Cannot start sqlldr or sqlplus

When I start sqlldr and got the following message:

#/usr/lib/oracle/xe/app/oracle/product/10.2.0/server/bin/sqlldr dtradeuser/dmstest errors=999999 control=/var/mail2/dms.ctl

Message 2100 not found; No message file for product=RDBMS, facility=ULMessage 2100 not found; No
message file for product=RDBMS, facility=UL


To fix it, setup the environment variables

# export ORACLE_HOME=/usr/lib/oracle/xe/app/oracle/product/10.2.0/server
# export ORACLE_SID=XE