Merge pull request #14646 from deivid-rodriguez/provide_byebug_by_default

Improve debugging support

11 files changed, 489 insertions, 294 deletions

diff --git a/activesupport/lib/active_support/core_ext/kernel.rb b/activesupport/lib/active_support/core_ext/kernel.rb
index aa19aed43b..293a3b2619 100644
--- a/activesupport/lib/active_support/core_ext/kernel.rb
+++ b/activesupport/lib/active_support/core_ext/kernel.rb
@@ -1,5 +1,5 @@
 require 'active_support/core_ext/kernel/agnostics'
 require 'active_support/core_ext/kernel/concern'
-require 'active_support/core_ext/kernel/debugger'
+require 'active_support/core_ext/kernel/debugger' if RUBY_VERSION < '2.0.0'
 require 'active_support/core_ext/kernel/reporting'
 require 'active_support/core_ext/kernel/singleton_class'
diff --git a/activesupport/test/core_ext/kernel_test.rb b/activesupport/test/core_ext/kernel_test.rb
index 18b251173f..d8bf81d02b 100644
--- a/activesupport/test/core_ext/kernel_test.rb
+++ b/activesupport/test/core_ext/kernel_test.rb
@@ -137,4 +137,4 @@ class KernelDebuggerTest < ActiveSupport::TestCase
   ensure
     Object.send(:remove_const, :Rails)
   end
-end
+end if RUBY_VERSION < '2.0.0'
diff --git a/guides/source/debugging_rails_applications.md b/guides/source/debugging_rails_applications.md
index 0e10d1b697..042ffa7d8d 100644
--- a/guides/source/debugging_rails_applications.md
+++ b/guides/source/debugging_rails_applications.md
@@ -240,430 +240,560 @@ The contents of the block, and therefore the string interpolation, is only
 evaluated if debug is enabled. This performance savings is only really 
 noticeable with large amounts of logging, but it's a good practice to employ.
 
-Debugging with the `debugger` gem
+Debugging with the `byebug` gem
 ---------------------------------
 
-When your code is behaving in unexpected ways, you can try printing to logs or the console to diagnose the problem. Unfortunately, there are times when this sort of error tracking is not effective in finding the root cause of a problem. When you actually need to journey into your running source code, the debugger is your best companion.
+When your code is behaving in unexpected ways, you can try printing to logs or
+the console to diagnose the problem. Unfortunately, there are times when this
+sort of error tracking is not effective in finding the root cause of a problem.
+When you actually need to journey into your running source code, the debugger
+is your best companion.
 
-The debugger can also help you if you want to learn about the Rails source code but don't know where to start. Just debug any request to your application and use this guide to learn how to move from the code you have written deeper into Rails code.
+The debugger can also help you if you want to learn about the Rails source code
+but don't know where to start. Just debug any request to your application and
+use this guide to learn how to move from the code you have written deeper into
+Rails code.
 
 ### Setup
 
-You can use the `debugger` gem to set breakpoints and step through live code in Rails. To install it, just run:
+You can use the `byebug` gem to set breakpoints and step through live code in
+Rails. To install it, just run:
 
 ```bash
-$ gem install debugger
+$ gem install byebug
 ```
 
-Rails has had built-in support for debugging since Rails 2.0. Inside any Rails application you can invoke the debugger by calling the `debugger` method.
+Inside any Rails application you can then invoke the debugger by calling the
+`byebug` method.
 
 Here's an example:
 
 ```ruby
 class PeopleController < ApplicationController
   def new
-    debugger
+    byebug
     @person = Person.new
   end
 end
 ```
 
-If you see this message in the console or logs:
+### The Shell
+
+As soon as your application calls the `byebug` method, the debugger will be
+started in a debugger shell inside the terminal window where you launched your
+application server, and you will be placed at the debugger's prompt `(byebug)`.
+Before the prompt, the code around the line that is about to be run will be
+displayed and the current line will be marked by '=>'. Like this:
 
 ```
-***** Debugger requested, but was not available: Start server with --debugger to enable *****
+[1, 10] in /PathTo/project/app/controllers/posts_controller.rb
+    3:
+    4:   # GET /posts
+    5:   # GET /posts.json
+    6:   def index
+    7:     byebug
+=>  8:     @posts = Post.find_recent
+    9:
+   10:     respond_to do |format|
+   11:       format.html # index.html.erb
+   12:       format.json { render json: @posts }
+
+(byebug)
 ```
 
-Make sure you have started your web server with the option `--debugger`:
+If you got there by a browser request, the browser tab containing the request
+will be hung until the debugger has finished and the trace has finished
+processing the entire request.
+
+For example:
 
 ```bash
-$ rails server --debugger
 => Booting WEBrick
-=> Rails 4.0.0 application starting on http://0.0.0.0:3000
-=> Debugger enabled
-...
-```
+=> Rails 4.1.0 application starting in development on http://0.0.0.0:3000
+=> Run `rails server -h` for more startup options
+=> Notice: server is listening on all interfaces (0.0.0.0). Consider using 127.0.0.1 (--binding option)
+=> Ctrl-C to shutdown server
+[2014-04-11 13:11:47] INFO  WEBrick 1.3.1
+[2014-04-11 13:11:47] INFO  ruby 2.1.1 (2014-02-24) [i686-linux]
+[2014-04-11 13:11:47] INFO  WEBrick::HTTPServer#start: pid=6370 port=3000
 
-TIP: In development mode, you can dynamically `require \'debugger\'` instead of restarting the server, even if it was started without `--debugger`.
 
-### The Shell
+Started GET "/" for 127.0.0.1 at 2014-04-11 13:11:48 +0200
+  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT "schema_migrations".* FROM "schema_migrations"
+Processing by PostsController#index as HTML
 
-As soon as your application calls the `debugger` method, the debugger will be started in a debugger shell inside the terminal window where you launched your application server, and you will be placed at the debugger's prompt `(rdb:n)`. The _n_ is the thread number. The prompt will also show you the next line of code that is waiting to run.
+[3, 12] in /PathTo/project/app/controllers/posts_controller.rb
+    3:
+    4:   # GET /posts
+    5:   # GET /posts.json
+    6:   def index
+    7:     byebug
+=>  8:     @posts = Post.find_recent
+    9:
+   10:     respond_to do |format|
+   11:       format.html # index.html.erb
+   12:       format.json { render json: @posts }
 
-If you got there by a browser request, the browser tab containing the request will be hung until the debugger has finished and the trace has finished processing the entire request.
+(byebug)
+```
 
-For example:
+Now it's time to explore and dig into your application. A good place to start is
+by asking the debugger for help. Type: `help`
 
-```bash
-@posts = Post.all
-(rdb:7)
 ```
+(byebug) help
 
