Harden parser error handling and builtin usage checks

ruby/shell/builtins.rb

@@ -24,6 +24,11 @@ module Shell
     #################
 
     def builtin_bg(args)
+      if args.empty?
+        logger.warn "Usage: bg <command>"
+        return -1
+      end
+
       cmd = args.shift
       job_control.exec_command(cmd, args, background: true)
     end
@@ -67,10 +72,16 @@ module Shell
     end
 
     def builtin_export(args)
+      if args.count != 1 || args.first.nil? || !args.first.include?("=")
+        logger.warn "Usage: export NAME=value"
+        return -1
+      end
+
       # only supports one variable and doesn't support quoting
       name, *value_parts = args.first.strip.split("=")
       if name.nil? || name.empty?
         logger.warn "#{red("[ERROR]")} Invalid export command"
+        return -1
       else
         ENV[name] = value_parts.join("=").gsub(/\$\w+/) { |m| ENV[m[1..]] || "" }
       end

ruby/shell/cli.rb

@@ -21,7 +21,9 @@ module Shell
       if options[:command]
         logger.verbose "Executing command: #{options[:command]}"
         print_logs
-        exit repl.process_command(options[:command])
+        status = repl.process_command(options[:command])
+        print_logs
+        exit status
       elsif $stdin.isatty
         repl.start(options: options)
       end

ruby/shell/repl.rb

@@ -70,7 +70,7 @@ module Shell
         end
       end
       result
-    rescue Errno => e
+    rescue StandardError => e
       warn "#{red("[ERROR]")} #{e.message}"
       -1
     end

ruby/shell/string_parser.rb

@@ -21,6 +21,9 @@ module Shell
               next
             when "&"
               if line[i + 1] == "&"
+                if command.strip.empty?
+                  raise ArgumentError, "syntax error near unexpected token `&&`"
+                end
                 commands << {command: command, op: next_op}
                 command = +""
                 next_op = :and
@@ -63,6 +66,10 @@ module Shell
           i += 1
         end
 
+        if next_op == :and && command.strip.empty?
+          raise ArgumentError, "syntax error: expected command after `&&`"
+        end
+
         commands << {command: command, op: next_op}
         commands
       end

ruby/test/shell_test.rb

@@ -1,5 +1,6 @@
 require "minitest/autorun"
 require "etc"
+require "open3"
 require "timeout"
 $LOAD_PATH.unshift(File.expand_path("..", __dir__))
 require_relative "../shell/job_control"
@@ -130,6 +131,34 @@ class ShellTest < Minitest::Test
     assert_equal "`echo hi`", %x(#{A1_PATH} -c 'echo "\\`echo hi\\`"').chomp
   end
 
+  def test_reports_parse_errors_without_ruby_backtrace
+    _stdout, stderr, status = Open3.capture3(A1_PATH, "-c", "echo \"unterminated")
+    refute status.success?
+    refute_match(/\.rb:\d+:in /, stderr)
+  end
+
+  def test_export_without_args_does_not_raise_nomethoderror
+    _stdout, stderr, status = Open3.capture3(A1_PATH, "-c", "export")
+    refute status.success?
+    refute_match(/NoMethodError|undefined method/, stderr)
+  end
+
+  def test_bg_without_command_reports_usage_error
+    _stdout, stderr, status = Open3.capture3(A1_PATH, "-c", "bg")
+    refute status.success?
+    assert_match(/Usage: bg <command>/, stderr)
+  end
+
+  def test_rejects_empty_command_around_and_operator
+    _stdout1, stderr1, status1 = Open3.capture3(A1_PATH, "-c", "&& echo hi")
+    refute status1.success?
+    assert_match(/syntax/i, stderr1)
+
+    _stdout2, stderr2, status2 = Open3.capture3(A1_PATH, "-c", "echo hi &&")
+    refute status2.success?
+    assert_match(/syntax/i, stderr2)
+  end
+
   #################################
   ### Execution and job control ###
   #################################