ZIP

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The Zip archive format was created by the late Phil Katz of PKWare, Inc. It is the most popular of the compression formats.

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[edit] Introduction

A ZIP file performs two functions. It assembles a set of files together into one file, called an archive, and it compresses the file to make it smaller (although compression can be disabled for faster unarchiving). The data in the ZIP file will generally be extracted prior or during its use. It is also possible to have part of the file uncompressed and other parts compressed. This is usually done in two steps where the uncompressed file(s) are added first and then the compressed data. Uncompressed data can be view or read without unzipping the file if it is done first, although there will be some amount of binary data at the beginning of the file prior to the text.

Phil began by creating archive utilities to create and extract files in the .ARC format used by the older Arc utility from Seaware. After legal battles with Seaware, Phil created a new archive format called Zip, and placed the format in the public domain so anyone could created archivers that used it.

Katz's original program, PKZip, still exists, as well as a Windows GUI version. But at this point, the most popular Zip archive utility is WinZip from Nico Mak.

[edit] Sources for Zip

Many archive utilities, both freeware and shareware, exist to handle Zip archives. As of Windows XP, Windows has built-in support for viewing and extracting Zip files in a feature called Zip Folders. It can also create them. Support for ZIP files is built into Windows XP, Windows MCP, Windows Vista, Windows 7 and later.

Info-Zip is an open source command line version that is available for many different operating systems. Info-Zip has many commands to create and manipulate zip files. There is a GUI front end for Info-Zip called WIZ for Windows and MacZip for MacOS X. The full zip CLI installation includes: zip, zipnote, zipsplit, zipcloak, unzip, zipinfo, funzip, and unzipsfx. (The last is used to make self-executables).

There is also an open source option at Yahoo widgets where there is a simple utility called ZIPPER which works effortlessly. You simply drag your file into the window for zipping or unzipping.

[edit] Embedded zip

More and more tools are now using zip to collect files and compress them while hiding this fact from the users. The tools themselves are able to work in these compressed files with no visible decompress step. eBook (eDocument) File formats that use this technique include ePUB, CBZ, SGF, ODT and DOCX. TIF can also be a compressed collection of images that use zip format although other compression methods can also be used.

The file format FB2 can also typically read a zip file by programs like FBReader or Cool Reader so you may see a file in the form of filename.fb2.zip. This double extension trick is non-standard and can lead to problems with some systems.

A zip file can also embed itself in an EXE file which will cause it to self extract. A small extraction executable is prepended to the to the zip file to accomplish this. Zip programs such as info-zip provide this stub executable that can be added to an existing zip.

Generally when a file uses embedded zip a standard zip program can still be used to inspect the contents or even extract them.

[edit] 7zip

Another popular free zip utility is 7zip. It can view a zipped file and permits you to drag files in or out of it. This tool can be very handy for tweaking an ePub file, even without changing the file extension. It will often compress higher than other zip tools. It can also use a higher compression native mode called 7ZIP or 7z, and can work with RAR files as well. It supports the following formats:

  • Packing / unpacking: 7z, XZ, BZIP2, GZIP, TAR, ZIP and WIM
  • Unpacking only: ARJ, CAB, CHM, CPIO, CramFS, DEB, DMG, FAT, HFS, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, RAR, RPM, SquashFS, UDF, VHD, WIM, XAR and Z.

Note that in windows this tool will add itself to the main selection menu.

[edit] GZIP

GZIP is a variation of the zip format (.gz) that was developed by the GNU team. It is designed to be zipped or unzipped on the fly (perhaps in a pipe) and only supports one file. Often the file is a tar (.tar) format which is an container (archive) format. When used together the file extension is usually .tgz.

Gzip is also the compressed format used by ABW where is it called ZABW. It is also the compression method used in zText files where it read directly without a separate decompression.

[edit] Comments

The Zip file also supports the ability to add a comment. The Info-Zip command ZipNote can read and write this field in the zip file. It is an uncompressed section of the file so that unzipping is not necessary. There is no prescribed use for this entry but some are beginning to use it for metadata for the contained item, such as a CBZ content.

[edit] For more information

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