08-03-2011
tar and gzip extraction issues
Not sure if this is really in the right forum but here goes....
Looking for a way to extract individual compressed files from a compressed tarball WITHOUT tar -zxvf and then recompressing. Basically we need to be able to chunk out an individual compressed file while it still remains compressed.
For whatever reason, when we tar out the single, then recompress it's destroying the file integrity. The files are so large that we can't just decompress the whole then pick out pieces, so that's out.
We had looked at going in and extracting the bits and writing them elsewhere, but from what I understand about how tar and gzip work, we'd get garbage because the gzip algorithm is predicated on the overall file rather than the pieces when working with a tarball. To gunzip the individual pieces would rely upon the initial Huffman double encoded "rosetta stone" that was generated from the overall tarball structure, right? Without that "rosetta stone", we'd not be able to accurately decompressed the individual .gzs....and a reverse algorithm wouldn't work because the initial was encoded off of patterns that were present in the whole but may not be present in the individual.
I'm a bit of a n00b, so I just need to check and make sure I've absorbed this all correctly. But, in case I've processed it all incorrectly and if there is a way or a script that can accomplish this, please point me in the right direction. Thanks.
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tar(n) Tar file handling tar(n)
__________________________________________________________________________________________________________________________________________________
NAME
tar - Tar file creation, extraction & manipulation
SYNOPSIS
package require Tcl 8.4
package require tar ?0.6?
::tar::contents tarball
::tar::stat tarball ?file?
::tar::untar tarball args
::tar::get tarball fileName
::tar::create tarball files args
::tar::add tarball files args
::tar::remove tarball files
_________________________________________________________________
DESCRIPTION
::tar::contents tarball
Returns a list of the files contained in tarball. The order is not sorted and depends on the order files were stored in the archive.
::tar::stat tarball ?file?
Returns a nested dict containing information on the named ?file? in tarball, or all files if none is specified. The top level are
pairs of filename and info. The info is a dict with the keys "mode uid gid size mtime type linkname uname gname devmajor devminor
% ::tar::stat tarball.tar
foo.jpg {mode 0644 uid 1000 gid 0 size 7580 mtime 811903867 type file linkname {} uname user gname wheel devmajor 0 devminor 0}
::tar::untar tarball args
Extracts tarball. -file and -glob limit the extraction to files which exactly match or pattern match the given argument. No error is
thrown if no files match. Returns a list of filenames extracted and the file size. The size will be null for non regular files.
Leading path seperators are stripped so paths will always be relative.
-dir dirName
Directory to extract to. Uses pwd if none is specified
-file fileName
Only extract the file with this name. The name is matched against the complete path stored in the archive including directo-
ries.
-glob pattern
Only extract files patching this glob style pattern. The pattern is matched against the complete path stored in the archive.
-nooverwrite
Dont overwrite files that already exist
-nomtime
Leave the file modification time as the current time instead of setting it to the value in the archive.
-noperms
In Unix, leave the file permissions as the current umask instead of setting them to the values in the archive.
% foreach {file size} [::tar::untar tarball.tar -glob *.jpg] {
puts "Extracted $file ($size bytes)"
}
::tar::get tarball fileName
Returns the contents of fileName from the tarball
% set readme [::tar::get tarball.tar doc/README] {
% puts $readme
}
::tar::create tarball files args
Creates a new tar file containing the files. files must be specified as a single argument which is a proper list of filenames.
-dereference
Normally create will store links as an actual link pointing at a file that may or may not exist in the archive. Specifying
this option will cause the actual file point to by the link to be stored instead.
% ::tar::create new.tar [glob -nocomplain file*]
% ::tar::contents new.tar
file1 file2 file3
::tar::add tarball files args
Appends files to the end of the existing tarball. files must be specified as a single argument which is a proper list of filenames.
-dereference
Normally add will store links as an actual link pointing at a file that may or may not exist in the archive. Specifying this
option will cause the actual file point to by the link to be stored instead.
-prefix string
Normally add will store files under exactly the name specified as argument. Specifying a ?-prefix? causes the string to be
prepended to every name.
-quick The only sure way to find the position in the tarball where new files can be added is to read it from start, but if tarball
was written with a "blocksize" of 1 (as this package does) then one can alternatively find this position by seeking from the
end. The ?-quick? option tells add to do the latter.
::tar::remove tarball files
Removes files from the tarball. No error will result if the file does not exist in the tarball. Directory write permission and free
disk space equivalent to at least the size of the tarball will be needed.
% ::tar::remove new.tar {file2 file3}
% ::tar::contents new.tar
file3
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category tar of
the Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883]. Please also report any ideas for enhancements you may have for
either package and/or documentation.
KEYWORDS
archive, tape archive, tar
CATEGORY
File formats
tar 0.6 tar(n)