path: root/guides/source/debugging_rails_applications.md

**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.**

Debugging Rails Applications
============================

This guide introduces techniques for debugging Ruby on Rails applications.

After reading this guide, you will know:

* The purpose of debugging.
* How to track down problems and issues in your application that your tests aren't identifying.
* The different ways of debugging.
* How to analyze the stack trace.

--------------------------------------------------------------------------------

View Helpers for Debugging
--------------------------

One common task is to inspect the contents of a variable. Rails provides three different ways to do this:

* `debug`
* `to_yaml`
* `inspect`

### `debug`

The `debug` helper will return a \<pre> tag that renders the object using the YAML format. This will generate human-readable data from any object. For example, if you have this code in a view:

```html+erb
<%= debug @article %>
<p>
  <b>Title:</b>
  <%= @article.title %>
</p>
```

You'll see something like this:

```yaml
--- !ruby/object Article
attributes:
  updated_at: 2008-09-05 22:55:47
  body: It's a very helpful guide for debugging your Rails app.
  title: Rails debugging guide
  published: t
  id: "1"
  created_at: 2008-09-05 22:55:47
attributes_cache: {}


Title: Rails debugging guide
```

### `to_yaml`

Alternatively, calling `to_yaml` on any object converts it to YAML. You can pass this converted object into the `simple_format` helper method to format the output. This is how `debug` does its magic.

```html+erb
<%= simple_format @article.to_yaml %>
<p>
  <b>Title:</b>
  <%= @article.title %>
</p>
```

The above code will render something like this:

```yaml
--- !ruby/object Article
attributes:
updated_at: 2008-09-05 22:55:47
body: It's a very helpful guide for debugging your Rails app.
title: Rails debugging guide
published: t
id: "1"
created_at: 2008-09-05 22:55:47
attributes_cache: {}

Title: Rails debugging guide
```

### `inspect`

Another useful method for displaying object values is `inspect`, especially when working with arrays or hashes. This will print the object value as a string. For example:

```html+erb
<%= [1, 2, 3, 4, 5].inspect %>
<p>
  <b>Title:</b>
  <%= @article.title %>
</p>
```

Will render:

```
[1, 2, 3, 4, 5]

Title: Rails debugging guide
```

The Logger
----------

It can also be useful to save information to log files at runtime. Rails maintains a separate log file for each runtime environment.

### What is the Logger?

Rails makes use of the `ActiveSupport::Logger` class to write log information. Other loggers, such as `Log4r`, may also be substituted.

You can specify an alternative logger in `config/application.rb` or any other environment file, for example:

```ruby
config.logger = Logger.new(STDOUT)
config.logger = Log4r::Logger.new("Application Log")
```

Or in the `Initializer` section, add _any_ of the following

```ruby
Rails.logger = Logger.new(STDOUT)
Rails.logger = Log4r::Logger.new("Application Log")
```

TIP: By default, each log is created under `Rails.root/log/` and the log file is named after the environment in which the application is running.

### Log Levels

When something is logged, it's printed into the corresponding log if the log
level of the message is equal to or higher than the configured log level. If you
want to know the current log level, you can call the `Rails.logger.level`
method.

The available log levels are: `:debug`, `:info`, `:warn`, `:error`, `:fatal`,
and `:unknown`, corresponding to the log level numbers from 0 up to 5,
respectively. To change the default log level, use

```ruby
config.log_level = :warn # In any environment initializer, or
Rails.logger.level = 0 # at any time
```

This is useful when you want to log under development or staging without flooding your production log with unnecessary information.

TIP: The default Rails log level is `debug` in all environments.

### Sending Messages

To write in the current log use the `logger.(debug|info|warn|error|fatal|unknown)` method from within a controller, model, or mailer:

```ruby
logger.debug "Person attributes hash: #{@person.attributes.inspect}"
logger.info "Processing the request..."
logger.fatal "Terminating application, raised unrecoverable error!!!"
```

Here's an example of a method instrumented with extra logging:

```ruby
class ArticlesController < ApplicationController
  # ...

  def create
    @article = Article.new(article_params)
    logger.debug "New article: #{@article.attributes.inspect}"
    logger.debug "Article should be valid: #{@article.valid?}"

    if @article.save
      logger.debug "The article was saved and now the user is going to be redirected..."
      redirect_to @article, notice: 'Article was successfully created.'
    else
      render :new
    end
  end

  # ...

  private
    def article_params
      params.require(:article).permit(:title, :body, :published)
    end
end
```

