Don't spawn the shell to read the prefix.

Change-Id: Id093e5e4dccbf4ba659eb76f5694a822e58068a6
Reviewed-on: http://review.couchbase.org/35006
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>