Cannot connect database as sys as sysdba using sqlplus I am new to oracle server 18c (currently taking online course). My problem is sql*plus where I tried to use sqlplus then sys as sysdba (it did not work) and the error appeared below. 


However, if I use sqlplus / as sysdba (it worked, and I don't know why)


I have tried this command: lsnrctl start, but again not working. In addition, I have checked my listener.ora and tnsnames.oraca



I have attached my environment set up in .bash_profile


Please help me this. Thank you
If the password contains special characters such as @, enter the password between double quotes:

$ sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Mon Nov 11 15:41:40 2019
Version 19.5.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.5.0.0.0

SQL&gt; connect sys/a@abcd as sysdba
ERROR:
ORA-12154: TNS:could not resolve the connect identifier specified


Warning: You are no longer connected to ORACLE.
SQL&gt; connect sys/"a@abcd" as sysdba
Connected.
SQL&gt;