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Full Discussion: compress/decompress don't take you back to the same file
Top Forums UNIX for Dummies Questions & Answers compress/decompress don't take you back to the same file Post 302283195 by chebarbudo on Monday 2nd of February 2009 09:10:55 PM
Old 02-02-2009
Thanks Lakris,
You are right. I knew very few about tars before I had this problem.
So I explored in details the content of the archives using
Code:
tar -tvf system.tgz > system.lst
tar -tvf system.01.tgz > system.01.lst
diff system.lst system.01.lst

1) Path, attributes and size are strictly identical before and after decompression/recompression.

2) Dates can be different:
- regular files, block and character devices, hard links and named pipes always keep the same date
- symbolic links never keep the same date. Actually they get the current time of decompression.
- I was not able to build any theory about directories

3) Owners are different in some cases. I assume that only UID/GID is kept for the owner. So if the archive comes from another system (which is my case), UID/GID can result in a different username/groupname.

If anyone has serious knowledge about tars, can he/she confirm what I said?
Can this explain the difference between the original archive and the one resulting of a decompression/recompression?

Thanks for your help
Santiago
 

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LEARN ABOUT DEBIAN

advzip

AdvanceCOMP ZIP Compression Utility(1)			      General Commands Manual			    AdvanceCOMP ZIP Compression Utility(1)

NAME

       advzip - AdvanceCOMP ZIP Compression Utility

SYNOPSIS

       advzip [-a, --add] [-x, --extract] [-l, --list]
	    [-z, --recompress] [-t, --test] [-0, --shrink-store]
	    [-1, --shrink-fast] [-2, --shrink-normal] [-3, --shrink-extra]
	    [-4, --shrink-insane] [-N, --not-zip] [-p, --pedantic] [-q, --quiet]
	    [-h, --help] [-V, --version] ARCHIVES... [FILES...]
DESCRIPTION

       The main purpose of this utility is to recompress and test the zip archives to get the smallest possible size.

       For recompression the 7-Zip (www.7-zip.com) Deflate implementation is used. This implementation generally gives 5-10% more compression than
       the zLib Deflate implementation.

       For experimental purpose also the 7-Zip LZMA algorithm is available with the -N option. In this case, the generated zips WILL NOT BE USABLE
       by  any	other  program.  To make them usable you need to recompress them without the -N option. Generally this algorithm gives 10-20% more
       compression than the 7-Zip Deflate implementation.

OPTIONS

       -a, --add ARCHIVE FILES...
	      Create the specified archive with the specified files. You must specify only one archive.

       -x, --extract ARCHIVE
	      Extract all the files on the specified archive. You must specify only one archive.

       -l, --list ARCHIVES...
	      List the content of the specified archives.

       -z, --recompress ARCHIVES...
	      Recompress the specified archives. If the -1, -2, -3 options are specified, it's used the smallest file choice  from:  the  previous
	      compressed  data,  the  new  compression	and the uncompressed format. If the -0 option is specified the archive is always rewritten
	      without any compression.

       -t, --test ARCHIVES...
	      Test the specified archives. The tests may be extended with the -p option.

       -N, --not-zip
	      Use the LZMA algorithm when compressing. The generated zips will not be readable by any other application!

       -p, --pedantic
	      Be pedantic on the zip tests. If this flag is enabled some more extensive tests on the zip integrity are done. These tests are  gen-
	      erally not done by other zip utilities.

       -0, --shrink-store
	      Disable  the  compression. The file is only stored and not compressed. This option is very useful to expand the archives of .png and
	      .mp3 files. These files are already compressed, trying to compress them another time is really a waste of time and resource.

       -1, --shrink-fast
	      Set the compression level to "fast".

       -2, --shrink-normal
	      Set the compression level to "normal". This is the default level of compression.

       -3, --shrink-extra
	      Set the compression level to "extra".

       -4, --shrink-insane
	      Set the compression level to "insane".

COPYRIGHT

       This file is Copyright (C) 2002 Andrea Mazzoleni, Filipe Estima

SEE ALSO

       advpng(1), advmng(1), advdef(1)

													    AdvanceCOMP ZIP Compression Utility(1)
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