check which version of rubygems installed on ubuntu?

Solution 1:

Just an idea, list the installed packages and grep for ruby or gems or whatever :)

dpkg --get-selections

Solution 2:

Either gem -v or gem --version would output the version number.

For more information:

$ gem -h
RubyGems is a sophisticated package manager for Ruby.  This is a
basic help message containing pointers to more information.

  Usage:
    gem -h/--help
    gem -v/--version
    gem command [arguments...] [options...]

  Examples:
    gem install rake
    gem list --local
    gem build package.gemspec
    gem help install

  Further help:
    gem help commands            list all 'gem' commands
    gem help examples            show some examples of usage
    gem help platforms           show information about platforms
    gem help <COMMAND>           show help on COMMAND
                                   (e.g. 'gem help install')
    gem server                   present a web page at
                                 http://localhost:8808/
                                 with info about installed gems
  Further information:
    http://rubygems.rubyforge.org

I've seen on another question that you have the gem app broken. And that you have updated it with gem update --system. If this is the case, you can look on /usr/lib/ruby/gems/1.8/gems/rubygems-update-x.x.x and take the directory with the highest value of x.x.x.