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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?

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

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