Use Thread.current

example.rb

>> Thread.current[:your_variable] = 3
=> 3
>> puts Thread.current[:your_variable].to_i + 1
4
=> nil
>> Thread.new { puts Thread.current[:your_variable].to_i + 1 }.join
1
=> #<Thread:0x007fa68d94d870@(irb):18 dead>
>>