[root@localhost tmp]# cat /etc/redhat-release
CentOS Linux release 7.5.1804 (Core) 
[root@localhost tmp]# cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"


[root@localhost tmp]# rpm -i oracle-database-ee-21c-1.0-1.ol8.x86_64.rpm                                                                      
warning: oracle-database-ee-21c-1.0-1.ol8.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID ad986da3: NOKEY                                          
error: Failed dependencies:
        oracle-database-preinstall-21c is needed by oracle-database-ee-21c-1.0-1.x86_64

[root@localhost tmp]# curl -o oracle-database-preinstall-19c-1.0-3.el7.x86_64.rpm https://yum.oracle.com/repo/OracleLinux/OL7/latest/x86_64/getPackage/oracle-database-preinstall-19c-1.0-3.el7.x86_64.rpm
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 27152  100 27152    0     0  47854      0 --:--:-- --:--:-- --:--:-- 47887
[root@localhost tmp]# rpm -i oracle-database-preinstall-19c-1.0-3.el7.x86_64.rpm 
warning: oracle-database-preinstall-19c-1.0-3.el7.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID ec551f03: NOKEY
[root@localhost tmp]# rpm -i oracle-database-ee-21c-1.0-1.ol8.x86_64.rpm --nodeps                                                                                                                                                            
warning: oracle-database-ee-21c-1.0-1.ol8.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID ad986da3: NOKEY                                                                                                                                 
[INFO] Executing post installation scripts...                                                                                                                                                                                                
[INFO] Oracle home installed successfully and ready to be configured.                                                                                                                                                                        
To configure a sample Oracle Database you can execute the following service configuration script as root: /etc/init.d/oracledb_ORCLCDB-21c configure                                                                                         

[root@localhost tmp]# /etc/init.d/oracledb_ORCLCDB-21c configure
Configuring Oracle Database ORCLCDB.
Prepare for db operation
8% complete
Copying database files
31% complete
Creating and starting Oracle instance
32% complete
36% complete
40% complete
43% complete
46% complete
Completing Database Creation
51% complete
54% complete
Creating Pluggable Databases
58% complete
77% complete
Executing Post Configuration Actions
100% complete
Database creation complete. For details check the logfiles at:
 /opt/oracle/cfgtoollogs/dbca/ORCLCDB.
Database Information:
Global Database Name:ORCLCDB
System Identifier(SID):ORCLCDB
Look at the log file "/opt/oracle/cfgtoollogs/dbca/ORCLCDB/ORCLCDB.log" for further details.

Database configuration completed successfully. The passwords were auto generated, you must change them by connecting to the database using 'sqlplus / as sysdba' as the oracle user.

[oracle@localhost ~]$ sqlplus
-bash: sqlplus: command not found
[oracle@localhost ~]$ echo $PATH
/usr/lib64/qt-3.3/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/oracle/.local/bin:/home/oracle/bin
[oracle@localhost ~]$ export PATH=/opt/oracle/product/21c/dbhome_1/bin:$PATH
[oracle@localhost ~]$ sqlplus / as sysdba
Error 6 initializing SQL*Plus
SP2-0667: Message file sp1<lang>.msb not found
SP2-0750: You may need to set ORACLE_HOME to your Oracle software directory
[oracle@localhost ~]$ export ORACLE_HOME=/opt/oracle/product/21c/dbhome_1/
[oracle@localhost ~]$ sqlplus / as sysdba

SQL*Plus: Release 21.0.0.0.0 - Production on Sun Aug 15 06:53:51 2021
Version 21.3.0.0.0

Copyright (c) 1982, 2021, Oracle.  All rights reserved.

ERROR:
ORA-12162: TNS:net service name is incorrectly specified


Enter user-name: ^C
[oracle@localhost ~]$ export ORACLE_SID=ORCLCDB
[oracle@localhost ~]$ sqlplus / as sysdba

SQL*Plus: Release 21.0.0.0.0 - Production on Sun Aug 15 06:54:12 2021
Version 21.3.0.0.0

Copyright (c) 1982, 2021, Oracle.  All rights reserved.


Connected to:
Oracle Database 21c Enterprise Edition Release 21.0.0.0.0 - Production
Version 21.3.0.0.0

SQL> show parameter sga

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
allow_group_access_to_sga            boolean     FALSE
lock_sga                             boolean     FALSE
pre_page_sga                         boolean     TRUE
sga_max_size                         big integer 1136M
sga_min_size                         big integer 0
sga_target                           big integer 0
SQL> show parameter pga

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
pga_aggregate_limit                  big integer 2G
pga_aggregate_target                 big integer 0