How to restart Oracle

Shutdown

$ sqlplus '/ as sysdba'
SQL> shutdown

Options:
shutdown - waits for sessions end
shutdown immediate - rollbacks and ends sessions (waits for rollback finish)
shutdown abort - kills everything - usually database has to perform some recovery on startup.

Startup

$ sqlplus '/ as sysdba'
SQL> startup

References:
https://dba.stackexchange.com/questions/42774/how-to-quickly-startup-shutdown-oracle-11
https://docs.oracle.com/cd/E17781_01/server.112/e18804/startup.htm#ADMQS124
https://www.cyberciti.biz/faq/how-do-i-start-oracle-service-in-unix/