Ruby and Rails FAQ(s)

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Knowing these stuff will be handy when in need and so do I post them here. If you think of (m)any stuff that should be helpful please do add them/it through comments to this post and I shall add them/it to my list of FAQs sooner.
  1. How to find the current version of Ruby installed?
    $ruby -v or  
    $ruby --version
  2. How to find the current version of RubyGem installed?
    $gem -v or  
    $gem --version
  3. How to find the current version of Rails installed?
    $rails -v or  
    $rails --version
  4. How to find environment information of RubyGems?
    $gem env
  5. How to update the current version of Rails installed?
    $sudo gem update rails
  6. How to install rails and its dependencies?
    $sudo gem install rails --include-dependencies
  7. How to find the list of plugins installed as part of gem in use?
    $gem list --local
  8. [More to come...]
Books I follow(ed) for Ruby are:
The Ruby Programming Language   Beginning Ruby: From Novice to Professional, Second Edition

Books I follow(ed) for Rails:
Simply Rails 2.0     Rails for Java Developers     Foundation Rails 2     Agile Web Development with Rails, Third Edition