To be able to admin Oracle instance from sqlplus command line on Ubuntu server, for first you need to set the enviroment variables:

source /u01/app/oracle/product/11.2.0/xe/bin/

At this point, is possible to login as sysdba using:

sqlplus / as sysdba

if you get insufficient privileges error, you need to add your user to dba group:

usermod [username] -G oracle, dba

a this point is possible to startup and shutdown the Oracle instance on sqlplus prompt using: