To check or set Environment: Go to ORACLE DBA / General / Environment...

 

Connecting to database using sqlplus

$ sqlplus / as sysdba

$ sqlplus "/ as sysdba"

Using aliace

$ s

$ sqlplus /nolog
SQL> conn / as sysdba

$ sqlplus /nolog
SQL> conn sys@orcl as sysdba

Connect remotely using oracle client installed on the local computer

$ sqlplus sys/sys_password@IPADDR/INSTANCE_NAME as sysdba

$ sqlplus sys/sys@10.0.0.1/orcl as sysdba

$ export NLS_LANG=AMERICAN_RUSSIA.AL32UTF8             # set codepage, if you have troubles with reading of the output messages (unreadable characters)