What will this command do when run from shell prompt?

This piece of code is called a fork bomb. Like a virus, it will continually multiply and subdivide itself, asking for more and more resources from the computer, until the entire computer is trying to process this code.

fork: retry: Resource temporarily unavailable

As a result, the computer has no resources left for any other programs or processes, and will freeze or crash. Fortunately, rebooting your computer should cure it.

Its called fork bomb, It is a bash function which calls itself twice and so on. You might have to reset your computer to get out of it.

It will create a loop runing a command within itself until it fills up the system memory and leave computer unusable.

it will consume all the resources

it acquires all the resurces of the Server to maximum.

"lag so badly".

Please do not run this command in your system for testing purpose.

Otherwise It will be force rebooting your system.

This command will keep on running in loop till all the resources of the system is being consumed.

Derivation of the command:

':()' function is defined with name ':'

'{:|:& }' Run that function :() on loop , feed output ':' value to ':'  again and run it on background '&'.

';' command seperator.

':' Call function again. 

Not to try this command for curiosity