What is a fork bomb and how can it be implemented in the fish shell? I've heard about implementations in bash and zsh, but not fish.
A fork bomb is a type of denial-of-service attack that can crash or freeze a computer by rapidly creating an enormous number of processes. Each process in turn creates more processes until the system becomes overwhelmed and unresponsive.

Here is an example of a fork bomb in the fish shell:
``` 
while true; fork
end
``` 
This code creates an infinite loop that continuously forks new processes until the system can no longer handle them. The fork command in the loop creates a new instance of the current shell process, effectively doubling the number of processes running on the system each time the loop executes.

It's important to note that running a fork bomb can cause serious harm to a computer system, including crashing the system or rendering it unusable. Therefore, it should never be used maliciously or without proper authorization.

If you accidentally run a fork bomb on your system, you can try to stop it by using the kill command to terminate the shell process running the loop. However, in some cases the system may be so overwhelmed that you may need to force a reboot to regain control of the machine.
If someone makes a fork bomb run at the very beginning of the startup, is it possible to save the PC?
Yes, but it might require some effort. 

If you boot from an operating system installed in a different drive or partition than your forked up operating system, the fork bomb will not be triggered. You can then edit the file in your forked up operating system which specifies which programs run at at startup to remove the fork bomb program. You will then be able to boot into your formerly forked up partition with no problems. The file or files you have to edit vary by operating system. And the technique for looking at and editing the files in the forked up operating system partition from the other operating system varies.

Some operating systems also allow you to boot while disabling some of the programs that run on startup, though the method for doing this also varies. 

Another option could be booting from another operating system, copying out all the files you want to save, then wiping your forked up operating system partition or drive and installing a new operating system; this would also completely remove the fork bomb.  If you're curious, I can give more specific advice pertaining to removing a fork bomb from a specific operating system.