Let the syntax do the talking
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In Rails 4.x how do I step through Ruby code using a debugger?

Rubygems offers a gem called debugger.

When I create a new rails project I should see the debugger gem listed in the Gemfile.

Although the debugger entry is commented out, I consider that entry to be evidence that the debugger should behave properly when I want to use it to step through Ruby syntax in my Rails app.

dan@hp ~/x611/app/views/posts $ cd /tmp
dan@hp /tmp $ 
dan@hp /tmp $ 
dan@hp /tmp $ which ruby
dan@hp /tmp $ 
dan@hp /tmp $ which rails
dan@hp /tmp $ 
dan@hp /tmp $ 
dan@hp /tmp $ rails -v
Rails 4.1.8
dan@hp /tmp $ 
dan@hp /tmp $ 
dan@hp /tmp $ ruby -v
ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-linux]
dan@hp /tmp $ 
dan@hp /tmp $ rails new mydemo -d sqlite3
      create  README.rdoc
      create  Rakefile
      create  .gitignore
      create  Gemfile
      create  app
      create  app/assets/javascripts/application.js
      create  app/assets/stylesheets/application.css
      create  app/controllers/application_controller.rb
      create  app/helpers/application_helper.rb
      create  app/views/layouts/application.html.erb
      create  app/assets/images/.keep
      create  app/mailers/.keep
      create  app/models/.keep
      create  app/controllers/concerns/.keep
      create  app/models/concerns/.keep
      create  bin
      create  bin/bundle
      create  bin/rails
      create  bin/rake
      create  config
      create  config/routes.rb
      create  config/application.rb
      create  config/environment.rb
      create  config/secrets.yml
      create  config/environments
      create  config/environments/development.rb
      create  config/environments/production.rb
      create  config/environments/test.rb
      create  config/initializers
      create  config/initializers/assets.rb
      create  config/initializers/backtrace_silencers.rb
      create  config/initializers/cookies_serializer.rb
      create  config/initializers/filter_parameter_logging.rb
      create  config/initializers/inflections.rb
      create  config/initializers/mime_types.rb
      create  config/initializers/session_store.rb
      create  config/initializers/wrap_parameters.rb
      create  config/locales
      create  config/locales/en.yml
      create  config/boot.rb
      create  config/database.yml
      create  db
      create  db/seeds.rb
      create  lib
      create  lib/tasks
      create  lib/tasks/.keep
      create  lib/assets
      create  lib/assets/.keep
      create  log
      create  log/.keep
      create  public
      create  public/404.html
      create  public/422.html
      create  public/500.html
      create  public/favicon.ico
      create  public/robots.txt
      create  test/fixtures
      create  test/fixtures/.keep
      create  test/controllers
      create  test/controllers/.keep
      create  test/mailers
      create  test/mailers/.keep
      create  test/models
      create  test/models/.keep
      create  test/helpers
      create  test/helpers/.keep
      create  test/integration
      create  test/integration/.keep
      create  test/test_helper.rb
      create  tmp/cache
      create  tmp/cache/assets
      create  vendor/assets/javascripts
      create  vendor/assets/javascripts/.keep
      create  vendor/assets/stylesheets
      create  vendor/assets/stylesheets/.keep
         run  bundle install
Fetching gem metadata from
Resolving dependencies...
Using rake 10.4.0
Using i18n 0.6.11
Using json 1.8.1
Using minitest 5.4.3
Using thread_safe 0.3.4
Using tzinfo 1.2.2
Using activesupport 4.1.8
Using builder 3.2.2
Using erubis 2.7.0
Using actionview 4.1.8
Using rack 1.5.2
Using rack-test 0.6.2
Using actionpack 4.1.8
Using mime-types 2.4.3
Using mail 2.6.3
Using actionmailer 4.1.8
Using activemodel 4.1.8
Using arel
Using activerecord 4.1.8
Using bundler 1.7.7
Using coffee-script-source 1.8.0
Using execjs 2.2.2
Using coffee-script 2.3.0
Using thor 0.19.1
Using railties 4.1.8
Using coffee-rails 4.0.1
Using hike 1.2.3
Using multi_json 1.10.1
Using jbuilder 2.2.5
Using jquery-rails 3.1.2
Using tilt 1.4.1
Using sprockets 2.12.3
Installing sprockets-rails 2.2.2
Using rails 4.1.8
Using rdoc 4.1.2
Using sass 3.2.19
Using sass-rails 4.0.5
Using sdoc 0.4.1
Using spring 1.2.0
Installing sqlite3 1.3.10
Using turbolinks 2.5.2
Using uglifier 2.5.3
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
         run  bundle exec spring binstub --all
* bin/rake: spring inserted
* bin/rails: spring inserted
dan@hp /tmp $ 
dan@hp /tmp $ cd mydemo
dan@hp /tmp/mydemo $ 
dan@hp /tmp/mydemo $ gem install debugger
Fetching: columnize-0.8.9.gem (100%)
Successfully installed columnize-0.8.9
Fetching: debugger-ruby_core_source-1.3.7.gem (100%)
Successfully installed debugger-ruby_core_source-1.3.7
Fetching: debugger-linecache-1.2.0.gem (100%)
Successfully installed debugger-linecache-1.2.0
Fetching: debugger-1.6.8.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing debugger:
	ERROR: Failed to build gem native extension.

