You may try:
Below works under Solaris10+Oracle11g
bash-3.2$ exp scott/tiger file=/export/home/oracle/exp/scott.dmp tables=emp_dump, dept_dump
You may also try:
bash-3.2$ expdp scott/tiger parfile=/export/home/oracle/exp/scott_par.txt
You may need this:
SQL> grant read,write on directory DATA_PUMP_DIR to scott;
If above did not ring a bell, you may need to have further detailed information, like Oracle,linux version, or further warning message if there is any posted.