Ruby Trivia 3

Presented at the Berlin Ruby User Group (RUG::B) on November 5, 2015.

Transcript

1. Ruby
   Trivia
   3
   

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2. What is the value of the
   global variable $_?
   Question 1:
   

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3. The String last read by gets.
   Answer 1:
   

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4. How can you list all

   global variables?
   Bonus Question:
   

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5. How can you list all

   global variables?
   Bonus Question:
   Answer: Use the Kernel#global_variables method.
   

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6. How many global variables
   does Ruby define?
   global_variables.count
   Bonus Question:
   

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7. How many global variables
   does Ruby define?
   global_variables.count
   Bonus Question:
   Answer: 54.
   

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8. What does Ruby’s -n switch do?
   Question 2:
   

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9. Causes Ruby to assume the following loop
   around your script, which makes it iterate over
   file name arguments like sed -n or awk.
   while gets
   ...
   end
   Answer 2:
   

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10. What does Ruby’s -p switch do?
    Bonus Question:
    

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11. Acts like the -n switch, but prints the
    value of variable $_ at the each end of
    the loop. For example:
    ruby -p -e '$_.tr! "a-z", "A-Z"' < file
    Bonus Question:
    

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12. What thread-local variable can
    only store four possible values?
    Question 3:
    

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13. What thread-local variable can
    only store four possible values?
    Question 3:
    Hint #1: Those values are 0, 1, 2, and 3.
    

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14. What thread-local variable can
    only store four possible values?
    Question 3:
    Hint #1: Those values are 0, 1, 2, and 3.
    Hint #2: The value is 0 by default and can only increase.
    

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15. $SAFE
    Answer 3:
    Trick question because it looks like a global variable,
    even though it behaves like a thread-local variable.
    

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16. $SAFE
    Answer 3:
    There used to be $SAFE = 4 but it was removed in Ruby 2.1.
    Supposedly, it was only ever used by one company in Japan.
    

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17. How can you check if
    an object is trusted?
    Bonus Question:
    

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18. How can you check if
    an object is trusted?
    Bonus Question:
    Answer: Use the Kernel#tainted? method.
    

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19. What happens if you do this?
    module Kernel
    def tainted?
    return false
    end
    end
    Question 4:
    

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20. The tainted? method will
    always return false, but Ruby
    will still track tainted state via
    an internal FL_TAINT flag.
    Answer 4:
    

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21. How can you mark a

    tainted object as safe?
    Question 5:
    

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22. Kernel#untaint
    a.k.a. Kernel#trust
    $SAFE = 1
    foo = gets.trust
    eval(foo)
    Answer 5:
    

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23. Thanks for playing!
    Follow @sferik on Twitter for
    more Ruby trivia and practica.
    

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