including variables in copy command

Hi ALL,

I am trying to use Copy command to copy data from table to a flat file.
Here is an example:

copy ( select * from employee where hire_date between begin_date and
end_date) to '/var/data/text.txt' delimiter ';';

But my problem is the query is not able to read variable begin_date and
end_date. It works fine when I passed the real value. Has anyone used
variable within a copy ?  Please let me know.

Thanks,

Dinesh