diff --git a/test/irb/test_cmd.rb b/test/irb/test_cmd.rb
index e8c959ec39956f..fd0a02a7afd55e 100644
--- a/test/irb/test_cmd.rb
+++ b/test/irb/test_cmd.rb
@@ -23,6 +23,9 @@ def setup
       save_encodings
       IRB.instance_variable_get(:@CONF).clear
       @is_win = (RbConfig::CONFIG['host_os'] =~ /mswin|msys|mingw|cygwin|bccwin|wince|emc/)
+      STDIN.singleton_class.define_method :tty? do
+        false
+      end
     end
 
     def teardown
@@ -31,6 +34,7 @@ def teardown
       Dir.chdir(@pwd)
       FileUtils.rm_rf(@tmpdir)
       restore_encodings
+      STDIN.singleton_class.remove_method :tty?
     end
 
     def execute_lines(*lines, conf: {}, main: self, irb_path: nil)
@@ -665,16 +669,6 @@ def test_whereami_alias
 
 
   class ShowCmdsTest < CommandTestCase
-    def setup
-      STDIN.singleton_class.define_method :tty? do
-        false
-      end
-    end
-
-    def teardown
-      STDIN.singleton_class.remove_method :tty?
-    end
-
     def test_show_cmds
       out, err = execute_lines(
         "show_cmds\n"
@@ -687,16 +681,6 @@ def test_show_cmds
   end
 
   class LsTest < CommandTestCase
-    def setup
-      STDIN.singleton_class.define_method :tty? do
-        false
-      end
-    end
-
-    def teardown
-      STDIN.singleton_class.remove_method :tty?
-    end
-
     def test_ls
       out, err = execute_lines(
         "class P\n",