When I do an updatedb and locate tnsnames.ora I get back a long list due to various Oracle DB installations. I need to add some entries to whichever is the "default" tnsnames.ora configured for the system.
How do I find out which of the many tnsnames.ora files is actually being used?
note: none of the 30 odd instances are under any "system" folders (e.g. /etc /usr etc) and:
Your output above is from the server that has your Oracle installations? Are you logged in as a user that accesses Oracle? I don't see how you could be getting access to anything Oracle based on that output, unless you do so via shell scripts that build your environment each time. It's been a while, but IIRC on *nix systems you must define ORACLE_HOME in order to access Oracle.
If you type:
from the above command prompt, what does it tell you? Seems to me it ought to tell you that sqlplus is an unknown command. If not, what does