ORA-01034: ORACLE not available

Hi All,

I have installed Oracle on my Solaris box. I can connect using sqlplus system@sid_name but when I use sqlplus then press enter and then enter the details I get the above error. I am trying to install something that is giving me the above error so although I can connect specifying the system@sid_name, I really need for it to work with just sqlplus then entering the details.

Screen output:

swse450:/# id
uid=0(root) gid=0(root)
swse450:/# set | grep ORA
ORACLE_BASE=/u01/app/oracle
ORACLE_HOME=/u01/app/oracle/10.2.0
ORACLE_SID=ars
ORA_NLS10=/u01/app/oracle/10.2.0/nls/data
swse450:/# echo $PATH
/usr/sbin:/usr/bin:/usr/local/bin:/u01/app/oracle/10.2.0/bin:/u01/app/oracle/10.2.0/client/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/etc:/usr/ccs/bin:/usr/openwin/bin:/export/home/oracle:/usr/local/sudo/bin:.
swse450:/# sqlplus

SQL*Plus: Release 10.2.0.1.0 - Production on Tue Feb 10 12:49:01 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Enter user-name: system
Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
SVR4 Error: 2: No such file or directory


Enter user-name:
ERROR:
ORA-01017: invalid username/password; logon denied


Enter user-name:
ERROR:
ORA-01017: invalid username/password; logon denied


SP2-0157: unable to CONNECT to ORACLE after 3 attempts, exiting SQL*Plus
swse450:/# sqlplus@ars
bash: sqlplus@ars: command not found
swse450:/# sqlplus system@ars

SQL*Plus: Release 10.2.0.1.0 - Production on Tue Feb 10 12:49:28 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Enter password:

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options


SQL> select name from v$database;

NAME
---------
ARS

SQL>

Thank you and kind regards
Danny