ORA-27101 while trying to login after shutdown immediate
I think you did the following steps and caught this error. 

SQL&gt; shut immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL&gt; exit
Disconnected from Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production


You tried to start SQL*PLUS and starting the database.

[oracle@myserver oracle]$ sqlplus

SQL*Plus: Release 12.2.0.1.0 Production on Tue Apr 24 00:20:59 2018

Copyright (c) 1982, 2016, Oracle.  All rights reserved.

Enter user-name: system
Enter password: 
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3701
Additional information: -688980181
Process ID: 0
Session ID: 0 Serial number: 0


Enter user-name: 

Enter user-name: sysdba
Enter password: 
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3701
Additional information: -688980181
Process ID: 0
Session ID: 0 Serial number: 0

Enter user-name: 


The problem is the database has not started to do the authentication for these supplied usernames. 

You could connect to the idle instance using as as sysdba as the username to start the instance.

Enter user-name: sys as sysdba
Enter password: 
Connected to an idle instance.

SQL&gt; startup
ORACLE instance started.

Total System Global Area  729808896 bytes
Fixed Size          8797056 bytes
Variable Size         452985984 bytes
Database Buffers      260046848 bytes
Redo Buffers            7979008 bytes
Database mounted.
Database opened.
SQL&gt;