What does "in" do?

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1 
select varname in opt1 opt2...optN
do
   case $varname in
      opt1)  command1;;
      opt2)  command2;;
      optN)  commandN;;
   esac
done

Looking at this skeleton for a select statement, I am baffled as to what exactly "in" does. Does it pass a reference off from the first variable to the second?

I apologize for such a beginner question.

2

It is part of the select syntax and it denotes the list of possibilities for the select statement. In this case there are two options (it is similar to how "in" in a for statement works)

3

in is a reserved word that have a special meaning to shell.

Below are few examples of compound commands where in is used:

for name [ [ in [ word ... ] ] ; ] do list ; done

select name [ in word ] ; do list ; done

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

In first 2 examples, words following in is expanded, generating a list of items.

In case command, first word is expanded and matched against pattern.