Only try building R files if there are some

Makefile

@@ -78,7 +78,7 @@ HTML_DST = \
 
 ## lesson-rmd:    : convert Rmarkdown files to markdown
 lesson-rmd: $(RMD_SRC)
-	@bin/knit_lessons.sh
+	@bin/knit_lessons.sh $(RMD_SRC)
 
 ## lesson-check   : validate lesson Markdown.
 lesson-check :

bin/knit_lessons.sh

 #!/usr/bin/env bash
 
-if [ -d "_episodes_rmd" ] ; then
+# Only try running R to translate files if there are some files present.
+# The Makefile passes in the names of files.
+
+if [ $# -ne 0 ] ; then
     Rscript -e "source('bin/generate_md_episodes.R')"
 fi