connect without connect string
We have 10204 environment and two databases running. Binaries are owned by id oracle and group is dba. There is application id at OS level (Solaris 10) called opera. The .profile of opera set the ORACLE_SID, ORACLE_HOME, and PATH and user is able to connect to sqlplus without the connect string. How come they are able to connect without the connect string as regular OS user ?
$ id
uid=100(opera) gid=100(operatis)
$ exit
root@tf-nic-p01:~# exit
logout
data:/opt/oracle> id
uid=1001(oracle) gid=101(dba)
I'm able to log on to server at OS level with id opera
$ sqlplus opera/opera1!