Rule

This is a Bash fork bomb, a malicious function definition that recursively calls itself:

        :() — defines a function named : (yes, just a colon).

    { :|:& } — the function's body:

        :|: — pipes the output of the function into another call of itself, creating two processes each time.

        & — runs the call in the background, meaning it doesn’t wait for completion.

    ; — ends the function definition.

    : — finally, this invokes the function once, starting the bomb.
  

    
function tombombadil() {
   tombombadil | tombombadil & 
}
tombombadil()


  

Each time it calls itself, it forks a background copy too and sends Gandalf deeper into the abyss

It just takes all your computers CPU and RAM by making endless copies of itself. Best case your PC gets sluggish, worst it crashes. Won't do any lasting damage. It's called a "Fork Bomb"

My professor had a Unix cheat sheet he gave out to people.

But someone made a joke version with malicious commands like this one. Was a riot in class.

Breaking MacOS is easy, if the fork bomb doesn't work just create a function which recursively writes files until the disk is full. Unless they fixed it in the past 5 years, at that point even when the OS is set to delete files directly, it will still fail to delete anything. I couldn't even remove anything with sudo rm -f, that just triggered a 'not enough space to remove file' error message.

Found that out the hard way when a program borked itself and filled the entire drive with log files until there was not even a single byte left. Fortunately I had a very small file in the bin, clearing that still worked and those few bytes were enough to be able to perform file deletes again. Never bothered to find out why MacOS behaved that way as I was able to ditch that OS entirely not long after, but perhaps somebody else knows.