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I would like to skip a large gzip file when I extract a tar file, but everytime tar starts to gunzip it to look inside it seems.

Even when I just try to peek inside tar seems to start gunzipping it, for example:

tar -tvf my.tar.gz --exclude="huge_mysql_file.gz"

Any tips how to skip entirely that file? I see gzip running when tar gets to that gz file.

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why do you think that tar is gunzipping the file? –  miracle173 Sep 30 '12 at 7:27
    
Try gzip -dc my.tar.gz | tar tf -. –  Thor Sep 30 '12 at 8:04
    
It always gets stuck at a large gz file, and when I look at top in another shell window I can see gzip running next to tar. –  giorgio79 Sep 30 '12 at 9:16
    
I don't really understand you problem here, but it seems that inside that .tar.gz are other .gz files? If that's the case, then the archive is wrong to begin with. a .tar.gz file, is a gziped tar-file, which contains a collection of uncompressed files. Everything else is pretty much done by an idiot. –  polemon Sep 30 '12 at 12:56
1  
As others have pointed out, tar has to read .tar.gz files sequentially, so it can't find out what's after "huge_mysql_file.gz" without reading it in to get to the next file in the archive. If you need to view the contents of a .tar.gz file frequently, you can always create a text file containing the files list. e.g. tar tvzf my.tar.gz > my.tar.gz.list. you can then read that .list file with less or grep it etc. –  cas Sep 30 '12 at 22:43
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up vote 7 down vote accepted

my.tar.gz is a gzipped tar file. tar ist short for 'tape archive'. So the file my.tar.gz has to be unzipped by gzip -d before the tar file can be read. There is no random access to the content of a gzip file. I do not really know the gzip file format but at least there is no option for gzip to do this. The 'gzip -d' started by tar to unzip the my.tar.gz file is the gzip you can see. tar does not decompress the file huge_mysql_file.gz but it has to read (and ignore) the file huge_mysql_file.gz because this is provided by the pipe to gunzip.

if the tar file is not gzipped and you list its content like in

tar -tvf my.tar --exclude="huge_mysql_file.gz"

only the header of huge_mysql_file.gz in the archive must be read to know its size. Then tar can skip the file without reading it and continue on the following file.

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Note that tar cannot do sequential access in this case because the archive is read from a pipe (from gzip), but for a tar in a regular file (uncompressed), it will not read the full content and will skip to the start of each file header in that archive (for -t to display the content) –  Stephane Chazelas Sep 30 '12 at 21:50
    
thx to @sch: I will check this and adjust my answer as soon as possible –  miracle173 Oct 1 '12 at 6:08
    
thx again @sch: I rewrote the post and hope now there are no more errors –  miracle173 Oct 7 '12 at 9:11
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