ORA-01034 error.

Today, my colleague said that a machine had a power outage during the holidays. After the machine was restarted, I logged on to the server using PLSQL:

A ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux_x86_64 Error: 2: No such file or directory

.

There are also many online statements about this error:

When Oracle is started;
When trying to create a database;
When trying to connect to the database;
When the client is running SQL/PL/SQL;
Backup/restoration of the database;
In other cases

This error will be reported.

The reasons include:

Improper Unix core parameter settings
Incorrect Oracle execution file permissions/Environment Variables
Client Communication cannot be properly handled
Database server crash/operating system crash/process kill
Oracle internal error
Errors Caused by specific SQL, PL/SQL
Insufficient Space
Firewall Problems
Other reasons

Error introduction from ORA-1034:

01034,000 00, "ORACLE not available"
// * Cause: Oracle was not started up. Possible causes include the following:
//-The SGA requires more space than was allocated for it.
//-The operating-system variable pointing to the instance is
// Improperly defined.
// * Action: Refer to accompanying messages for possible causes and correct
// The problem mentioned in the other messages.
// If Oracle has been initialized, then on some operating systems,
// Verify that Oracle was linked correctly. See the platform
// Specific Oracle documentation.

It can also be seen that the reason is: "Oracle was not started up". As to why it is not started, it may be one of the reasons described above.

But in fact, the reason for this problem is that Oracle is not started, the machine is not configured to automatically start the Oracle service, and sqlplus is in the idle status after login...

The problem may seem complicated, but the cause may be very simple. Of course, this is only a special case. At least it taught us to check whether Oracle has executed the startup command. If so, you can continue to view the logs. For example, to open the alert Log, You need to analyze the specific problem.