    /home/dan/.rbenv/versions/2.1.5/bin/ruby extconf.rb
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
/home/dan/.rbenv/versions/2.1.5/lib/ruby/2.1.0/fileutils.rb:1401:in `initialize': 
No such file or directory @ rb_sysopen - ./215/ruby_debug.h (Errno::ENOENT)
	from /home/dan/.rbenv/versions/2.1.5/lib/ruby/2.1.0/fileutils.rb:1401:in `open'
	from /home/dan/.rbenv/versions/2.1.5/lib/ruby/2.1.0/fileutils.rb:1401:in `copy_file'
	from /home/dan/.rbenv/versions/2.1.5/lib/ruby/2.1.0/fileutils.rb:483:in `copy_file'
	from /home/dan/.rbenv/versions/2.1.5/lib/ruby/2.1.0/fileutils.rb:400:in `block in cp'
	from extconf.rb:83:in `block in 
' from extconf.rb:82:in `each' from extconf.rb:82:in `
' extconf failed, exit code 1 dan@hp /tmp/mydemo $ dan@hp /tmp/mydemo $
If I were the product manager for Rails, I would remove the debugger gem declaration from default Gemfiles.

It is obviously broken.

I looked at this page:

I noticed this tip:

A fork of ruby-debug(19) that works on 1.9.2 and 1.9.3 and installs easily for rvm/rbenv rubies.

ruby >= 2.0 are not supported

For 2.X rubies, consider using byebug.

I found it:

I tried installing it:

dan@hp /tmp/mydemo $ 
dan@hp /tmp/mydemo $ gem install byebug
Fetching: slop-3.6.0.gem (100%)
Successfully installed slop-3.6.0
Fetching: byebug-3.5.1.gem (100%)
Building native extensions.  This could take a while...
Successfully installed byebug-3.5.1
2 gems installed
dan@hp /tmp/mydemo $ 
dan@hp /tmp/mydemo $ 

I did vi Gemfile:
gem 'byebug', group: [:development, :test]
Then I did this:
bundle install
The byebug gem worked very well with my combo of Ruby 2.1.5 and Rails 4.1.8:

class PostsController < ApplicationController
  def index

# console should stop here:

dan@hp /tmp/mydemo $ 
dan@hp /tmp/mydemo $ 
dan@hp /tmp/mydemo $ bin/rails s
=> Booting WEBrick
=> Rails 4.1.8 application starting in development on
=> Run `rails server -h` for more startup options
=> Notice: server is listening on all interfaces ( Consider using (--binding option)
=> Ctrl-C to shutdown server
[2014-11-29 22:04:09] INFO  WEBrick 1.3.1
[2014-11-29 22:04:09] INFO  ruby 2.1.5 (2014-11-13) [x86_64-linux]
[2014-11-29 22:04:09] INFO  WEBrick::HTTPServer#start: pid=15437 port=3000

Started GET "/posts/index" for at 2014-11-29 22:05:09 +0000
Processing by PostsController#index as HTML

[2, 11] in /tmp/mydemo/app/controllers/posts_controller.rb
    2:   def index
    3:     a=1
    4:     b=2
    6: # console should stop here:
    7: byebug
=>  9:     c=3
   10:   end
   11: end
(byebug) a
(byebug) help
byebug help v3.5.1
Type "help " for help on a specific command
Available commands:
(en|dis)able  bt         continue  down  exit    help     interrupt  list    p   putl    restart  show    thread     up   
backtrace     catch      delete    edit  finish  history  irb        method  pp  quit    save     source  trace      var  
break         condition  display   eval  frame   info     kill       next    ps  reload  set      step    undisplay  where
(byebug) delete
Delete all breakpoints? (y/n) y

In the future when I need to step through Rails code while Rails is responding to a request, I will use the byebug gem. Let the syntax do the talking
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