Add .cd - to return to prior working directory

[Finishes #1028]

lib/pry/commands/shell_command.rb

@@ -17,12 +17,7 @@ class Pry
 
     def process(cmd)
       if cmd =~ /^cd\s+(.+)/i
-        dest = $1
+        process_cd parse_destination($1)
-        begin
-          Dir.chdir File.expand_path(dest)
-        rescue Errno::ENOENT
-          raise CommandError, "No such directory: #{dest}"
-        end
       else
         pass_block(cmd)
 
@@ -37,6 +32,20 @@ class Pry
     def complete(search)
       super + Bond::Rc.files(search.split(" ").last || '')
     end
+
+    private
+
+      def parse_destination(dest)
+        return dest unless dest == "-"
+        state.old_pwd || raise(CommandError, "No prior directory available")
+      end
+
+      def process_cd(dest)
+        state.old_pwd = Dir.pwd
+        Dir.chdir File.expand_path(dest)
+      rescue Errno::ENOENT
+        raise CommandError, "No such directory: #{dest}"
+      end
   end
 
   Pry::Commands.add_command(Pry::Command::ShellCommand)

spec/commands/shell_command_spec.rb

@@ -0,0 +1,56 @@
+require 'helper'
+
+describe "Command::ShellCommand" do
+  describe 'cd' do
+    before do
+      @o = Object.new
+
+      @t = pry_tester(@o) do
+        def command_state
+          pry.command_state[Pry::Command::ShellCommand.match]
+        end
+      end
+    end
+
+    describe ".cd" do
+      before do
+        Dir.stubs(:chdir)
+      end
+
+      it "saves the current working directory" do
+        Dir.stubs(:pwd).returns("initial_path")
+
+        @t.eval ".cd new_path"
+        @t.command_state.old_pwd.should == "initial_path"
+      end
+
+      describe "given a path" do
+        it "sends the path to File.expand_path" do
+          Dir.expects(:chdir).with(File.expand_path("new_path"))
+          @t.eval ".cd new_path"
+        end
+      end
+
+      describe "given a dash" do
+        describe "given no prior directory" do
+          it "raises the correct error" do
+            lambda { @t.eval ".cd -" }.should.raise(StandardError).
+              message.should == "No prior directory available"
+          end
+        end
+
+        describe "given a prior directory" do
+          it "sends the user's last pry working directory to File.expand_path" do
+            Dir.stubs(:pwd).returns("initial_path")
+
+            Dir.expects(:chdir).with(File.expand_path("new_path"))
+            @t.eval ".cd new_path"
+
+            Dir.expects(:chdir).with(File.expand_path("initial_path"))
+            @t.eval ".cd -"
+          end
+        end
+      end
+    end
+  end
+end