compress/decompress don't take you back to the same file
Hi there,
I have a file system.tgz. I want to make changes inside this archive but before that, I want to be sure I recompress it in the exact same format.
So I did the following:
Code:
me:~# # Duplicate the archive ---------------------------------
me:~# cp system.tgz system.01.tgz
me:~# ls -l system.tgz system.01.tgz
-rw-r--r-- 1 root root 274650447 2009-02-02 22:41 system.01.tgz
-rw-r--r-- 1 root root 274650447 2009-02-02 21:32 system.tgz
me:~#
me:~# # Unzip the archive -------------------------------------
me:~# mkdir system.01/
me:~# tar -zxf system.01.tgz -C system.01/
me:~#
me:~# # Rezip the archive -------------------------------------
me:~# cd system.01/
me:~/system.01# tar -zcf system.01.tgz *
me:~/system.01# mv system.01.tgz ../
me:~/system.01# cd ..
me:~#
me:~# # Compare with previous archive -------------------------
me:~# ls -l system.tgz system.01.tgz
-rw-r--r-- 1 root root 274642873 2009-02-02 22:54 system.01.tgz
-rw-r--r-- 1 root root 274650447 2009-02-02 21:32 system.tgz
As you can see, the archive is not exactly as it was. Maybe I'm not using the same compression level? But I have no idea how to set this parameter.
Thanks for your help.
Santiago
Hi,
without analysing it too much,
have You examined that the pathnames stored in the files are the same? If You change the name of a directory it is compressed from it will affect the compression since size and sequence of characters differ and pathnames are stored in the compressed file.
Just a thought.
Ok, well that was just my first idea, that since tar contains META information, there are a lot of thing that could affect the final size of an archive. I wouldn't trust tar size alone as the criteria.
Are the files created with the same version of tar?
/L
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?
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02-02-2009
compress/decompress don't take you back to the same file