The sh(1) says somewhere at the beginning ...

----- cut here -----
[...]
     case $- in *i*)
          # commands for interactive use only
          ...
     esac
[...]
----- cut here -----

What is '$-'?

So far, I only encountered $1, $2, ...

--
Cu  Georges Heinesch, Luxembourg
    geohei@ibm.net - geohei@geocities.com
    http://www.geocities.com/yosemite/2480
    PGP 2.6.3i / 5.1i public key on request and on public servers

... Hakuna Matata ...