The Bash command :(){ :|:& };: will spawn processes to kernel death. Can you explain the syntax?

Question

I looked at this page and can't understand how this works.

This command "exponentially spawns subprocesses until your box locks up".

But why? What I understand less are the colons.

user@host$ :(){ :|:& };:

Answer 1

:(){ :|:& };: 

..defines a function named :, which spawns itself (twice, one pipes into the other), and backgrounds itself.

With line breaks:

:() {     :|:& }; : 

Renaming the : function to forkbomb:

forkbomb() {     forkbomb | forkbomb & }; forkbomb 

You can prevent such attacks by using ulimit to limit the number of processes-per-user:

$ ulimit -u 50 $ :(){ :|:& };: -bash: fork: Resource temporarily unavailable $ 

More permanently, you can use /etc/security/limits.conf (on Debian and others, at least), for example:

* hard nproc 50 

Of course that means you can only run 50 processes, you may want to increase this depending on what the machine is doing!