Export:
mysqldump -u username -p database_name > dump.sql
Import:
mysql -u username -p database_name < dump.sql
Live export/duplication (note you may need to enter the password on the line for the mysql command):
mysql -uuser -ppassword -e 'DROP DATABASE test_db;'
mysql -uuser -ppassword -e 'CREATE DATABASE test_db;'
mysqldump -uuser -ppassword live_db | mysql -uuser -ppassword test_db;