fix up nix-shell-wrapper for .binder feature

repo2docker/buildpacks/nix/nix-shell-wrapper

@@ -8,16 +8,25 @@ _term() {
 
 trap _term SIGTERM
 
+# if there is a binder/ sub-directory it takes precedence
+# files outside it are ignored
 # find binder sub-directory (if present)
-for dir in "./.binder" "./binder"; do
+binder_dir="./"
+for dir in "./binder" "./.binder" ; do
   if [ -e $dir ]; then
     binder_dir=$dir
+    break
   fi
 done
 
-# if there is a binder/ sub-directory it takes precedence
-# files outside it are ignored
-if [ -z $binder_dir ]; then
+# raise error if both binder and .binder are found
+if [[ -d "./binder" && -d "./.binder" ]]; then
+  echo "Error: Found both binder and .binder directories."
+  exit 1
+fi
+
+echo "binder_dir is: $binder_dir"
+
 nixpath="$binder_dir/default.nix";
 if [ -f $binder_dir/start ]; then
   chmod u+x $binder_dir/start
@@ -32,15 +41,6 @@ if [ -z $binder_dir ]; then
 else
   nix-shell $nixpath --command "$*" &
 fi
-else
-  nixpath="./default.nix";
-  if [ -f ./start ]; then
-    chmod u+x ./start
-    nix-shell $nixpath --command "./start $*" &
-  else
-    nix-shell $nixpath --command "$*" &
-  fi
-fi
 
 PID=$!
 wait "$PID"