Hi DBA's

i am looking for a script that can be run from sqlplus to connect to multiple databases.
where the script has service name as variable and can prompt me to enter service name everytime i run the script.
i have installed oracle client on the windows machine which is using the tnsnames.ora to connect to the DB's which are on linux.

This task can be achived by typing

sql> conn system/password@testdb

but i am looking for something like this
sql> conn.sql
Enter DB_NAME : testdb (or whatever the DB i am trying to connect)

is there a way this can be acheived.