报错：

[oracle@localhost ~]$ sqlplus sys/password

SQL*Plus: Release 12.1.0.2.0 Production on Tue Mar 3 09:59:53 2020

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

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: 3640
Additional information: 1371676159
Process ID: 0
Session ID: 0 Serial number: 0

1.查询得知，由于Oracle服务未启动导致，
但启动服务后继续报错

SQL> startup 
SP2-0640: Not connected

2.首先查看监听服务已经启动成功

LSNRCTL> status 
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 12.1.0.2.0 - Production
Start Date                03-MAR-2020 10:20:44
Uptime                    0 days 0 hr. 1 min. 0 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Parameter File   /u01/app/oracle/product/12.1.0/dbhome_1/network/admin/listener.ora
Listener Log File         /u01/app/oracle/diag/tnslsnr/localhost/listener/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=localhost)(PORT=1521)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
The listener supports no services
The command completed successfully

3.检查环境变量配置，发现~/.bash_profile和listener.ora里的ORACLE_SID不一样，修改一致后重启监听继续无效，后来发现需要先source ~/.bash_profile文件使之生效；
再次重启监听后，访问成功

[oracle@localhost ~]$ sqlplus /nolog

SQL*Plus: Release 12.1.0.2.0 Production on Tue Mar 3 10:21:54 2020

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

SQL> conn sys/password as sysdba
Connected to an idle instance.
SQL> startup
ORACLE instance started.

Total System Global Area  511705088 bytes
Fixed Size                  2926176 bytes
Variable Size             377489824 bytes
Database Buffers          125829120 bytes
Redo Buffers                5459968 bytes
Database mounted.
Database opened.