Here's an example of the log generated when this controller action is executed:

```
Started POST "/articles" for 127.0.0.1 at 2018-10-18 20:09:23 -0400
Processing by ArticlesController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"XLveDrKzF1SwaiNRPTaMtkrsTzedtebPPkmxEFIU0ordLjICSnXsSNfrdMa4ccyBjuGwnnEiQhEoMN6H1Gtz3A==", "article"=>{"title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs.", "published"=>"0"}, "commit"=>"Create Article"}
New article: {"id"=>nil, "title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs.", "published"=>false, "created_at"=>nil, "updated_at"=>nil}
Article should be valid: true
   (0.0ms)  begin transaction
  ↳ app/controllers/articles_controller.rb:31
  Article Create (0.5ms)  INSERT INTO "articles" ("title", "body", "published", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["title", "Debugging Rails"], ["body", "I'm learning how to print in logs."], ["published", 0], ["created_at", "2018-10-19 00:09:23.216549"], ["updated_at", "2018-10-19 00:09:23.216549"]]
  ↳ app/controllers/articles_controller.rb:31
   (2.3ms)  commit transaction
  ↳ app/controllers/articles_controller.rb:31
The article was saved and now the user is going to be redirected...
Redirected to http://localhost:3000/articles/1
Completed 302 Found in 4ms (ActiveRecord: 0.8ms)
```

Adding extra logging like this makes it easy to search for unexpected or unusual behavior in your logs. If you add extra logging, be sure to make sensible use of log levels to avoid filling your production logs with useless trivia.

### Verbose Query Logs

When looking at database query output in logs, it may not be immediately clear why multiple database queries are triggered when a single method is called:

```
irb(main):001:0> Article.pamplemousse
  Article Load (0.4ms)  SELECT "articles".* FROM "articles"
  Comment Load (0.2ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 2]]
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 3]]
=> #<Comment id: 2, author: "1", body: "Well, actually...", article_id: 1, created_at: "2018-10-19 00:56:10", updated_at: "2018-10-19 00:56:10">
```

After running `ActiveRecord::Base.verbose_query_logs = true` in the `rails console` session to enable verbose query logs and running the method again, it becomes obvious what single line of code is generating all these discrete database calls:

```
irb(main):003:0> Article.pamplemousse
  Article Load (0.2ms)  SELECT "articles".* FROM "articles"
  ↳ app/models/article.rb:5
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
  ↳ app/models/article.rb:6
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 2]]
  ↳ app/models/article.rb:6
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 3]]
  ↳ app/models/article.rb:6
=> #<Comment id: 2, author: "1", body: "Well, actually...", article_id: 1, created_at: "2018-10-19 00:56:10", updated_at: "2018-10-19 00:56:10">
```

Below each database statement you can see arrows pointing to the specific source filename (and line number) of the method that resulted in a database call. This can help you identify and address performance problems caused by N+1 queries: single database queries that generates multiple additional queries.

Verbose query logs are enabled by default in the development environment logs after Rails 5.2.

WARNING: We recommend against using this setting in production environments. It relies on Ruby's `Kernel#caller` method which tends to allocate a lot of memory in order to generate stacktraces of method calls.

### Tagged Logging

When running multi-user, multi-account applications, it's often useful
to be able to filter the logs using some custom rules. `TaggedLogging`
in Active Support helps you do exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications.

```ruby
logger = ActiveSupport::TaggedLogging.new(Logger.new(STDOUT))
logger.tagged("BCX") { logger.info "Stuff" }                            # Logs "[BCX] Stuff"
logger.tagged("BCX", "Jason") { logger.info "Stuff" }                   # Logs "[BCX] [Jason] Stuff"
logger.tagged("BCX") { logger.tagged("Jason") { logger.info "Stuff" } } # Logs "[BCX] [Jason] Stuff"
```

### Impact of Logs on Performance
Logging will always have a small impact on the performance of your Rails app,
        particularly when logging to disk. Additionally, there are a few subtleties:

Using the `:debug` level will have a greater performance penalty than `:fatal`,
      as a far greater number of strings are being evaluated and written to the
      log output (e.g. disk).

Another potential pitfall is too many calls to `Logger` in your code:

```ruby
logger.debug "Person attributes hash: #{@person.attributes.inspect}"
```

In the above example, there will be a performance impact even if the allowed
output level doesn't include debug. The reason is that Ruby has to evaluate
these strings, which includes instantiating the somewhat heavy `String` object
and interpolating the variables.
Therefore, it's recommended to pass blocks to the logger methods, as these are
only evaluated if the output level is the same as — or included in — the allowed level
(i.e. lazy loading). The same code rewritten would be:

```ruby
logger.debug {"Person attributes hash: #{@person.attributes.inspect}"}
```

The contents of the block, and therefore the string interpolation, are only
evaluated if debug is enabled. This performance savings are only really
noticeable with large amounts of logging, but it's a good practice to employ.

