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From: Murthy Gandikota <murthy64@hotmail.com>
To: Gary Johnson <garyjohn@spocom.com>,
        "cygwin@cygwin.com"	<cygwin@cygwin.com>
Subject: RE: tar zxvf won't  work with Redhat generated compressed tar file
Date: Mon, 20 Apr 2015 23:52:42 +0000
In-Reply-To: <20150420233848.GB5293@phoenix>
References: <COL130-W80F61D41DBC766A5C40206C2E00@phx.gbl> <969800560.20150420222609@yandex.ru> <COL130-W78804B6BA04135A92CC2FAC2E00@phx.gbl> <509379965.20150420235036@yandex.ru> <COL130-W65B445CB84F1010DEEDA6EC2E00@phx.gbl> <55356CBB.9030601@redhat.com> <COL130-W530B6F57CC449D3CD22F44C2E00@phx.gbl> <COL130-W103DBAFD945896AC6F7E51C2E00@phx.gbl>,<20150420233848.GB5293@phoenix>
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----------------------------------------
> Date: Mon, 20 Apr 2015 16:38:49 -0700
> From: garyjohn@spocom.com
> To: cygwin@cygwin.com
> Subject: Re: tar zxvf won't work with Redhat generated compressed tar file
>
> On 2015-04-20, Murthy Gandikota wrote:
>
>>>> Then it looks like you have a DOUBLE-compressed file. That is, someone
>>>> took a .tar.gz file, and ran THAT through gzip again (which seldom does
>>>> anything except make a LARGER file - because the first round of
>>>> compression removed any redundancy). Tar cannot read a
>>>> double-compressed stream, but breaking things into two steps lets you
>>>> get back to a single compressed stream, where the tar call then
>>>> auto-decompresses because you weren't supplying an explicit 'z' the
>>>> second time around. To prove it, try:
>>>>
>>>> gunzip <mytar.gz>mytar
>>>> tar zxvf mytar
>>>>
>>>> and if it still untars with an explicit decompression, then you have
>>>> proven that your original file was double-compressed. Also, if I'm
>>>> right about double compression, then mytar.gz would likely be slightly
>>>> larger than mytar (rather than the usual case of the .gz being
>>>> noticeably smaller).
>>>>
>>>> --
>>>> Eric Blake eblake redhat com +1-919-301-3266
>>>> Libvirt virtualization library http://libvirt.org
>>>>
>>>
>>> Yes, Sir. It did untar properly. So the problem is with the
>>> server compressing during the download (I get the file from a
>>> http server)?
>>> Thanks a lot for your help
>
>> It would be doubly nice if cygwin handled it ;-) It seems gunzip
>> had no problem recognizing the file as double compressed.
>> Thanks all for the help.
>
> I think you misunderstand. Gunzip decompresses once. As Eric
> wrote, tar recognizes compressed files and will decompress them
> automatically even without specifying a compression flag such as
> 'z'. The second decompression is done by tar, not by gunzip.
>
> Regards,
> Gary
>

Got it! Thanks a ton.

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