nixpkgs/pkgs/development/libraries/fontconfig/make-fonts-cache.nix

{ runCommand, lib, writeText, fontconfig, fontDirectories }:

runCommand "fc-cache"
  rec {
    buildInputs = [ fontconfig.bin ];
    passAsFile = [ "fontDirs" ];
    fontDirs = ''
      <!-- Font directories -->
      ${lib.concatStringsSep "\n" (map (font: "<dir>${font}</dir>") fontDirectories)}
    '';
  }
  ''
    export FONTCONFIG_FILE=$(pwd)/fonts.conf

    cat > fonts.conf << EOF
    <?xml version='1.0'?>
    <!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
    <fontconfig>
      <include>${fontconfig.out}/etc/fonts/fonts.conf</include>
      <cachedir>$out</cachedir>
    EOF
    cat "$fontDirsPath" >> fonts.conf
    echo "</fontconfig>" >> fonts.conf

    mkdir -p $out
    fc-cache -sv
  ''