3 files changed, 62 insertions, 5 deletions
diff --git a/lib/rubygems/command.rb b/lib/rubygems/command.rb
index badc21023a..f4688d793b 100644
--- a/lib/rubygems/command.rb
+++ b/lib/rubygems/command.rb
@@ -630,7 +630,11 @@ RubyGems is a package manager for Ruby.
   Usage:
     gem -h/--help
     gem -v/--version
-    gem command [arguments...] [options...]
+    gem [global options...] command [arguments...] [options...]
+
+  Global options:
+    -C PATH                      run as if gem was started in <PATH>
+                                 instead of the current working directory
 
   Examples:
     gem install rake
diff --git a/lib/rubygems/command_manager.rb b/lib/rubygems/command_manager.rb
index 8d31d85b44..1bdbd50530 100644
--- a/lib/rubygems/command_manager.rb
+++ b/lib/rubygems/command_manager.rb
@@ -175,14 +175,20 @@ class Gem::CommandManager
     when "-v", "--version" then
       say Gem::VERSION
       terminate_interaction 0
+    when "-C" then
+      args.shift
+      start_point = args.shift
+      if Dir.exist?(start_point)
+        Dir.chdir(start_point) { invoke_command(args, build_args) }
+      else
+        alert_error clean_text("#{start_point} isn't a directory.")
+        terminate_interaction 1
+      end
     when /^-/ then
       alert_error clean_text("Invalid option: #{args.first}. See 'gem --help'.")
       terminate_interaction 1
     else
-      cmd_name = args.shift.downcase
-      cmd = find_command cmd_name
-      cmd.deprecation_warning if cmd.deprecated?
-      cmd.invoke_with_build_args args, build_args
+      invoke_command(args, build_args)
     end
   end
 
@@ -237,4 +243,11 @@ class Gem::CommandManager
       ui.backtrace e
     end
   end
+
+  def invoke_command(args, build_args)
+    cmd_name = args.shift.downcase
+    cmd = find_command cmd_name
+    cmd.deprecation_warning if cmd.deprecated?
+    cmd.invoke_with_build_args args, build_args
+  end
 end
diff --git a/test/rubygems/test_gem_command_manager.rb b/test/rubygems/test_gem_command_manager.rb
index 0b533329a9..6b34156187 100644
--- a/test/rubygems/test_gem_command_manager.rb
+++ b/test/rubygems/test_gem_command_manager.rb
@@ -126,6 +126,46 @@ class TestGemCommandManager < Gem::TestCase
     @command_manager.unregister_command :crash
   end
 
+  def test_process_args_with_c_flag
+    custom_start_point = File.join @tempdir, "nice_folder"
+    FileUtils.mkdir_p custom_start_point
+
+    execution_path = nil
+    use_ui @ui do
+      @command_manager[:install].when_invoked do
+        execution_path = Dir.pwd
+        true
+      end
+      @command_manager.process_args %W[-C #{custom_start_point} install net-scp-4.0.0.gem --local]
+    end
+
+    assert_equal custom_start_point, execution_path
+  end
+
+  def test_process_args_with_c_flag_without_path
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @command_manager.process_args %w[-C install net-scp-4.0.0.gem --local]
+      end
+    end
+
+    assert_match(/install isn't a directory./i, @ui.error)
+  end
+
+  def test_process_args_with_c_flag_path_not_found
+    custom_start_point = File.join @tempdir, "nice_folder"
+    FileUtils.mkdir_p custom_start_point
+    custom_start_point.tr!("_", "-")
+
+    use_ui @ui do
+      assert_raise Gem::MockGemUi::TermError do
+        @command_manager.process_args %W[-C #{custom_start_point} install net-scp-4.0.0.gem --local]
+      end
+    end
+
+    assert_match(/#{custom_start_point} isn't a directory./i, @ui.error)
+  end
+
   def test_process_args_bad_arg
     use_ui @ui do
       assert_raise Gem::MockGemUi::TermError do