@@ -1,12 +1,17 @@
 plugin_add_command() {
+  if [ "$#" -ne 2 ]; then
+    display_error "usage: asdf plugin-add <name> <git-url>"
+    exit 1
+  fi
+
   local plugin_name=$1
   local source_url=$2
   local plugin_path=$(get_plugin_path $plugin_name)
 
   mkdir -p $(asdf_dir)/plugins
 
   if [ -d $plugin_path ]; then
-    echo "Plugin named $plugin_name already added"
+    display_error "Plugin named $plugin_name already added"
     exit 1
   else
     git clone $source_url $plugin_path
 
@@ -1,4 +1,9 @@
 plugin_update_command() {
+  if [ "$#" -ne 1 ]; then
+    display_error "usage: asdf plugin-update {<name> | --all}"
+    exit 1
+  fi
+
   local plugin_name=$1
   if [ "$plugin_name" = "--all" ]; then
     for dir in $(asdf_dir)/plugins/*; do