-Now it's time to explore and dig into your application. A good place to start is by asking the debugger for help. Type: `help`
+byebug 2.7.0
 
-```
-(rdb:7) help
-ruby-debug help v0.10.2
 Type 'help <command-name>' for help on a specific command
 
 Available commands:
-backtrace  delete   enable  help    next  quit     show    trace
-break      disable  eval    info    p     reload   source  undisplay
-catch      display  exit    irb     pp    restart  step    up
-condition  down     finish  list    ps    save     thread  var
-continue   edit     frame   method  putl  set      tmate   where
+backtrace  delete   enable  help       list    pry next  restart  source     up
+break      disable  eval    info       method  ps        save     step       var
+catch      display  exit    interrupt  next    putl      set      thread
+condition  down     finish  irb        p       quit      show     trace
+continue   edit     frame   kill       pp      reload    skip     undisplay
 ```
 
-TIP: To view the help menu for any command use `help <command-name>` at the debugger prompt. For example: _`help var`_
-
-The next command to learn is one of the most useful: `list`. You can abbreviate any debugging command by supplying just enough letters to distinguish them from other commands, so you can also use `l` for the `list` command.
+TIP: To view the help menu for any command use `help <command-name>` at the
+debugger prompt. For example: _`help list`_. You can abbreviate any debugging
+command by supplying just enough letters to distinguish them from other
+commands, so you can also use `l` for the `list` command, for example.
 
-This command shows you where you are in the code by printing 10 lines centered around the current line; the current line in this particular case is line 6 and is marked by `=>`.
+To see the previous ten lines you should type `list-` (or `l-`)
 
 ```
-(rdb:7) list
+(byebug) l-
+
 [1, 10] in /PathTo/project/app/controllers/posts_controller.rb
    1  class PostsController < ApplicationController
-   2    # GET /posts
-   3    # GET /posts.json
-   4    def index
-   5      debugger
-=> 6      @posts = Post.all
-   7
-   8      respond_to do |format|
-   9        format.html # index.html.erb
-   10        format.json { render json: @posts }
-```
+   2    before_action :set_post, only: [:show, :edit, :update, :destroy]
+   3
+   4    # GET /posts
+   5    # GET /posts.json
+   6    def index
+   7      byebug
+   8      @posts = Post.find_recent
+   9
+   10      respond_to do |format|
 
-If you repeat the `list` command, this time using just `l`, the next ten lines of the file will be printed out.
-
-```
-(rdb:7) l
-[11, 20] in /PathTo/project/app/controllers/posts_controller.rb
-   11      end
-   12    end
-   13
-   14    # GET /posts/1
-   15    # GET /posts/1.json
-   16    def show
-   17      @post = Post.find(params[:id])
-   18
-   19      respond_to do |format|
-   20        format.html # show.html.erb
 ```
 
-And so on until the end of the current file. When the end of file is reached, the `list` command will start again from the beginning of the file and continue again up to the end, treating the file as a circular buffer.
+This way you can move inside the file, being able to see the code above and over
+the line where you added the `byebug` call.  Finally, to see where you are in
+the code again you can type `list=`
 
-On the other hand, to see the previous ten lines you should type `list-` (or `l-`)
-
-```
-(rdb:7) l-
-[1, 10] in /PathTo/project/app/controllers/posts_controller.rb
-   1  class PostsController < ApplicationController
-   2    # GET /posts
-   3    # GET /posts.json
-   4    def index
-   5      debugger
-   6      @posts = Post.all
-   7
-   8      respond_to do |format|
-   9        format.html # index.html.erb
-   10        format.json { render json: @posts }
 ```
+(byebug) list=
 
-This way you can move inside the file, being able to see the code above and over the line you added the `debugger`.
-Finally, to see where you are in the code again you can type `list=`
+[3, 12] in /PathTo/project/app/controllers/posts_controller.rb
+    3:
+    4:   # GET /posts
+    5:   # GET /posts.json
+    6:   def index
+    7:     byebug
+=>  8:     @posts = Post.find_recent
+    9:
+   10:     respond_to do |format|
+   11:       format.html # index.html.erb
+   12:       format.json { render json: @posts }
 
-```
-(rdb:7) list=
-[1, 10] in /PathTo/project/app/controllers/posts_controller.rb
-   1  class PostsController < ApplicationController
-   2    # GET /posts
-   3    # GET /posts.json
-   4    def index
-   5      debugger
-=> 6      @posts = Post.all
-   7
-   8      respond_to do |format|
-   9        format.html # index.html.erb
-   10        format.json { render json: @posts }
+(byebug)
 ```
 
 ### The Context
 
-When you start debugging your application, you will be placed in different contexts as you go through the different parts of the stack.
-
-The debugger creates a context when a stopping point or an event is reached. The context has information about the suspended program which enables a debugger to inspect the frame stack, evaluate variables from the perspective of the debugged program, and contains information about the place where the debugged program is stopped.
-
-At any time you can call the `backtrace` command (or its alias `where`) to print the backtrace of the application. This can be very helpful to know how you got where you are. If you ever wondered about how you got somewhere in your code, then `backtrace` will supply the answer.
-
-```
-(rdb:5) where
-    #0 PostsController.index
-       at line /PathTo/project/app/controllers/posts_controller.rb:6
-    #1 Kernel.send
-       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
-    #2 ActionController::Base.perform_action_without_filters
-       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
-    #3 ActionController::Filters::InstanceMethods.call_filters(chain#ActionController::Fil...,...)
-       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/filters.rb:617
+When you start debugging your application, you will be placed in different
+contexts as you go through the different parts of the stack.
+
+The debugger creates a context when a stopping point or an event is reached. The
+context has information about the suspended program which enables the debugger
+to inspect the frame stack, evaluate variables from the perspective of the
+debugged program, and contains information about the place where the debugged
+program is stopped.
+
+At any time you can call the `backtrace` command (or its alias `where`) to print
+the backtrace of the application. This can be very helpful to know how you got
+where you are. If you ever wondered about how you got somewhere in your code,
+then `backtrace` will supply the answer.
+
+```
+(byebug) where
+--> #0  PostsController.index
+      at /PathTo/project/test_app/app/controllers/posts_controller.rb:8
+    #1  ActionController::ImplicitRender.send_action(method#String, *args#Array)
+      at /PathToGems/actionpack-4.1.0/lib/action_controller/metal/implicit_render.rb:4
+    #2  AbstractController::Base.process_action(action#NilClass, *args#Array)
+      at /PathToGems/actionpack-4.1.0/lib/abstract_controller/base.rb:189
+    #3  ActionController::Rendering.process_action(action#NilClass, *args#NilClass)
+      at /PathToGems/actionpack-4.1.0/lib/action_controller/metal/rendering.rb:10
 ...
 ```
 
