Oracle 11g displaying ORA-01034, ORA-27101, and Linux “error 2” messages.

Question

When I try to connect to Oracle 11g using:

sqlplus noah/noah@orcl

I get these errors:

ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux Error: 2: No such file or directory
Process ID: 0
Session ID: 0 Serial number: 0

The service does start up when I connect as sysdba using:

sqlplus /nolog
conn / as sysdba
startup

Here's my listener.ora:

SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
  (GLOBAL_DBNAME = orcl)
  (ORACLE_HOME = /opt/oracle/112)
  (SID_NAME = orcl)
)
)

LISTENER =
 (DESCRIPTION =
  (ADDRESS = (PROTOCOL = TCP)(HOST = mylinux)(PORT = 1521))
)

And the tnsnames.ora:

ORCL =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = mylinux)(PORT = 1521))
(CONNECT_DATA =
  (SERVER = DEDICATED)
  (SERVICE_NAME = orcl)
)
)

Here is the sqlnet.ora:

SQLNET.AUTHENTICATION_SERVICES= (NTS)
NAMES.DIRECTORY_PATH= (TNSNAMES,HOSTNAME)

Could anyone give me a hand making this work?