How do I copy a hash in Ruby?

The clone method is Ruby’s standard, built-in way to do a shallow-copy:

h0 = {"John" => "Adams", "Thomas" => "Jefferson"}
# => {"John"=>"Adams", "Thomas"=>"Jefferson"}
h1 = h0.clone
# => {"John"=>"Adams", "Thomas"=>"Jefferson"}
h1["John"] = "Smith"
# => "Smith"
h1
# => {"John"=>"Smith", "Thomas"=>"Jefferson"}
h0
# => {"John"=>"Adams", "Thomas"=>"Jefferson"}

Note that the behavior may be overridden:

This method may have class-specific behavior. If so, that behavior will be documented under the #initialize_copy method of the class.