I was reading the man zip page and I found this examples:
[...] using the command
unzip -p backup | tar xf -
When [...] For example,
tar cf - . | zip | dd of=/dev/nrst0 obs=16k
is equivalent to
tar cf - . | zip - - | dd of=/dev/nrst0 obs=16k
I' like know the minus utility of these cases. Why two minus signs are written in the third case?