How do I copy a hash in Ruby?

As others have pointed out, clone will do it. Be aware that clone of a hash makes a shallow copy. That is to say:

h1 = {:a => 'foo'} 
h2 = h1.clone
h1[:a] << 'bar'
p h2                # => {:a=>"foobar"}

What's happening is that the hash's references are being copied, but not the objects that the references refer to.

If you want a deep copy then:

def deep_copy(o)
  Marshal.load(Marshal.dump(o))
end

h1 = {:a => 'foo'}
h2 = deep_copy(h1)
h1[:a] << 'bar'
p h2                # => {:a=>"foo"}

deep_copy works for any object that can be marshalled. Most built-in data types (Array, Hash, String, &c.) can be marshalled.

Marshalling is Ruby's name for serialization. With marshalling, the object--with the objects it refers to--is converted to a series of bytes; those bytes are then used to create another object like the original.

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.