What this command :(){ :|: & };: does?

Question

What does this shell command do in Linux

:(){ :|: & };:

How can it be used in Denial of Service attacks?

Answer 1

It's a fork bomb. I actually have that written on my whiteboard (as a joke) as I speak. Don't run it.

:()         # define a function named :, () defines a function in bash
{           
    : | :;  # the pipe needs two instances of this function, which forks two shells
}
;           # end function definition
:           # run it

So, the first run makes 2 subshells, which then each runs 2 more subshells...

: is a built in command in bash. It's kind of a "null" no-op command. It used to be the comment character, before there was a comment character. Now, it's got a small use as a no-op, but really used here because it's more cryptic, you look at :() and think WTH is that?