(using Info-ZIP 2.3 on MDK 9.1)

I want to know how big a zipfile would be if I were to make one, but I
don't actually want to make it (at least not on my filesystem).

If I do this:
   zip -q whatever.zip *db
or this:
   zip -q - *db >whatever.zip

I get a file 6911 bytes long (and I expect these results to be the
same).

But I don't actually want to make the zipfile on my filesystem (even
temporarily).  So I try this:

   zip -q - *db |wc -c

and it tells me it's 7231 bytes.

Hmmmm.  Shouldn't that be 6911???

Hi davidfilmer-ga
Interesting question! With the help of the InfoZip man page (man zip),
I found that the program behaves differently in two situations. The
first situation is when the output is sent to a file or to the
terminal, the second when zip is used as a filter.
In the second situation, it writes a different type of file:

   zip archives created in this manner can be extracted with  the 
program
   funzip  which  is  provided in the unzip package, or by gunzip
which is
   provided in the gzip package.

If you would have done
   zip -q - *db | dd of=whatever.zip
then the resulting file would be of the length that wc reports.

I couldn't find a way to directly let zip write the same type of file,
i.e. like in situation one, but at least the different filesize isn't
an error or so.

Regards,
aurelus-ga

aurelus, thanks for your comments. Your information proved accurate,
and highlighted a somewhat frustrating aspect of InfoZIP (ie, that in
order to generate a 'real' zipfile that matches the filesize of the
calculated value, I would be forced to use a 'compatibility' mode that
reduces the effeciency of the algorythm).

Unfortunately, since you don't appear to be a researcher, you can't
get "official" credit or payment for your assistance, but you have my
sincere appreciation.

FWIW, your assistance actually helps me develop a Scouting website
(http://troop606.com) which is a functional prototype of an
open-source project (http://scoutsite.net). The photo galleries offer
a "download everything as a zipfile" option, where the zipfile is
built and provided on-the-fly. But it's nice to tell people how big it
would be BEFORE they actually download it.

Thanks again!