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gzip produces different output from the same input

 
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# 1  
Old 06-29-2010
Question gzip produces different output from the same input
Hi there,

I'm puzzled. Compressing the same file (same name, same md5sum) at two different times will produce a different output. I mean the md5sum of the resulting .gz files are different.
Does it make any sens to any of you?
I'd like some explanations if you know what's going on.

Thanks
Santiago
# 2  
Old 06-29-2010
Yes. gzip saves the name of the original file,if possible, and timestamp. You can check it with the "file" command.
# 3  
Old 06-29-2010
Thanks pludi.
The thing is that I ran gzip for the exact same file twice:
Code:
santiago:~$ cp original copy
santiago:~$ gzip copy
santiago:~$ md5sum copy.gz
603343de9a6c295f00dc4759f6919031  copy.gz
santiago:~$ rm copy.gz
santiago:~$ cp original copy
santiago:~$ gzip copy
santiago:~$ md5sum copy.gz
e4da2538a243cb49cf6212a0a20a60ce  copy.gz

How can it be different?
# 4  
Old 06-29-2010
Because gzip saves, into the file, the modification time of the file, as well as the original file name if possible:
Code:
$ echo "TestTestTest" > test1.txt
$ stat test1.txt
  File: `test1.txt'
  Size: 13              Blocks: 8          IO Block: 4096   regular file
Device: 801h/2049d      Inode: 1872126     Links: 1
Access: (0644/-rw-r--r--)  Uid: (    1000/   pludi)   Gid: (  100/   users)
Access: 2010-06-29 14:04:56.000000000 +0200
Modify: 2010-06-29 14:04:56.000000000 +0200
Change: 2010-06-29 14:04:56.000000000 +0200
$ gzip -9c test1.txt > test1_1.txt.gz
$ touch test1.txt
$ stat test1.txt
  File: `test1.txt'
  Size: 13              Blocks: 8          IO Block: 4096   regular file
Device: 801h/2049d      Inode: 1872126     Links: 1
Access: (0644/-rw-r--r--)  Uid: (    1000/   pludi)   Gid: (  100/   users)
Access: 2010-06-29 14:05:14.000000000 +0200
Modify: 2010-06-29 14:05:14.000000000 +0200
Change: 2010-06-29 14:05:14.000000000 +0200
$ gzip -9c test1.txt > test1_2.txt.gz
$ md5sum test1_?.txt.gz
de2048a553d7f5fe9815f4d0f0c286e3  test1_1.txt.gz
02e66fb2b786c4585f012dff592010f3  test1_2.txt.gz
$ file test1_?.txt.gz
test1_1.txt.gz: gzip compressed data, was "test1.txt", from Unix, last modified: Tue Jun 29 14:04:56 2010, max compression
test1_2.txt.gz: gzip compressed data, was "test1.txt", from Unix, last modified: Tue Jun 29 14:05:14 2010, max compression

If it can't save the original files modification time (eg because it's being piped into gzip) it saves the creation time of the compressed file instead.
This User Gave Thanks to pludi For This Post:
ensameer
# 5  
Old 06-29-2010
Thanks again pludi
My problem is that I thought that copying a file actually kept the modification time wich is wrong.
I ran the same test with option -p to cp command and the conclusion is obvious:
Code:
santiago:~$ rm -f copy*
santiago:~$ cp -p original copy
santiago:~$ gzip copy
santiago:~$ md5sum copy.gz
8dab7a05d099f445698b04f0d1e59873  copy.gz
santiago:~$ file copy.gz
copy.gz: gzip compressed data, was "copy", from Unix, last modified: Tue Jun 29 14:48:31 2010
santiago:~$
santiago:~$ rm -f copy*
santiago:~$ cp -p original copy
santiago:~$ gzip copy
santiago:~$ md5sum copy.gz
8dab7a05d099f445698b04f0d1e59873  copy.gz
santiago:~$ file copy.gz
copy.gz: gzip compressed data, was "copy", from Unix, last modified: Tue Jun 29 14:48:31 2010

 
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