which runs the last command with all instances of hello replaced by world. !-1 can be any so called event designator, for instance !n to refer to any specific line number from history or !string for last command containing a particular string.
Seriously people, read the man page for bash. Your shell has so many cool features.
I know that this expansion exists, but never use it in practice.
Why? The last command is more general, you can just drop the g from the command I wrote to only replace the first occurrence. Besides, :[g]s/old/new should already be familiar to anyone who has used vim, so that is almost like having to learn zero new things.