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.

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 into the
underlying Rails code.

### Setup

You can use the `byebug` gem to set breakpoints and step through live code in
Rails. To install it, just run:

```bash
$ gem install byebug
```

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
    byebug
    @person = Person.new
  end
end
```

### 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:

```
[1, 10] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }

(byebug)
```

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
=> Booting Puma
=> Rails 5.1.0 application starting in development on http://0.0.0.0:3000
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.4.0 (ruby 2.3.1-p112), codename: Owl Bowl Brawl
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://localhost:3000
Use Ctrl-C to stop
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 ArticlesController#index as HTML

[3, 12] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }
(byebug)
```

Now it's time to explore your application. A good place to start is
by asking the debugger for help. Type: `help`

```
(byebug) help

  break      -- Sets breakpoints in the source code
  catch      -- Handles exception catchpoints
  condition  -- Sets conditions on breakpoints
  continue   -- Runs until program ends, hits a breakpoint or reaches a line
  debug      -- Spawns a subdebugger
  delete     -- Deletes breakpoints
  disable    -- Disables breakpoints or displays
  display    -- Evaluates expressions every time the debugger stops
  down       -- Moves to a lower frame in the stack trace
  edit       -- Edits source files
  enable     -- Enables breakpoints or displays
  finish     -- Runs the program until frame returns
  frame      -- Moves to a frame in the call stack
  help       -- Helps you using byebug
  history    -- Shows byebug's history of commands
  info       -- Shows several informations about the program being debugged
  interrupt  -- Interrupts the program
  irb        -- Starts an IRB session
  kill       -- Sends a signal to the current process
  list       -- Lists lines of source code
  method     -- Shows methods of an object, class or module
  next       -- Runs one or more lines of code
  pry        -- Starts a Pry session
  quit       -- Exits byebug
  restart    -- Restarts the debugged program
  save       -- Saves current byebug session to a file
  set        -- Modifies byebug settings
  show       -- Shows byebug settings
  source     -- Restores a previously saved byebug session
  step       -- Steps into blocks or methods one or more times
  thread     -- Commands to manipulate threads
  tracevar   -- Enables tracing of a global variable
  undisplay  -- Stops displaying all or some expressions when program stops
  untracevar -- Stops tracing a global variable
  up         -- Moves to a higher frame in the stack trace
  var        -- Shows variables and its values
  where      -- Displays the backtrace

(byebug)
```

To see the previous ten lines you should type `list-` (or `l-`).

```
(byebug) l-

[1, 10] in /PathTo/project/app/controllers/articles_controller.rb
   1  class ArticlesController < ApplicationController
   2    before_action :set_article, only: [:show, :edit, :update, :destroy]
   3
   4    # GET /articles
   5    # GET /articles.json
   6    def index
   7      byebug
   8      @articles = Article.find_recent
   9
   10     respond_to do |format|
```

This way you can move inside the file and see the code above the line where you
added the `byebug` call. Finally, to see where you are in the code again you can
type `list=`

```
(byebug) list=

[3, 12] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }
(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 the debugger
to inspect the frame stack, evaluate variables from the perspective of the
debugged program, and know 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  ArticlesController.index
      at /PathToProject/app/controllers/articles_controller.rb:8
    #1  ActionController::BasicImplicitRender.send_action(method#String, *args#Array)
      at /PathToGems/actionpack-5.1.0/lib/action_controller/metal/basic_implicit_render.rb:4
    #2  AbstractController::Base.process_action(action#NilClass, *args#Array)
      at /PathToGems/actionpack-5.1.0/lib/abstract_controller/base.rb:181
    #3  ActionController::Rendering.process_action(action, *args)
      at /PathToGems/actionpack-5.1.0/lib/action_controller/metal/rendering.rb:30
...
```

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.

```
(byebug) frame 2

[176, 185] in /PathToGems/actionpack-5.1.0/lib/abstract_controller/base.rb
   176:       # is the intended way to override action dispatching.
   177:       #
   178:       # Notice that the first argument is the method to be dispatched
   179:       # which is *not* necessarily the same as the action name.
   180:       def process_action(method_name, *args)
=> 181:         send_action(method_name, *args)
   182:       end
   183:
   184:       # Actually call the method associated with the action. Override
   185:       # 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.

You can also use `up [n]` 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 `thread` command (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 current
thread is marked with a plus (+) sign.
* `thread stop n`: stops thread _n_.
* `thread resume n`: resumes thread _n_.
* `thread switch n`: switches the current thread context to _n_.

This command is very helpful 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:

