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Possible Duplicate:
What does “--” (double-dash) mean?
Single dashes - for single-character options, but double dashes -- for words?

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?

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marked as duplicate by jasonwryan, Michael Mrozek Nov 10 '11 at 0:08

This question was marked as an exact duplicate of an existing question.

@jasonwryan That one doesn't seem related. This looks like a combination of Usage of dash (-) in place of a filename and What does "--" (double-dash) mean? though; I think I'm going with the latter since that's the actual question at the end – Michael Mrozek Nov 10 '11 at 0:07
Thanks both @jasonwryan and Michael. I found Michael's answer helpful – omar Nov 10 '11 at 0:30
There's no -- in this question. Its - -`, meaning create the ZIP file on standard output, and read the data to compress from standard input. So I'm pretty sure it's unix.stackexchange.com/questions/16357/…. – Mikel Jun 25 '12 at 17:14