The XE services are defined in the configuration in /opt/oracle/product/18c/dbhomeXE/network/admin/tnsnames.ora

CLI commands

Restart

$ systemctl restart oracle-xe-18c

or

$ /etc/init.d/oracle-xe-18c restart

$ export ORACLE_SID=XE
$ export ORACLE_ASK=NO
. /opt/oracle/product/18c/dbhomeXE/bin/oraenv