1. When invoking sp_ctrl, the following message prompts user to enter login and password:
Please supply a login and password for <servername>:
2. The output of "id" at OS prompt shows the following user invoked it:
The user who is trying to invoke sp_ctrl may not be a part of spadmin group.
Query the file /etc/group to see if the user who invoked sp_ctrl is a part of the spadmin group, and if not so, make it the part of spadmin. In this case it turned out that the user was not a part of the spadmin group:
$> cat /etc/group
The user oracle had to be added to the spadmin group and then sp_ctrl could be launched.