Recently I've been upgrading my RVM and Ruby versions. After upgrade I've encountered a problem connected to ruby-debugger. When starting Rails server I always ended with such a message:

You need to install ruby-debug to run the server in debugging mode. 
With gems, use 'gem install ruby-debug'

# Edit: this solutions fixes also this problem:

cannot load such file -- zlib

Ruby-debbug doesn't work to well with Ruby 1.9, so I use following combination in my gemsets:

group :development, :test do
  gem 'linecache19', :git => 'git://'
  gem 'ruby-debug-base19x', '~> 0.11.30.pre4'
  gem 'ruby-debug19'

Until now, it worked really well, but after the upgrade, the "You need to install ruby-debug" message kept showing again and again (even when the gems from list above were installed). To fix this issue, you need to run following commands:

rm -rf $rvm_path/usr
rvm pkg install zlib

# Posted in 5 lines instead of 1 for better visibility
sudo apt-get install build-essential openssl libreadline6 libreadline6-dev 
sudo apt-get install curl git-core zlib1g zlib1g-dev libssl-dev
sudo apt-get install libyaml-dev  libsqlite3-0 libsqlite3-dev sqlite3
sudo apt-get install libxml2-dev  libxslt-dev autoconf libc6-dev 
sudo apt-get install ncurses-dev automake libtool bison subversion

rvm reinstall 1.9.3-p194

After successful Ruby version reinstall, you should be able to run ruby software with debugger enabled.

# Update
Looks like, you can (as guys suggested in comments) debugger instead of ruby-debugger, however the method above fixes also problem with:

cannot load such file -- zlib