pdf}\l_tmpa_tl
\l_tmpa_tl
\ExplSyntaxOff
\end{document}