```
[3, 12] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }

(byebug) instance_variables
[:@_action_has_layout, :@_routes, :@_request, :@_response, :@_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/articles_controller.rb
   5     # GET /articles.json
   6     def index
   7       byebug
   8       @articles = Article.find_recent
   9
=> 10      respond_to do |format|
   11        format.html # index.html.erb
   12        format.json { render json: @articles }
   13      end
   14    end
   15
(byebug)
```

And then ask again for the instance_variables:

```
(byebug) instance_variables
[:@_action_has_layout, :@_routes, :@_request, :@_response, :@_lookup_context,
 :@_action_name, :@_response_body, :@marked_for_same_origin_verification,
 :@_config, :@articles]
```

Now `@articles` 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 will start an irb session within the context you invoked it.

The `var` method is the most convenient way to show variables and their values.
Let's have `byebug` help us with it.

```
(byebug) help var

  [v]ar <subcommand>

  Shows variables and its values


  var all      -- Shows local, global and instance variables of self.
  var args     -- Information about arguments of the current scope
  var const    -- Shows constants of an object.
  var global   -- Shows global variables.
  var instance -- Shows instance variables of self or a specific object.
  var local    -- Shows local variables in current scope.

```

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:

```
(byebug) var local
(byebug)
```

You can also inspect for an object method this way:

```
(byebug) var instance Article.new
@_start_transaction_state = {}
@aggregation_cache = {}
@association_cache = {}
@attributes = #<ActiveRecord::AttributeSet:0x007fd0682a9b18 @attributes={"id"=>#<ActiveRecord::Attribute::FromDatabase:0x007fd0682a9a00 @name="id", @value_be...
@destroyed = false
@destroyed_by_association = nil
@marked_for_destruction = false
@new_record = true
@readonly = false
@transaction_state = nil
```

You can also use `display` to start watching variables. This is a good way of
tracking the values of a variable while the execution goes on.

```
(byebug) display @articles
1: @articles = nil
```

The variables inside the displayed 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 let's 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. `next` is similar to
`step`, but while `step` stops at the next line of code executed, doing just a
single step, `next` moves to the next line without descending inside methods.

For example, consider the following situation:

```
Started GET "/" for 127.0.0.1 at 2014-04-11 13:39:23 +0200
Processing by ArticlesController#index as HTML

[1, 6] in /PathToProject/app/models/article.rb
   1: class Article < ApplicationRecord
   2:   def self.find_recent(limit = 10)
   3:     byebug
=> 4:     where('created_at > ?', 1.week.ago).limit(limit)
   5:   end
   6: end

(byebug)
```

If we use `next`, we won't go deep inside method calls. Instead, `byebug` will
go to the next line within the same context. In this case, it is the last line
of the current method, so `byebug` will return to the next line of the caller
method.

```
(byebug) next
[4, 13] in /PathToProject/app/controllers/articles_controller.rb
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     @articles = Article.find_recent
    8:
=>  9:     respond_to do |format|
   10:       format.html # index.html.erb
   11:       format.json { render json: @articles }
   12:     end
   13:   end

(byebug)
```

If we use `step` in the same situation, `byebug` will literally go to the next
Ruby instruction to be executed -- in this case, Active Support's `week` method.

```
(byebug) step

[49, 58] in /PathToGems/activesupport-5.1.0/lib/active_support/core_ext/numeric/time.rb
   49:
   50:   # Returns a Duration instance matching the number of weeks provided.
   51:   #
   52:   #   2.weeks # => 14 days
   53:   def weeks
=> 54:     ActiveSupport::Duration.weeks(self)
   55:   end
   56:   alias :week :weeks
   57:
   58:   # Returns a Duration instance matching the number of fortnights provided.
(byebug)
```

This is one of the best ways to find bugs in your code.

TIP: You can also use `step n` or `next n` to move forward `n` steps at once.

### Breakpoints

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:

* `break n`: set breakpoint in line number _n_ in the current source file.
* `break file:n [if expression]`: set breakpoint in line number _n_ inside
file named _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:n.

For example, in the previous situation

```
[4, 13] in /PathToProject/app/controllers/articles_controller.rb
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     @articles = Article.find_recent
    8:
=>  9:     respond_to do |format|
   10:       format.html # index.html.erb
   11:       format.json { render json: @articles }
   12:     end
   13:   end

(byebug) break 11
Successfully created breakpoint with id 1

```

Use `info breakpoints` to list breakpoints. If you supply a number, it lists
that breakpoint. Otherwise it lists all breakpoints.

