i saw a trick in a book:

in a linux shell, we can use &? to get the return value of a command. For example,we run a command,if it exit normally, the return value is 0. And then we type $?,we will get 0 in the screen.

i want to google this kind of usage, so i have to type these two symbol $? in the search blank.But the search engine just return 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 with this kind of keyword.

or if you can give me some advise of the usage of $? or sort of thing, that will be also appreciated.