Create a default command to run help if there are no arguments but to fail with non-zero exit code if the command is invalid.

Props to @ggilder.

bin/jekyll

@@ -12,7 +12,7 @@ program :name, 'jekyll'
 program :version, Jekyll::VERSION
 program :description, 'Jekyll is a blog-aware, static site generator in Ruby'
 
-default_command :help
+default_command :default
 
 global_option '-s', '--source [DIR]', 'Source directory (defaults to ./)'
 global_option '-d', '--destination [DIR]', 'Destination directory (defaults to ./_site)'
@@ -33,6 +33,16 @@ def normalize_options(options)
   options
 end
 
+command :default do |c|
+  c.action do |args, options|
+    if args.empty?
+      command(:help).run
+    else
+      Jekyll.logger.abort_with "Invalid command. Use --help for more information"
+    end
+  end
+end
+
 command :new do |c|
   c.syntax = 'jekyll new PATH'
   c.description = 'Creates a new Jekyll site scaffold in PATH'