You might get the bellow erros during Oracle DB installation, or, when trying to connect Oracle DB:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
One of the main causes of this error is due to a wrong setting in the hosts file under /etc folder.
To solve this error, first check what is the server IP address, by running the command:
# ifconfig
From the output, copy the value of the “inet addr” parameter (for example: 10.0.0.1).
Edit the hosts file under /etc folder. Add a new line, and set the host and IP, for example:
arik 10.0.0.1
Save and try again.
Good Luck