How to connect to oracle 12c from sqlplus without password in command line
This answer based on a1ex07 comment:

given the folowing detailes:

user name: demo
password: Pass1234
ip: 1.2.3.4
listener port: 1521
oracle SID: orcl
pdb service name: pdborcl


connect to core db from db host as sysdba: 


without need of password: sqlplus / as sysdba 
password manually: sqlplus sys as sysdba 
password in command:  sqlplus sys/Pass1234 as sysdba


connect to core db from db host or remote as sysdba: 


password manually:  sqlplus sys@\"1.2.3.4:1521/orcl\" as sysdba
password in command: sqlplus sys/Pass1234@1.2.3.4:1521/orcl as sysdba


connect to pdb from db host or remote as sysdba: 


password manually:  sqlplus sys@\"1.2.3.4:1521/pdborcl\" as sysdba
password in command: sqlplus sys/Pass1234@1.2.3.4:1521/pdborcl as sysdba


connect to pdb from db host or remote as demo (regular user): 


password manually:  sqlplus demo@\"1.2.3.4:1521/pdborcl\"
password in command: sqlplus demo/Pass1234@1.2.3.4:1521/pdborcl