tar zxvf won't work with Redhat generated compressed tar file

[Eric Blake]

On 04/20/2015 03:08 PM, Murthy Gandikota wrote:

>>
>>> $ tar xvf mytar.gz
>>> tar: This does not look like a tar archive
>>> tar: Skipping to next header
>>> tar: Exiting with failure status due to previous errors
>>
>> Then what makes you think it is actually a tar archive?
>> What
>>
>> gunzip> mytar < mytar.gz; file mytar
>>
>> says?
>>

> Sorry, didn't notice the top-posting.... here is the  output to the command
> 
> mytar: gzip compressed data, last modified: Tue Mar 24 03:50:58 2015, from Unix

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

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