Extract file from tar home/

[Suvayu Ali]

On Wednesday 09 June 2010 11:13 AM, Patrick O'Callaghan wrote:
> On Wed, 2010-06-09 at 00:39 -0430, Patrick O'Callaghan wrote:
>> On Tue, 2010-06-08 at 21:55 -0500, Robert Nichols wrote:
>>> There is no way other than linear search to find a file in a tar
>>> archive, so tar always has to read** from the beginning of the archive
>>> until it comes to a file you want.
>>
>> IIRC for uncompressed tarballs this is not strictly the case. The layout
>> is basically metadata-file-metadata-file-... so it's possible to seek
>> over intermediate files in strides (as the metadata includes the file
>> size). For compressed tarballs of course this won't work, which is what
>> I was trying to say.
>>
>>> Even after it has extracted everything you
>>> asked for, tar will continue to the end of the archive looking for a
>>> possible later version of one of the files you wanted, appended with a
>>> --concatenate operation after the original archive was created.
>>
>> A good point which I hadn't considered.
>
> On further reflexion and a close reading of "info tar", we find the
> following:
>
> `--occurrence[=NUMBER]'
>       This option can be used in conjunction with one of the subcommands
>       `--delete', `--diff', `--extract' or `--list' when a list of files
>       is given either on the command line or via `-T' option.
>
>       This option instructs `tar' to process only the NUMBERth
>       occurrence of each named file.  NUMBER defaults to 1, so
>
>            tar -x -f archive.tar --occurrence filename
>
>       will extract the first occurrence of the member `filename' from
>       `archive.tar' and will terminate without scanning to the end of
>       the archive.
>
> So in case the under discussion (i.e. when there's no possibility of an
> updated version of the target appearing later in the tarball), the best
> approach would be:
>
> tar xf tarball.tar.gz --ocurrence target
>
> thus avoiding the unnecessary processing of the rest of the tarball.
>

This discussion has been very enlightening. Thank you all. :)

> poc
>

-- 
Suvayu

Open source is the future. It sets us free.