ignore cp if file already exists

scripts/slides_to_html.sh

@@ -11,7 +11,7 @@ do
   file_out=`echo $filename | sed -e 's/\.md/\.html/g'`
   sed -e "s|SLIDEHEAD|$headEscapedForSed|g" $filename | pandoc -s --mathjax -t revealjs -o html/$file_out --slide-level=$SLIDE_LEVEL --highlight-style=$HIGHLIGHT_STYLE -V revealjs-url=https://unpkg.com/reveal.js@5.2.0 --template=./templates/template.revealjs --css custom.css
   if [ -f ./templates/custom.css ]; then
-    cp ./templates/custom.css ./html/custom.css
+    cp -n ./templates/custom.css ./html/custom.css
   fi
   echo output $file_out