You can use exec or run command to execute the pig script using the grunt shell. In case of run command, all the commands present in script file will be available in the grunt history, which is not the case with exec command.
First you need to go ahead and load the data file in PigStorage.
product = LOAD 'hdfs://localhost:9000/product_dir/products.csv' USING PigStorage(',') as (product_id:int, product_name:chararray, price:int);
Next you can go ahead and execute the script file, which is stored in HDFS.