How to Google a question mark?
I saw a trick in a book:
In a linux shell, we can use $? to get the return value of a command. For example, after we run a command, if it exits normally, the return value is 0. Then when we type $?, we will get 0 in the screen.
I want to google this kind of usage, so I have to type these two symbols $? in the search blank. But the search engine just returns nothing to me...
I have looked at the Google help page, but still can't find a solution.
So my question is: How can I search for this kind of keyword?
Or if you can give me some advise on the usage of $? or that sort of thing, that will be also appreciated.