Common ENV variables

puts ENV['PATH']     # lists OS PATH variable

puts ENV['RUBYOPT']  # Any command line switches here will be added to any switches specified on the command line. 

puts ENV['RUBYPATH'] # When used with the -S switch on the command line, the paths listed in RUBYPATH will be added to the
                     # paths searched when looking for Ruby scripts. The paths in RUBYPATH precede the paths listed in PATH. 

puts ENV['RUBYLIB']  # The list of paths here will be added to the list of paths Ruby uses to search for libraries included 
                     # in the program with the require method. The paths in RUBYLIB will be searched before other directories. 

puts $LOAD_PATH      # list directories that are searched on LOAD_PATH configured in /config/environment.rb