Writing shell script to add/update/remove data from flat file

I wrote a shell script with help from this list to add data from ACII file. This works like a charm. I used the ISQL, LOAD, and INSERT command to do this. My question is that how would I update or remove the data using the flat file? In ascii file, first character designates whether I want to remove or update. Please help me if this is possible using shell script and ISQL comands. Some suggested that I could write C and do this using 4gl. But the problem is that I don't know C but if you have sample example, I would really appreciate. I need to do this right away. Thanks alot. Ruchi.