```
(byebug) info breakpoints
Num Enb What
1   y   at /PathToProject/app/controllers/articles_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.

```
(byebug) delete 1
(byebug) info breakpoints
No breakpoints.
```

You can also enable or disable breakpoints:

* `enable breakpoints [n [m [...]]]`: allows a specific breakpoint list or all
breakpoints to stop your program. This is the default state when you create a
breakpoint.
* `disable breakpoints [n [m [...]]]`: make certain (or all) breakpoints 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 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 [n]`: resumes program execution at the address where your script last
stopped; any breakpoints set at that address are bypassed. The optional argument
`n` allows you to specify a line number to set a one-time breakpoint which is
deleted when that breakpoint is reached.
* `finish [n]`: 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:n]`: edit file named _file_ using the editor specified by the
EDITOR environment variable. A specific line _n_ can also be given.

### Quitting

To exit the debugger, use the `quit` command (abbreviated to `q`). Or, type `q!`
to bypass the `Really quit? (y/n)` prompt and exit unconditionally.

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

`byebug` has a few available options to tweak its behavior:

```
(byebug) help set

  set <setting> <value>

  Modifies byebug settings

  Boolean values take "on", "off", "true", "false", "1" or "0". If you
  don't specify a value, the boolean setting will be enabled. Conversely,
  you can use "set no<setting>" to disable them.

  You can see these environment settings with the "show" command.

  List of supported settings:

  autosave       -- Automatically save command history record on exit
  autolist       -- Invoke list command on every stop
  width          -- Number of characters per line in byebug's output
  autoirb        -- Invoke IRB on every stop
  basename       -- <file>:<line> information after every stop uses short paths
  linetrace      -- Enable line execution tracing
  autopry        -- Invoke Pry on every stop
  stack_on_error -- Display stack trace when `eval` raises an exception
  fullpath       -- Display full file names in backtraces
  histfile       -- File where cmd history is saved to. Default: ./.byebug_history
  listsize       -- Set number of source lines to list by default
  post_mortem    -- Enable/disable post-mortem mode
  callstyle      -- Set how you want method call parameters to be displayed
  histsize       -- Maximum number of commands that can be stored in byebug history
  savefile       -- File where settings are saved to. Default: ~/.byebug_save
```

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 callstyle short
set listsize 25
```

Debugging with the `web-console` gem
------------------------------------

Web Console is a bit like `byebug`, but it runs in the browser. In any page you
are developing, you can request a console in the context of a view or a
controller. The console would be rendered next to your HTML content.

### Console

Inside any controller action or view, you can invoke the console by
calling the `console` method.

For example, in a controller:

```ruby
class PostsController < ApplicationController
  def new
    console
    @post = Post.new
  end
end
```

Or in a view:

```html+erb
<% console %>

<h2>New Post</h2>
```

This will render a console inside your view. You don't need to care about the
location of the `console` call; it won't be rendered on the spot of its
invocation but next to your HTML content.

The console executes pure Ruby code: You can define and instantiate
custom classes, create new models, and inspect variables.

NOTE: Only one console can be rendered per request. Otherwise `web-console`
will raise an error on the second `console` invocation.

### Inspecting Variables

You can invoke `instance_variables` to list all the instance variables
available in your context. If you want to list all the local variables, you can
do that with `local_variables`.

### Settings

* `config.web_console.whitelisted_ips`: Authorized list of IPv4 or IPv6
addresses and networks (defaults: `127.0.0.1/8, ::1`).
* `config.web_console.whiny_requests`: Log a message when a console rendering
is prevented (defaults: `true`).

Since `web-console` evaluates plain Ruby code remotely on the server, don't try
to use it in production.

Debugging Memory Leaks
----------------------

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.

### Valgrind

[Valgrind](http://valgrind.org/) is an 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.

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:

* [Query Trace](https://github.com/ruckus/active-record-query-trace/tree/master) Adds query
origin tracing to your logs.
* [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.
* [Pry](https://github.com/pry/pry) An IRB alternative and runtime developer console.

References
----------

* [byebug Homepage](https://github.com/deivid-rodriguez/byebug)
* [web-console Homepage](https://github.com/rails/web-console)