-You move anywhere you want in this trace (thus changing the context) by using the `frame _n_` command, where _n_ is the specified frame number.
+The current frame is marked with `-->`. You can move anywhere you want in this
+trace (thus changing the context) by using the `frame _n_` command, where _n_ is
+the specified frame number. If you do that, `byebug` will display your new
+context.
 
 ```
-(rdb:5) frame 2
-#2 ActionController::Base.perform_action_without_filters
-       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
+(byebug) frame 2
+
+[184, 193] in /PathToGems/actionpack-4.1.0/lib/abstract_controller/base.rb
+   184:       # is the intended way to override action dispatching.
+   185:       #
+   186:       # Notice that the first argument is the method to be dispatched
+   187:       # which is *not* necessarily the same as the action name.
+   188:       def process_action(method_name, *args)
+=> 189:         send_action(method_name, *args)
+   190:       end
+   191:
+   192:       # Actually call the method associated with the action. Override
+   193:       # this method if you wish to change how action methods are called,
+
+(byebug)
 ```
 
-The available variables are the same as if you were running the code line by line. After all, that's what debugging is.
+The available variables are the same as if you were running the code line by
+line. After all, that's what debugging is.
 
-Moving up and down the stack frame: You can use `up [n]` (`u` for abbreviated) and `down [n]` commands in order to change the context _n_ frames up or down the stack respectively. _n_ defaults to one. Up in this case is towards higher-numbered stack frames, and down is towards lower-numbered stack frames.
+You can also use `up [n]` (`u` for abbreviated) and `down [n]` commands in order
+to change the context _n_ frames up or down the stack respectively. _n_ defaults
+to one. Up in this case is towards higher-numbered stack frames, and down is
+towards lower-numbered stack frames.
 
 ### Threads
 
-The debugger can list, stop, resume and switch between running threads by using the command `thread` (or the abbreviated `th`). This command has a handful of options:
+The debugger can list, stop, resume and switch between running threads by using
+the command `thread` (or the abbreviated `th`). This command has a handful of
+options:
 
 * `thread` shows the current thread.
-* `thread list` is used to list all threads and their statuses. The plus + character and the number indicates the current thread of execution.
+* `thread list` is used to list all threads and their statuses. The plus +
+character and the number indicates the current thread of execution.
 * `thread stop _n_` stop thread _n_.
 * `thread resume _n_` resumes thread _n_.
 * `thread switch _n_` switches the current thread context to _n_.
 
-This command is very helpful, among other occasions, when you are debugging concurrent threads and need to verify that there are no race conditions in your code.
+This command is very helpful, among other occasions, when you are debugging
+concurrent threads and need to verify that there are no race conditions in your
+code.
 
 ### Inspecting Variables
 
-Any expression can be evaluated in the current context. To evaluate an expression, just type it!
-
-This example shows how you can print the instance_variables defined within the current context:
-
-```
-@posts = Post.all
-(rdb:11) instance_variables
-["@_response", "@action_name", "@url", "@_session", "@_cookies", "@performed_render", "@_flash", "@template", "@_params", "@before_filter_chain_aborted", "@request_origin", "@_headers", "@performed_redirect", "@_request"]
-```
-
-As you may have figured out, all of the variables that you can access from a controller are displayed. This list is dynamically updated as you execute code. For example, run the next line using `next` (you'll learn more about this command later in this guide).
-
-```
-(rdb:11) next
-Processing PostsController#index (for 127.0.0.1 at 2008-09-04 19:51:34) [GET]
-  Session ID: BAh7BiIKZmxhc2hJQzonQWN0aW9uQ29udHJvbGxlcjo6Rmxhc2g6OkZsYXNoSGFzaHsABjoKQHVzZWR7AA==--b16e91b992453a8cc201694d660147bba8b0fd0e
-  Parameters: {"action"=>"index", "controller"=>"posts"}
-/PathToProject/posts_controller.rb:8
-respond_to do |format|
+Any expression can be evaluated in the current context. To evaluate an
+expression, just type it!
+
+This example shows how you can print the instance_variables defined within the
+current context:
+
+```
+[3, 12] in /PathTo/project/app/controllers/posts_controller.rb
+    3:
+    4:   # GET /posts
+    5:   # GET /posts.json
+    6:   def index
+    7:     byebug
+=>  8:     @posts = Post.find_recent
+    9:
+   10:     respond_to do |format|
+   11:       format.html # index.html.erb
+   12:       format.json { render json: @posts }
+
+(byebug) instance_variables
+[:@_action_has_layout, :@_routes, :@_headers, :@_status, :@_request,
+ :@_response, :@_env, :@_prefixes, :@_lookup_context, :@_action_name,
+ :@_response_body, :@marked_for_same_origin_verification, :@_config]
+```
+
+As you may have figured out, all of the variables that you can access from a
+controller are displayed. This list is dynamically updated as you execute code.
+For example, run the next line using `next` (you'll learn more about this
+command later in this guide).
+
+```
+(byebug) next
+[5, 14] in /PathTo/project/app/controllers/posts_controller.rb
+   5     # GET /posts.json
+   6     def index
+   7       byebug
+   8       @posts = Post.find_recent
+   9
+=> 10       respond_to do |format|
+   11         format.html # index.html.erb
+   12        format.json { render json: @posts }
+   13      end
+   14    end
+   15
+(byebug)
 ```
 
 And then ask again for the instance_variables:
 
