Running Oracle Export Utility within a Shell script
Try changing the tables parameter to this. 
You only need parentheses when you have more than one table. 

TABLES='SYSTEM_PARAMETERS' \

$ exp tables=system.plan_table file=plan_table.dmp statistics=none

Export: Release 11.2.0.2.0 - Production on Tue Feb 17 08:59:05 2015

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Username: system
Password:

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in UTF8 character set and AL16UTF16 NCHAR character set
server uses WE8MSWIN1252 character set (possible charset conversion)

About to export specified tables via Conventional Path ...
. . exporting table                     PLAN_TABLE          4 rows exported
Export terminated successfully without warnings.