SQL & PL/SQL

I am using some environment variables to connect to a sql*plus session in unix.
they are exported as part of a file which contains the following and gets executed through the .profile.

export db_username=xxx
export db_password=xxx
export db_name=xxx

i have created a file abc.sql which contains the following

connect $db_username/$db_password@$db_name

Now, I have a shell script which uses this file inside it to connect to a sql*plus session and execute some queries.

The contents of the shell script :

API_CONNECT_SQL=abc.sql
sqlplus /nolog << THEEND
WHENEVER SQLERROR EXIT SQL.SQLCODE ROLLBACK
WHENEVER OSERROR EXIT FAILURE ROLLBACK
set timing on
@${API_CONNECT_SQL}
---- do something
EXIT
THEEND

Now, the sql*plus session is able to get db_username and db_password, but not the db_name. It says "TNS service name not found".

If I hardcode some database name in place of db_name, it connects properly.

Please let me know the problem and solution as well.

Any help appreciated.

thanks,
Vijay