 ```
-(rdb:11) instance_variables.include? "@posts"
+(byebug) instance_variables.include? "@posts"
 true
 ```
 
-Now `@posts` is included in the instance variables, because the line defining it was executed.
+Now `@posts` is included in the instance variables, because the line defining it
+was executed.
 
-TIP: You can also step into **irb** mode with the command `irb` (of course!). This way an irb session will be started within the context you invoked it. But be warned: this is an experimental feature.
+TIP: You can also step into **irb** mode with the command `irb` (of course!).
+This way an irb session will be started within the context you invoked it. But
+be warned: this is an experimental feature.
 
-The `var` method is the most convenient way to show variables and their values:
+The `var` method is the most convenient way to show variables and their values.
+Let's let `byebug` to help us with it.
 
 ```
-var
-(rdb:1) v[ar] const <object>            show constants of object
-(rdb:1) v[ar] g[lobal]                  show global variables
-(rdb:1) v[ar] i[nstance] <object>       show instance variables of object
-(rdb:1) v[ar] l[ocal]                   show local variables
+(byebug) help var
+v[ar] cl[ass]                   show class variables of self
+v[ar] const <object>            show constants of object
+v[ar] g[lobal]                  show global variables
+v[ar] i[nstance] <object>       show instance variables of object
+v[ar] l[ocal]                   show local variables
 ```
 
-This is a great way to inspect the values of the current context variables. For example:
+This is a great way to inspect the values of the current context variables. For
+example, to check that we have no local variables currently defined.
 
 ```
-(rdb:9) var local
-  __dbg_verbose_save => false
+(byebug) var local
+(byebug)
 ```
 
 You can also inspect for an object method this way:
 
 ```
-(rdb:9) var instance Post.new
-@attributes = {"updated_at"=>nil, "body"=>nil, "title"=>nil, "published"=>nil, "created_at"...
+(byebug) var instance Post.new
+@_start_transaction_state = {}
+@aggregation_cache = {}
+@association_cache = {}
+@attributes = {"id"=>nil, "created_at"=>nil, "updated_at"=>nil}
 @attributes_cache = {}
-@new_record = true
+@changed_attributes = nil
+...
 ```
 
-TIP: The commands `p` (print) and `pp` (pretty print) can be used to evaluate Ruby expressions and display the value of variables to the console.
+TIP: The commands `p` (print) and `pp` (pretty print) can be used to evaluate
+Ruby expressions and display the value of variables to the console.
 
-You can use also `display` to start watching variables. This is a good way of tracking the values of a variable while the execution goes on.
+You can use also `display` to start watching variables. This is a good way of
+tracking the values of a variable while the execution goes on.
 
 ```
-(rdb:1) display @recent_comments
-1: @recent_comments =
+(byebug) display @posts
+1: @posts = nil
 ```
 
-The variables inside the displaying list will be printed with their values after you move in the stack. To stop displaying a variable use `undisplay _n_` where _n_ is the variable number (1 in the last example).
+The variables inside the displaying list will be printed with their values after
+you move in the stack. To stop displaying a variable use `undisplay _n_` where
+_n_ is the variable number (1 in the last example).
 
 ### Step by Step
 
-Now you should know where you are in the running trace and be able to print the available variables. But lets continue and move on with the application execution.
+Now you should know where you are in the running trace and be able to print the
+available variables. But lets continue and move on with the application
+execution.
 
-Use `step` (abbreviated `s`) to continue running your program until the next logical stopping point and return control to the debugger.
+Use `step` (abbreviated `s`) to continue running your program until the next
+logical stopping point and return control to the debugger.
 
-TIP: You can also use `step+ n` and `step- n` to move forward or backward `n` steps respectively.
+You may also use `next` which is similar to step, but function or method calls
+that appear within the line of code are executed without stopping.
 
-You may also use `next` which is similar to step, but function or method calls that appear within the line of code are executed without stopping. As with step, you may use plus sign to move _n_ steps.
+TIP: You can also use `step n` or `next n` to move forwards `n` steps at once.
 
-The difference between `next` and `step` is that `step` stops at the next line of code executed, doing just a single step, while `next` moves to the next line without descending inside methods.
+The difference between `next` and `step` is that `step` stops at the next line
+of code executed, doing just a single step, while `next` moves to the next line
+without descending inside methods.
 
-For example, consider this block of code with an included `debugger` statement:
+For example, consider the following situation:
 
 ```ruby
-class Author < ActiveRecord::Base
-  has_one :editorial
-  has_many :comments
+Started GET "/" for 127.0.0.1 at 2014-04-11 13:39:23 +0200
+Processing by PostsController#index as HTML
 
-  def find_recent_comments(limit = 10)
-    debugger
-    @recent_comments ||= comments.where("created_at > ?", 1.week.ago).limit(limit)
-  end
-end
+[1, 8] in /home/davidr/Proyectos/test_app/app/models/post.rb
+   1: class Post < ActiveRecord::Base
+   2:
+   3:   def self.find_recent(limit = 10)
+   4:     byebug
+=> 5:     where('created_at > ?', 1.week.ago).limit(limit)
+   6:   end
+   7:
+   8: end
+
+(byebug)
 ```
 
-TIP: You can use the debugger while using `rails console`. Just remember to `require "debugger"` before calling the `debugger` method.
+If we use  `next`, we want go deep inside method calls. Instead, byebug will go
+to the next line within the same context. In this case, this is the last line of
+the method, so `byebug` will jump to next next line of the previous frame.
 
