File archiver

A file archiver is a computer program that combines a number of files together into one archive file, or a series of archive files, for easier transportation or storage. Many file archivers employ Archive formats that provide lossless data compression to reduce the size of the archive which is often useful for transferring a large number of individual files over a high latency network like the Internet.

The most basic archivers just take a list of files and concatenate their contents sequentially into the archive. In addition the archive must also contain some information about at least the names and lengths of the originals, so that proper reconstruction is possible. Most archivers also store metadata about a file that the operating system provides, such as timestamps, ownership and access control.

The process of making an archive file is called "archiving" or "packing". Reconstructing the original files from the archive is termed "unarchiving", "unpacking" or "extracting".

Discussion

Unix Archiver Tools

Unlike integrated archival and compression tools like PKZIP, Winzip, and WinRAR, the Unix tools ar, tar, cpio (for "archiver", "tape archiver" and "copy in/out" respectively) act as archivers but not compressors. Users of the Unix tools typically add compression by compressing the result of packing (and uncompressing before unpacking), most often using the gzip or bzip2 programs. In fact modern tar programs include an option to automatically call a (de)compression program, so that it looks just as if tar itself could handle compressed archives. This approach has two advantages:

*It follows the Unix toolbox concept that each program should accomplish a "single" but well-done task. Once a better compressor is developed, users may use that immediately, without having to give up their archiver.
*Since the whole archive is compressed, redundancy between archived files can be detected and eliminated. An archiver compressing each archived file in isolation cannot exploit these inter-file redundancies.

Its main disadvantage is that extracting one file from a compressed archive requires all the files before it to be decompressed, which may take many minutes for a large archive. Altering the underlying archive is even more inconvenient, requiring the entire file to be uncompressed, altered and then recompressed. Archivers with integrated compression perform these operations much more quickly.

See also

* Comparison of file archivers
* Archive format
* List of archive formats
* Comparison of archive formats

External links

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