How can you check if fontspec (with LuaLaTeX) can load a font?

Question

I want to check if fontspec (with LuaLaTeX) can load a certain font and if not try to load another one. For pdflatex I did by checking if the package file exists; but I don't find an easy way to do it with fontspec.

Answer 1

Updated answer

The patch originally described here is not needed (and may not work in recent releases).

As documented in the fontspec manual, fontspec now has an

\IfFontExistsTF{font name}{yes code}{no code}

command that allows you to test if a font exists.

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Original answer

You can trap the error and make it a warning, just setting a flag, then test for that and load something else, see the code below. Note this is poking into internal interfaces so if it breaks at a fontspec update, don't complain:-)

\documentclass{article}

\usepackage{fontspec}

\newif\ifgoodfont
\makeatletter
\ExplSyntaxOn
\cs_set:Nn \__fontspec_load_font:
 {
  \__fontspec_font_set:Nnn    \l_fontspec_font
     { \__fontspec_fullname:n {\l_fontspec_fontname_up_tl} } {\f@size pt}
  \__fontspec_font_if_null:NT \l_fontspec_font { 
\global\goodfontfalse
\__fontspec_warning:nx {font-not-found} {\l_fontspec_fontname_up_tl} }
  \__fontspec_set_font_type:
  \__fontspec_font_gset:Nnn   \l_fontspec_font
     { \__fontspec_fullname:n {\l_fontspec_fontname_up_tl} } {\f@size pt}
  \l_fontspec_font % this is necessary for LuaLaTeX to check the scripts properly
 }

\cs_set:Nn \__fontspec_load_fontname:n
 {
    \__fontspec_load_external_fontoptions:Nn \l_fontspec_fontname_tl {#1}
    \prop_get:NVNF \g__fontspec_fontopts_prop \l_fontspec_fontname_tl \l__fontspec_fontopts_clist
     { \clist_clear:N \l__fontspec_fontopts_clist }
    \__fontspec_font_set:Nnn \l_fontspec_font {\__fontspec_fullname:n {\l_fontspec_fontname_tl}} {\f@size pt}
    \__fontspec_font_if_null:NT \l_fontspec_font { 
\global\goodfontfalse
\__fontspec_warning:nx {font-not-found} {#1} }
 }


\ExplSyntaxOff
\makeatother

{\scrollmode\global\goodfonttrue
\setmainfont{zzzz}
}

\ifgoodfont
\typeout{(zzzz found (strange:-)}
\else
\typeout{zzzz not found, trying TeX Gyre Bonum}
\global\goodfonttrue
\setmainfont{TeX Gyre Bonum}
\ifgoodfont
\typeout{OK}
\else
\typeout{not found that either, stopping}
\stop
\fi
\fi


\begin{document}
zzz
\end{document}