 ```
-$ rails console
-Loading development environment (Rails 4.0.0)
->> require "debugger"
-=> []
->> author = Author.first
-=> #<Author id: 1, first_name: "Bob", last_name: "Smith", created_at: "2008-07-31 12:46:10", updated_at: "2008-07-31 12:46:10">
->> author.find_recent_comments
-/PathTo/project/app/models/author.rb:11
-)
-```
+(byebug) next
+Next went up a frame because previous frame finished
 
-With the code stopped, take a look around:
+[4, 13] in /PathTo/project/test_app/app/controllers/posts_controller.rb
+    4:   # GET /posts
+    5:   # GET /posts.json
+    6:   def index
+    7:     @posts = Post.find_recent
+    8:
+=>  9:     respond_to do |format|
+   10:       format.html # index.html.erb
+   11:       format.json { render json: @posts }
+   12:     end
+   13:   end
 
-```
-(rdb:1) list
-[2, 9] in /PathTo/project/app/models/author.rb
-   2    has_one :editorial
-   3    has_many :comments
-   4
-   5    def find_recent_comments(limit = 10)
-   6      debugger
-=> 7      @recent_comments ||= comments.where("created_at > ?", 1.week.ago).limit(limit)
-   8    end
-   9  end
+(byebug)
 ```
 
-You are at the end of the line, but... was this line executed? You can inspect the instance variables.
+If we use `step` in the same situation, we will literally go the next ruby
+instruction to be executed. In this case, the activesupport's `week` method.
 
 ```
-(rdb:1) var instance
-@attributes = {"updated_at"=>"2008-07-31 12:46:10", "id"=>"1", "first_name"=>"Bob", "las...
-@attributes_cache = {}
-```
+(byebug) step
 
-`@recent_comments` hasn't been defined yet, so it's clear that this line hasn't been executed yet. Use the `next` command to move on in the code:
+[50, 59] in /PathToGems/activesupport-4.1.0/lib/active_support/core_ext/numeric/time.rb
+   50:     ActiveSupport::Duration.new(self * 24.hours, [[:days, self]])
+   51:   end
+   52:   alias :day :days
+   53:
+   54:   def weeks
+=> 55:     ActiveSupport::Duration.new(self * 7.days, [[:days, self * 7]])
+   56:   end
+   57:   alias :week :weeks
+   58:
+   59:   def fortnights
 
+(byebug)
 ```
-(rdb:1) next
-/PathTo/project/app/models/author.rb:12
-@recent_comments
-(rdb:1) var instance
-@attributes = {"updated_at"=>"2008-07-31 12:46:10", "id"=>"1", "first_name"=>"Bob", "las...
-@attributes_cache = {}
-@comments = []
-@recent_comments = []
-```
-
-Now you can see that the `@comments` relationship was loaded and @recent_comments defined because the line was executed.
 
-If you want to go deeper into the stack trace you can move single `steps`, through your calling methods and into Rails code. This is one of the best ways to find bugs in your code, or perhaps in Ruby or Rails.
+This is one of the best ways to find bugs in your code, or perhaps in Ruby on
+Rails.
 
 ### Breakpoints
 
-A breakpoint makes your application stop whenever a certain point in the program is reached. The debugger shell is invoked in that line.
+A breakpoint makes your application stop whenever a certain point in the program
+is reached. The debugger shell is invoked in that line.
 
-You can add breakpoints dynamically with the command `break` (or just `b`). There are 3 possible ways of adding breakpoints manually:
+You can add breakpoints dynamically with the command `break` (or just `b`).
+There are 3 possible ways of adding breakpoints manually:
 
 * `break line`: set breakpoint in the _line_ in the current source file.
-* `break file:line [if expression]`: set breakpoint in the _line_ number inside the _file_. If an _expression_ is given it must evaluated to _true_ to fire up the debugger.
-* `break class(.|\#)method [if expression]`: set breakpoint in _method_ (. and \# for class and instance method respectively) defined in _class_. The _expression_ works the same way as with file:line.
+* `break file:line [if expression]`: set breakpoint in the _line_ number inside
+the _file_. If an _expression_ is given it must evaluated to _true_ to fire up
+the debugger.
+* `break class(.|\#)method [if expression]`: set breakpoint in _method_ (. and
+\# for class and instance method respectively) defined in _class_. The
+_expression_ works the same way as with file:line.
+
+
+For example, in the previous situation
 
 ```
-(rdb:5) break 10
-Breakpoint 1 file /PathTo/project/vendor/rails/actionpack/lib/action_controller/filters.rb, line 10
+[4, 13] in /PathTo/project/app/controllers/posts_controller.rb
+    4:   # GET /posts
+    5:   # GET /posts.json
+    6:   def index
+    7:     @posts = Post.find_recent
+    8:
+=>  9:     respond_to do |format|
+   10:       format.html # index.html.erb
+   11:       format.json { render json: @posts }
+   12:     end
+   13:   end
+
+(byebug) break 11
+Created breakpoint 1 at /PathTo/project/app/controllers/posts_controller.rb:11
+
 ```
 
-Use `info breakpoints _n_` or `info break _n_` to list breakpoints. If you supply a number, it lists that breakpoint. Otherwise it lists all breakpoints.
+Use `info breakpoints _n_` or `info break _n_` to list breakpoints. If you
+supply a number, it lists that breakpoint. Otherwise it lists all breakpoints.
 
 ```
-(rdb:5) info breakpoints
+(byebug) info breakpoints
 Num Enb What
-  1 y   at filters.rb:10
+1   y   at /PathTo/project/app/controllers/posts_controller.rb:11
 ```
 
-To delete breakpoints: use the command `delete _n_` to remove the breakpoint number _n_. If no number is specified, it deletes all breakpoints that are currently active..
+To delete breakpoints: use the command `delete _n_` to remove the breakpoint
+number _n_. If no number is specified, it deletes all breakpoints that are
+currently active.
 
