Rezip

A script to rewrite a zip archive, possibly changing the compression level.

Introduction

This script is meant to rewrite a zip archive, offering control on the options used to read and rewrite the archive itself.

The intended use of the script is to change the way in which components files are stored in the zip archive, and particularly their compression level. Specifically, it is possible to rewrite an archive so that its component are only stored, rather than deflated. Alternatively, it is possible to rewrite an archive file to attain maximum compression of its component files.

This functionality can be particularly useful when working with the git revision control system. Git, as many revision control systems, uses a delta format to efficiently store multiple versions of the same file. However, many software packages nowadays rely on zip based storage formats from which it is impossible to extract deltas when compression is used. In fact, a side effect of compression is to spread a local change throughout a large part of the file. For instance, this is the case with Libreoffice/Openoffice (that use the ODF format), MS Office (when using the docx format), Java (using Jar), SweetHome3D (using sh3d files) and more.

By using this script, compression can be removed when checking a file into the revision control system and reintroduced on checkout.

Usage with git

In order to automatically invoke the script from git, one can rely on the clean/smudge functionality.

For instance, one may introduce the following in the git configuration

[filter "opendocument"]
        clean = rezip -p ODF_UNCOMPRESS2
        smudge = rezip -p ODF_COMPRESS2

and then have a .gitattributes or a .git/info/attributes file with

*.odt filter=opendocument
*.ods filter=opendocument
*.odg filter=opendocument
*.odp filter=opendocument
*.odm filter=opendocument

Bugs and limitations

The script is hackish. This is because zip has different ways of storing file attributes (unix, fat, etc.). When unzipping and rezipping on a given host the way in which the attributs are stored may get changed to that corresponding to the particular host operating system and setting. Unfortunately, in zip/unzip there is no explicit way to change this behavior. To cope with this issue, some workarounds are employed. Specifically, extended attributes are dropped and an explicit umask setting is used.

Nonetheless, things might not be fully portable between different linux or unix variants, and certainly are not portable to Windows.

One day, all this will be rewritten to use a proper zip library allowing a finer control of attributes and such.