heroku running problem

I am learning ruby from this guide http://ruby.railstutorial.org/ruby-on-rails-tutorial-book#sec:deploying, and I am trying to install and make heroku to work. I added heroku to the PATH variable, (I am using ubuntu) b开发者_如何学Goun when i type #command: herokuit returns this:

"/usr/lib/ruby/1.8/rubygems.rb:779:in 'report_activate_error': Could not find RubyGem heroku (>=0) (Gem::LoadError)
from /usr/lib/ruby/1.8/rubygems.rb:214:in 'activate'
...
...
..."

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Looks like you haven't installed the Heroku gem;

sudo gem install heroku