 ```
-(rdb:5) delete 1
-(rdb:5) info breakpoints
+(byebug) delete 1
+(byebug) info breakpoints
 No breakpoints.
 ```
 
 You can also enable or disable breakpoints:
 
-* `enable breakpoints`: allow a list _breakpoints_ or all of them if no list is specified, to stop your program. This is the default state when you create a breakpoint.
+* `enable breakpoints`: allow a list _breakpoints_ or all of them if no list is
+specified, to stop your program. This is the default state when you create a
+breakpoint.
 * `disable breakpoints`: the _breakpoints_ will have no effect on your program.
 
 ### Catching Exceptions
 
-The command `catch exception-name` (or just `cat exception-name`) can be used to intercept an exception of type _exception-name_ when there would otherwise be is no handler for it.
+The command `catch exception-name` (or just `cat exception-name`) can be used to
+intercept an exception of type _exception-name_ when there would otherwise be no
+handler for it.
 
 To list all active catchpoints use `catch`.
 
 ### Resuming Execution
 
-There are two ways to resume execution of an application that is stopped in the debugger:
-
-* `continue` [line-specification] \(or `c`): resume program execution, at the address where your script last stopped; any breakpoints set at that address are bypassed. The optional argument line-specification allows you to specify a line number to set a one-time breakpoint which is deleted when that breakpoint is reached.
-* `finish` [frame-number] \(or `fin`): execute until the selected stack frame returns. If no frame number is given, the application will run until the currently selected frame returns. The currently selected frame starts out the most-recent frame or 0 if no frame positioning (e.g up, down or frame) has been performed. If a frame number is given it will run until the specified frame returns.
+There are two ways to resume execution of an application that is stopped in the
+debugger:
+
+* `continue` [line-specification] \(or `c`): resume program execution, at the
+address where your script last stopped; any breakpoints set at that address are
+bypassed. The optional argument line-specification allows you to specify a line
+number to set a one-time breakpoint which is deleted when that breakpoint is
+reached.
+* `finish` [frame-number] \(or `fin`): execute until the selected stack frame
+returns. If no frame number is given, the application will run until the
+currently selected frame returns. The currently selected frame starts out the
+most-recent frame or 0 if no frame positioning (e.g up, down or frame) has been
+performed. If a frame number is given it will run until the specified frame
+returns.
 
 ### Editing
 
 Two commands allow you to open code from the debugger into an editor:
 
-* `edit [file:line]`: edit _file_ using the editor specified by the EDITOR environment variable. A specific _line_ can also be given.
-* `tmate _n_` (abbreviated `tm`): open the current file in TextMate. It uses n-th frame if _n_ is specified.
+* `edit [file:line]`: edit _file_ using the editor specified by the EDITOR
+environment variable. A specific _line_ can also be given.
 
 ### Quitting
 
-To exit the debugger, use the `quit` command (abbreviated `q`), or its alias `exit`.
+To exit the debugger, use the `quit` command (abbreviated `q`), or its alias
+`exit`.
 
-A simple quit tries to terminate all threads in effect. Therefore your server will be stopped and you will have to start it again.
+A simple quit tries to terminate all threads in effect. Therefore your server
+will be stopped and you will have to start it again.
 
 ### Settings
 
-The `debugger` gem can automatically show the code you're stepping through and reload it when you change it in an editor. Here are a few of the available options:
-
-* `set reload`: Reload source code when changed.
-* `set autolist`: Execute `list` command on every breakpoint.
-* `set listsize _n_`: Set number of source lines to list by default to _n_.
-* `set forcestep`: Make sure the `next` and `step` commands always move to a new line
+`byebug` has a few available options to tweak its behaviour:
 
-You can see the full list by using `help set`. Use `help set _subcommand_` to learn about a particular `set` command.
+* `set autoreload`: Reload source code when changed (default: true).
+* `set autolist`: Execute `list` command on every breakpoint (default: true).
+* `set listsize _n_`: Set number of source lines to list by default to _n_
+(default: 10)
+* `set forcestep`: Make sure the `next` and `step` commands always move to a new
+line.
 
-TIP: You can save these settings in an `.rdebugrc` file in your home directory. The debugger reads these global settings when it starts.
+You can see the full list by using `help set`. Use `help set _subcommand_` to
+learn about a particular `set` command.
 
-Here's a good start for an `.rdebugrc`:
+TIP: You can save these settings in an `.byebugrc` file in your home directory.
+The debugger reads these global settings when it starts. For example:
 
