ORA-12154: TNS:could not resolve the connect identifier specified

Error " ORA-12154: TNS:could not resolve the connect identifier specified " or " ORA-12505: TNS:listener does not currently know of SID given in connect descriptor " is a very generic message. It does not tell the true problem.

For example, I had a wrong port number 1521 in tnsnames.ora file. tnsping reads the string from tnsnames.ora without a complaint:

$  tnsping ebsdev
......
Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION= (ADDRESS=(PROTOCOL=tcp)(HOST=db_server1d.domain.com)(PORT=1521)) (CONNECT_DATA= (SID=EBSDEV)))
OK (20 msec)

But SQL*Plus does not connect to the database:

$ sqlplus apps/appsPWD@ebsdev
......
ERROR:
ORA-12505: TNS:listener does not currently know of SID given in connect descriptor

The real problem is that port 1521 is the wrong port entered in tnsnames.ora file.

In Oracle EBS environment, SQL*Plus uses env variable $TWO_TASK as default to make database connection, and database connection string is not necessary.

$  echo $TWO_TASK
EBSDEV

Below two lines work fine:
$ sqlplus apps/appsPWD
SQL>

$ sqlplus apps/appsPWD@ebsdev
SQL>

If that variable is set to something else, SQL*Plus will give ORA-12154 error:

$ export TWO_TASK=dbdb
$ sqlplus apps/appsPWD
......
ERROR:
ORA-12154: TNS:could not resolve the connect identifier specified