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diff --git a/railties/lib/rails/app_loader.rb b/railties/lib/rails/app_loader.rb
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+++ b/railties/lib/rails/app_loader.rb
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+# frozen_string_literal: true
+
+require "pathname"
+require "rails/version"
+
+module Rails
+  module AppLoader # :nodoc:
+    extend self
+
+    RUBY = Gem.ruby
+    EXECUTABLES = ["bin/rails", "script/rails"]
+    BUNDLER_WARNING = <<EOS
+Beginning in Rails 4, Rails ships with a `rails` binstub at ./bin/rails that
+should be used instead of the Bundler-generated `rails` binstub.
+
+If you are seeing this message, your binstub at ./bin/rails was generated by
+Bundler instead of Rails.
+
+You might need to regenerate your `rails` binstub locally and add it to source
+control:
+
+ rails app:update:bin           # Bear in mind this generates other binstubs
+                                # too that you may or may not want (like yarn)
+
+If you already have Rails binstubs in source control, you might be
+inadverently overwriting them during deployment by using bundle install
+with the --binstubs option.
+
+If your application was created prior to Rails 4, here's how to upgrade:
+
+  bundle config --delete bin    # Turn off Bundler's stub generator
+  rails app:update:bin          # Use the new Rails executables
+  git add bin                   # Add bin/ to source control
+
+You may need to remove bin/ from your .gitignore as well.
+
+When you install a gem whose executable you want to use in your app,
+generate it and add it to source control:
+
+  bundle binstubs some-gem-name
+  git add bin/new-executable
+
+EOS
+
+    def exec_app
+      original_cwd = Dir.pwd
+
+      loop do
+        if exe = find_executable
+          contents = File.read(exe)
+
+          if contents =~ /(APP|ENGINE)_PATH/
+            exec RUBY, exe, *ARGV
+            break # non reachable, hack to be able to stub exec in the test suite
+          elsif exe.end_with?("bin/rails") && contents.include?("This file was generated by Bundler")
+            $stderr.puts(BUNDLER_WARNING)
+            Object.const_set(:APP_PATH, File.expand_path("config/application", Dir.pwd))
+            require File.expand_path("../boot", APP_PATH)
+            require "rails/commands"
+            break
+          end
+        end
+
+        # If we exhaust the search there is no executable, this could be a
+        # call to generate a new application, so restore the original cwd.
+        Dir.chdir(original_cwd) && return if Pathname.new(Dir.pwd).root?
+
+        # Otherwise keep moving upwards in search of an executable.
+        Dir.chdir("..")
+      end
+    end
+
+    def find_executable
+      EXECUTABLES.find { |exe| File.file?(exe) }
+    end
+  end
+end