 ```bash
-set autolist
 set forcestep
 set listsize 25
 ```
@@ -671,35 +801,59 @@ set listsize 25
 Debugging Memory Leaks
 ----------------------
 
-A Ruby application (on Rails or not), can leak memory - either in the Ruby code or at the C code level.
+A Ruby application (on Rails or not), can leak memory - either in the Ruby code
+or at the C code level.
 
-In this section, you will learn how to find and fix such leaks by using tool such as Valgrind.
+In this section, you will learn how to find and fix such leaks by using tool
+such as Valgrind.
 
 ### Valgrind
 
-[Valgrind](http://valgrind.org/) is a Linux-only application for detecting C-based memory leaks and race conditions.
+[Valgrind](http://valgrind.org/) is a Linux-only application for detecting
+C-based memory leaks and race conditions.
 
-There are Valgrind tools that can automatically detect many memory management and threading bugs, and profile your programs in detail. For example, if a C extension in the interpreter calls `malloc()` but doesn't properly call `free()`, this memory won't be available until the app terminates.
+There are Valgrind tools that can automatically detect many memory management
+and threading bugs, and profile your programs in detail. For example, if a C
+extension in the interpreter calls `malloc()` but doesn't properly call
+`free()`, this memory won't be available until the app terminates.
 
-For further information on how to install Valgrind and use with Ruby, refer to [Valgrind and Ruby](http://blog.evanweaver.com/articles/2008/02/05/valgrind-and-ruby/) by Evan Weaver.
+For further information on how to install Valgrind and use with Ruby, refer to
+[Valgrind and Ruby](http://blog.evanweaver.com/articles/2008/02/05/valgrind-and-ruby/)
+by Evan Weaver.
 
 Plugins for Debugging
 ---------------------
 
-There are some Rails plugins to help you to find errors and debug your application. Here is a list of useful plugins for debugging:
-
-* [Footnotes](https://github.com/josevalim/rails-footnotes) Every Rails page has footnotes that give request information and link back to your source via TextMate.
-* [Query Trace](https://github.com/ntalbott/query_trace/tree/master) Adds query origin tracing to your logs.
-* [Query Reviewer](https://github.com/nesquena/query_reviewer) This rails plugin not only runs "EXPLAIN" before each of your select queries in development, but provides a small DIV in the rendered output of each page with the summary of warnings for each query that it analyzed.
-* [Exception Notifier](https://github.com/smartinez87/exception_notification/tree/master) Provides a mailer object and a default set of templates for sending email notifications when errors occur in a Rails application.
-* [Better Errors](https://github.com/charliesome/better_errors) Replaces the standard Rails error page with a new one containing more contextual information, like source code and variable inspection.
-* [RailsPanel](https://github.com/dejan/rails_panel) Chrome extension for Rails development that will end your tailing of development.log. Have all information about your Rails app requests in the browser - in the Developer Tools panel. Provides insight to db/rendering/total times, parameter list, rendered views and more.
+There are some Rails plugins to help you to find errors and debug your
+application. Here is a list of useful plugins for debugging:
+
+* [Footnotes](https://github.com/josevalim/rails-footnotes) Every Rails page has
+footnotes that give request information and link back to your source via
+TextMate.
+* [Query Trace](https://github.com/ntalbott/query_trace/tree/master) Adds query
+origin tracing to your logs.
+* [Query Reviewer](https://github.com/nesquena/query_reviewer) This rails plugin
+not only runs "EXPLAIN" before each of your select queries in development, but
+provides a small DIV in the rendered output of each page with the summary of
+warnings for each query that it analyzed.
+* [Exception Notifier](https://github.com/smartinez87/exception_notification/tree/master)
+Provides a mailer object and a default set of templates for sending email
+notifications when errors occur in a Rails application.
+* [Better Errors](https://github.com/charliesome/better_errors) Replaces the
+standard Rails error page with a new one containing more contextual information,
+like source code and variable inspection.
+* [RailsPanel](https://github.com/dejan/rails_panel) Chrome extension for Rails
+development that will end your tailing of development.log. Have all information
+about your Rails app requests in the browser - in the Developer Tools panel.
+Provides insight to db/rendering/total times, parameter list, rendered views and
+more.
 
 References
 ----------
 
 * [ruby-debug Homepage](http://bashdb.sourceforge.net/ruby-debug/home-page.html)
 * [debugger Homepage](https://github.com/cldwalker/debugger)
+* [byebug Homepage](https://github.com/deivid-rodriguez/byebug)
 * [Article: Debugging a Rails application with ruby-debug](http://www.sitepoint.com/debug-rails-app-ruby-debug/)
 * [Ryan Bates' debugging ruby (revised) screencast](http://railscasts.com/episodes/54-debugging-ruby-revised)
 * [Ryan Bates' stack trace screencast](http://railscasts.com/episodes/24-the-stack-trace)
diff --git a/railties/lib/rails/commands/console.rb b/railties/lib/rails/commands/console.rb
index f6bdf129d6..555d8f31e1 100644
--- a/railties/lib/rails/commands/console.rb
+++ b/railties/lib/rails/commands/console.rb
@@ -18,7 +18,14 @@ module Rails
           opt.on("-e", "--environment=name", String,
                   "Specifies the environment to run this console under (test/development/production).",
                   "Default: development") { |v| options[:environment] = v.strip }
-          opt.on("--debugger", 'Enable the debugger.') { |v| options[:debugger] = v }
+          opt.on("--debugger", 'Enable the debugger.') do |v|
+            if RUBY_VERSION < '2.0.0'
+              options[:debugger] = v
+            else
+              puts "=> Notice: debugger option is ignored since ruby 2.0 and " \
+                   "it will be removed in future versions"
+            end
+          end
           opt.parse!(arguments)
         end
 
@@ -69,12 +76,25 @@ module Rails
       Rails.env = environment
     end
 
-    def debugger?
-      options[:debugger]
+    if RUBY_VERSION < '2.0.0'
+      def debugger?
+        options[:debugger]
+      end
+
+      def require_debugger
+        require 'debugger'
+        puts "=> Debugger enabled"
+      rescue LoadError
+        puts "You're missing the 'debugger' gem. Add it to your Gemfile, bundle it and try again."
+        exit(1)
+      end
     end
 
     def start
-      require_debugger if debugger?
+      if RUBY_VERSION < '2.0.0'
+        require_debugger if debugger?
+      end
+
       set_environment! if environment?
 
       if sandbox?
@@ -89,13 +109,5 @@ module Rails
       end
       console.start
     end
-
-    def require_debugger
-      require 'debugger'
