title: How to get rid of this error when trying to run ruby-debug on a ruby script
body: <p>I followed the instructions from <a href="https://github.com/mark-moseley/ruby-debug/blob/master/README" rel="nofollow">here</a> but still have trouble figuring out the problem. Would be great to hear some suggestions. Thanks in advance!</p>
  
  <pre><code>stanley@ubuntu:~/Github/webdev_class/ruby$ gem install ruby-debug19
  Fetching: archive-tar-minitar-0.5.2.gem (100%)
  Fetching: ruby_core_source-0.1.5.gem (100%)
  Fetching: linecache19-0.5.12.gem (100%)
  Building native extensions.  This could take a while...
  Fetching: ruby-debug-base19-0.11.25.gem (100%)
  Building native extensions.  This could take a while...
  Fetching: ruby-debug19-0.11.6.gem (100%)
  Successfully installed archive-tar-minitar-0.5.2
  Successfully installed ruby_core_source-0.1.5
  Successfully installed linecache19-0.5.12
  Successfully installed ruby-debug-base19-0.11.25
  Successfully installed ruby-debug19-0.11.6
  5 gems installed
  Installing ri documentation for archive-tar-minitar-0.5.2...
  Building YARD (yri) index for archive-tar-minitar-0.5.2...
  Installing ri documentation for ruby_core_source-0.1.5...
  Building YARD (yri) index for ruby_core_source-0.1.5...
  Installing ri documentation for linecache19-0.5.12...
  Building YARD (yri) index for linecache19-0.5.12...
  Installing ri documentation for ruby-debug-base19-0.11.25...
  Building YARD (yri) index for ruby-debug-base19-0.11.25...
  Installing ri documentation for ruby-debug19-0.11.6...
  Building YARD (yri) index for ruby-debug19-0.11.6...
  Installing RDoc documentation for archive-tar-minitar-0.5.2...
  Installing RDoc documentation for ruby_core_source-0.1.5...
  Installing RDoc documentation for linecache19-0.5.12...
  Installing RDoc documentation for ruby-debug-base19-0.11.25...
  Installing RDoc documentation for ruby-debug19-0.11.6...
  stanley@ubuntu:~/Github/webdev_class/ruby$ ruby-debug ex39.rb
  ruby-debug: command not found
  stanley@ubuntu:~/Github/webdev_class/ruby$ ruby-debug19
  ruby-debug19: command not found
  stanley@ubuntu:~/Github/webdev_class/ruby$ rdebug ex40.rb
  /home/stanley/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `require': /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/ruby-debug-base19-0.11.25/lib/ruby_debug.so: undefined symbol: ruby_current_thread - /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/ruby-debug-base19-0.11.25/lib/ruby_debug.so (LoadError)
      from /home/stanley/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `rescue in require'
      from /home/stanley/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:35:in `require'
      from /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/ruby-debug-base19-0.11.25/lib/ruby-debug-base.rb:1:in `&lt;top (required)&gt;'
      from /home/stanley/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
      from /home/stanley/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
      from /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/ruby-debug19-0.11.6/cli/ruby-debug.rb:5:in `&lt;top (required)&gt;'
      from /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/ruby-debug19-0.11.6/bin/rdebug:108:in `require_relative'
      from /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/ruby-debug19-0.11.6/bin/rdebug:108:in `&lt;top (required)&gt;'
      from /home/stanley/.rvm/gems/ruby-1.9.3-p194/bin/rdebug:19:in `load'
      from /home/stanley/.rvm/gems/ruby-1.9.3-p194/bin/rdebug:19:in `&lt;main&gt;'
  </code></pre>
  
  <p>----------Update---------------------------</p>
  
  <p>I tried uninstalling and reinstalling ruby-debug and linecache, and still failed.
  Here's the log of what happened:</p>
  
  <pre><code>stanley@ubuntu:~/Github/webdev_class/ruby$ gem install linecache
  Building native extensions.  This could take a while...
  ERROR:  Error installing linecache:
      ERROR: Failed to build gem native extension.
  
          /home/stanley/.rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
  Can't handle 1.9.x yet
  *** 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:
      --with-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/home/stanley/.rvm/rubies/ruby-1.9.3-p194/bin/ruby
  
  
  Gem files will remain installed in /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/linecache-0.46 for inspection.
  Results logged to /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/linecache-0.46/ext/gem_make.out
  stanley@ubuntu:~/Github/webdev_class/ruby$ gem uninstall linecache
  INFO:  gem "linecache" is not installed
  stanley@ubuntu:~/Github/webdev_class/ruby$ gem install linecache
  Building native extensions.  This could take a while...
  ERROR:  Error installing linecache:
      ERROR: Failed to build gem native extension.
  
          /home/stanley/.rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
  Can't handle 1.9.x yet
  *** 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:
      --with-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/home/stanley/.rvm/rubies/ruby-1.9.3-p194/bin/ruby
  
  
  Gem files will remain installed in /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/linecache-0.46 for inspection.
  Results logged to /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/linecache-0.46/ext/gem_make.out
  stanley@ubuntu:~/Github/webdev_class/ruby$ gem uninstall ruby-debug
  INFO:  gem "ruby-debug" is not installed
  stanley@ubuntu:~/Github/webdev_class/ruby$ gem install ruby-debug
  Building native extensions.  This could take a while...
  ERROR:  Error installing ruby-debug:
      ERROR: Failed to build gem native extension.
  
          /home/stanley/.rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
  Can't handle 1.9.x yet
  *** 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:
      --with-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/home/stanley/.rvm/rubies/ruby-1.9.3-p194/bin/ruby
  
  
  Gem files will remain installed in /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/linecache-0.46 for inspection.
  Results logged to /home/stanley/.rvm/gems/ruby-1.9.3-p194/gems/linecache-0.46/ext/gem_make.out
  </code></pre>