Basic IF Command Question

Hi,

I have a months worth of data that I need to separate into weekly files. There is a date column with dates in the following format: YYYYMMDD.

I'm thinking I can create the weekly files by using a grep command combined with an IF command and specify each day of the specific week I'm extracting within the if command. Does this seem efficient? How would I set up the IF command?

cwl

This is extraordinarily difficult because the data does not include the week number within the year.
It is much easier to group the data by Month or Month-Day within the Year.

If weeks are important we would need to know:
What Operating System and version you are running?
What Shell you prefer?
Whether this is forward-looking processing which can be changed to include the Week Number in the data or retrospective processing which needs to derive the week number from a date in a data file?

Week numbering is itself a contentious topic. Some detailed examples would help (particularly around the change of year).
Beware of the accountancy "week 53".