-      puts "=> Debugger enabled"
-    rescue LoadError
-      puts "You're missing the 'debugger' gem. Add it to your Gemfile, bundle it and try again."
-      exit(1)
-    end
   end
 end
diff --git a/railties/lib/rails/commands/server.rb b/railties/lib/rails/commands/server.rb
index fec4962fb5..6146b6c1db 100644
--- a/railties/lib/rails/commands/server.rb
+++ b/railties/lib/rails/commands/server.rb
@@ -18,7 +18,14 @@ module Rails
           opts.on("-c", "--config=file", String,
                   "Use custom rackup configuration file") { |v| options[:config] = v }
           opts.on("-d", "--daemon", "Make server run as a Daemon.") { options[:daemonize] = true }
-          opts.on("-u", "--debugger", "Enable the debugger") { options[:debugger] = true }
+          opts.on("-u", "--debugger", "Enable the debugger") do
+            if RUBY_VERSION < '2.0.0'
+              options[:debugger] = true
+            else
+              puts "=> Notice: debugger option is ignored since ruby 2.0 and " \
+                   "it will be removed in future versions"
+            end
+          end
           opts.on("-e", "--environment=name", String,
                   "Specifies the environment to run this server under (test/development/production).",
                   "Default: development") { |v| options[:environment] = v }
@@ -75,7 +82,9 @@ module Rails
 
     def middleware
       middlewares = []
-      middlewares << [Rails::Rack::Debugger] if options[:debugger]
+      if RUBY_VERSION < '2.0.0'
+        middlewares << [Rails::Rack::Debugger] if options[:debugger]
+      end
       middlewares << [::Rack::ContentLength]
 
       # FIXME: add Rack::Lock in the case people are using webrick.
diff --git a/railties/lib/rails/generators/rails/app/templates/Gemfile b/railties/lib/rails/generators/rails/app/templates/Gemfile
index a9b6787894..756c334772 100644
--- a/railties/lib/rails/generators/rails/app/templates/Gemfile
+++ b/railties/lib/rails/generators/rails/app/templates/Gemfile
@@ -23,8 +23,12 @@ source 'https://rubygems.org'
 # gem 'capistrano-rails', group: :development
 
 <% unless defined?(JRUBY_VERSION) -%>
-# Use debugger
+# To use a debugger
+  <%- if RUBY_VERSION < '2.0.0' -%>
 # gem 'debugger', group: [:development, :test]
+  <%- else -%>
+# gem 'byebug', group: [:development, :test]
+  <%- end -%>
 <% end -%>
 
 <% if RUBY_PLATFORM.match(/bccwin|cygwin|emx|mingw|mswin|wince/) -%>
diff --git a/railties/lib/rails/generators/rails/plugin/templates/Gemfile b/railties/lib/rails/generators/rails/plugin/templates/Gemfile
index f0a832f783..1f704db510 100644
--- a/railties/lib/rails/generators/rails/plugin/templates/Gemfile
+++ b/railties/lib/rails/generators/rails/plugin/templates/Gemfile
@@ -38,6 +38,10 @@ end
 
 <% end -%>
 <% unless defined?(JRUBY_VERSION) -%>
-# To use debugger
-# gem 'debugger'
+# To use a debugger
+  <%- if RUBY_VERSION < '2.0.0' -%>
+# gem 'debugger', group: [:development, :test]
+  <%- else -%>
+# gem 'byebug', group: [:development, :test]
+  <%- end -%>
 <% end -%>
diff --git a/railties/lib/rails/rack.rb b/railties/lib/rails/rack.rb
index d1ee96f7fd..886f0e52e1 100644
--- a/railties/lib/rails/rack.rb
+++ b/railties/lib/rails/rack.rb
@@ -1,6 +1,6 @@
 module Rails
   module Rack
-    autoload :Debugger,      "rails/rack/debugger"
+    autoload :Debugger,      "rails/rack/debugger"   if RUBY_VERSION < '2.0.0'
     autoload :Logger,        "rails/rack/logger"
     autoload :LogTailer,     "rails/rack/log_tailer"
   end
diff --git a/railties/test/commands/console_test.rb b/railties/test/commands/console_test.rb
index a34beaedb3..1273f9d4c2 100644
--- a/railties/test/commands/console_test.rb
+++ b/railties/test/commands/console_test.rb
@@ -19,14 +19,8 @@ class Rails::ConsoleTest < ActiveSupport::TestCase
     assert console.sandbox?
   end
 
-  def test_debugger_option
-    console = Rails::Console.new(app, parse_arguments(["--debugger"]))
-    assert console.debugger?
-  end
-
   def test_no_options
     console = Rails::Console.new(app, parse_arguments([]))
-    assert !console.debugger?
     assert !console.sandbox?
   end
 
@@ -36,13 +30,6 @@ class Rails::ConsoleTest < ActiveSupport::TestCase
     assert_match(/Loading \w+ environment \(Rails/, output)
   end
 
-  def test_start_with_debugger
-    rails_console = Rails::Console.new(app, parse_arguments(["--debugger"]))
-    rails_console.expects(:require_debugger).returns(nil)
-
-    silence_stream(STDOUT) { rails_console.start }
-  end
-
   def test_start_with_sandbox
     app.expects(:sandbox=).with(true)
     FakeConsole.expects(:start)
@@ -52,6 +39,25 @@ class Rails::ConsoleTest < ActiveSupport::TestCase
     assert_match(/Loading \w+ environment in sandbox \(Rails/, output)
   end
 
+  if RUBY_VERSION < '2.0.0'
+    def test_debugger_option
+      console = Rails::Console.new(app, parse_arguments(["--debugger"]))
+      assert console.debugger?
+    end
+
+    def test_no_options_does_not_set_debugger_flag
+      console = Rails::Console.new(app, parse_arguments([]))
+      assert !console.debugger?
+    end
+
+    def test_start_with_debugger
+      rails_console = Rails::Console.new(app, parse_arguments(["--debugger"]))
+      rails_console.expects(:require_debugger).returns(nil)
+
+      silence_stream(STDOUT) { rails_console.start }
+    end
+  end
+
   def test_console_with_environment
     start ["-e production"]
     assert_match(/\sproduction\s/, output)
diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index b7bc721247..007dd886da 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -305,14 +305,17 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_file "Gemfile", /gem 'jbuilder'/
   end
 
-  def test_inclusion_of_debugger
+  def test_inclusion_of_a_debugger
     run_generator
     if defined?(JRUBY_VERSION)
       assert_file "Gemfile" do |content|
+        assert_no_match(/byebug/, content)
         assert_no_match(/debugger/, content)
       end
-    else
+    elsif RUBY_VERSION < '2.0.0'
       assert_file "Gemfile", /# gem 'debugger'/
+    else
+      assert_file "Gemfile", /# gem 'byebug'/
     end
   end
 
diff --git a/railties/test/generators/plugin_generator_test.rb b/railties/test/generators/plugin_generator_test.rb
index 7a2701f813..853af80111 100644
--- a/railties/test/generators/plugin_generator_test.rb
+++ b/railties/test/generators/plugin_generator_test.rb
@@ -64,14 +64,17 @@ class PluginGeneratorTest < Rails::Generators::TestCase
     assert_file "test/integration/navigation_test.rb", /ActionDispatch::IntegrationTest/
   end
 
-  def test_inclusion_of_debugger
+  def test_inclusion_of_a_debugger
     run_generator [destination_root, '--full']
     if defined?(JRUBY_VERSION)
       assert_file "Gemfile" do |content|
+        assert_no_match(/byebug/, content)
         assert_no_match(/debugger/, content)
       end
-    else
+    elsif RUBY_VERSION < '2.0.0'
       assert_file "Gemfile", /# gem 'debugger'/
+    else
+      assert_file "Gemfile", /# gem 'byebug'/
     end
   end