mac geekery

Answer

Spotlight does this for you, really. As files are accessed, Spotlight updates its database with the last access time for every file. All you need to do is query that database to get the right answer from it and then pass the files to shell tools to do the rest. It’s another handy one-liner.

The tool to use to query Spotlight is mdfind. By using Spotlight’s query syntax you can construct a query that will always ask for the proper time period. Read the link to see how I came up with:

$ mdfind '(kMDLastUsedDate > $time.this_month)'

This will return a list of files used in the last month. It’s downhill from there. Send that to xargs and then to du to get the space used:

$ mdfind -0 '(kMDItemLastUsedDate > $time.this_week)' | xargs -0 du -csk

So, we’re telling mdfind to find on the given criteria and then null-terminate the output so we can handle quotes and other fun. xargs then looks for nulls and passes them as arguments to du which gets a summary of each item in kilobytes as well as a grand total. You can pipe it further to tail -1 to get the total line only if that’s what you’re interested in.

Of course, as-is we have a small problem in that every file on the system is checked, including system files. This doesn’t sound like what you want. Handily, mdfind can anchor its search by doing something like the following:

$ mdfind -onlyin /Shared\ Items -0 '(kMDItemLastUsedDate > $time.this_week)' | xargs -0 du -csk

Now it only searches “Shared Items” for the files. That’s probably more along the lines of what you want.