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That -D doesn't work is expected - it works as expected, but
probably not as *you* expected. I had to look at the code
to see what it really means. Basically, -D is turned on by
default (like the man page says) and -D is defined as a
toggle: this means that specifying -D will change the value
from 0 to 1 or 1 to 0, depending on what it is at the
moment. Given that -D is on by default, specifying -D
actually means that *no* directory should be created.
That's may be a surprising, but that's how it is.

With the Debian package, running jack -w foo -f bar works
fine. Just don't specify -D.
100:tbm@deception: ~/tmp/jack] jack --force -w foo -f x
This is jack 3.1.1 (C)2004 Arne Zellentin <zarne@users.sf.net>
....
102:tbm@deception: ~/tmp/jack] ll foo
total 4
drwxr-xr-x 2 tbm tbm 4096 2006-02-27 12:30 jack-c00b930e