Extract file from tar home/

[Ed Greshko]

On 06/09/2010 08:46 AM, Patrick O'Callaghan wrote:
> On Tue, 2010-06-08 at 16:36 -0700, Suvayu Ali wrote:
>   
>>> To extract only a specific file:
>>> $ tar xf bobg.tar.gz the/file/you/want
>>>
>>>       
>> I think the OP's worry is not whether it can be done, but he wants to 
>> avoid the time and CPU cycles involved in the gunzip step. Since he
>> has his entire home directory in the tarball, even extracting a single
>> file requires tar to decompress the entire tarball before it can
>> extract that one file.
>>     
> Without looking at the source code one can't be sure, but I'd be
> surprised if that were literally true. IOW I doubt that tar decompresses
> everything to a temp file and then searches for the target. It should
> only be necessary to decompress up to the position of the target file
> (recall that tar originally meant "tape archiver", i.e. it's very
> focussed on doing things in a single sequential pass). I assumed that
> was what the OP wanted to do, and I believe the solution offered
> achieves it. Furthermore, given that the Gzip compression algorithm is
> stream-based, there cannot by definition be any substantially more
> efficient way of doing it other than decompressing the stream up to the
> point of interest.
>
> The alternative archive method (compress each file and then tar up the
> result), would be somewhat easier to extract in this specific case, but
> at the cost of a poorer compression ratio. It's the old space/time
> tradeoff once again.
>
>
>   
If one does a "tar -xf bobg.tar.gz the/file/you/want"  it is easy to see
that gzip is consuming the highest cpu and that it is using a pipe file
as is tar.  No temp files are created.

I don't know about anyone else here....  But if I have a large tar
archive I include as the first file in the archive a "ls" of what is in
the archive.  So when I forget what is really in the archive I simply
first extract the "ls" find what I want and then do the extract as you
have indicated.  To me, archiving a bunch of smaller tars, zips, etc.
doesn't make much sense as it makes it harder for me to find what I am
looking for, i.e. lost.

-- 
As far as we know, our computer has never had an undetected error. --
Weisert 葛斯克 愛德華 / 台北市八德路四段

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