:() defines a new function called : { :|:& } is the body of the function, where we call the : function recursively, piping (|) its output to another call to :,

:() defines a new function called : { :|:& } is the body of the function, where we call the : function recursively, piping (|) its output to another call to :, then backgrounding the whole thing (&) ; indicate the end of a statement and the start of a new one : and finally the last : calls the function we defined to start the chain Essentially each time the function is ran, it creates 2 new copies of itself, which each create 2 copies of themselves, etc. until your OS stops responding and crashes. Nowadays many shells recognize this particular fork bomb and refuse to execute it