I get:
irb(main):001:0> ENV.keys.size
=> 15
irb(main):002:0> ENV.keys[0..14]
=> ["TERM_PROGRAM", "TERM", "SHELL", "CLICOLOR",  
"TERM_PROGRAM_VERSION", "USER", "__CF_USER_TEXT_ENCODING", "PATH",  
"RI", "PWD", "SHLVL", "HOME", "LOGNAME", "SECURITYSESSIONID", "_"]

Which is a smaller set of environmental variables quite different  
than what is described in the Pickaxe Book. Did the Ruby powers trim  
them? Just curious.

--
Conscience is thoroughly well-bred and soon leaves off talking to  
those who do not wish to hear it.
-Samuel Butler, writer (1835-1902)