File archiver
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A file archiver combines a number of files together into an archive file, or series of archive files, often for easy transportation or storage. Many file archivers use data compression in order to temporarily reduce the size of the computer files.
The process of making one such container or archive file is called archiving or packing. Reconstructing the original files from the archive is termed unarchiving or unpacking.
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 restore any meta-information about a file that an operating system provides (like ownership, timestamps, access control).
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Data compression
Most archivers incorporate lossless data compression so the files are identical before and after archiving. As well, compressed archives take less space or bandwidth and therefore make for faster and cheaper transport and storage.
Types of archive formats
Due to the many different compression algorithms used, a number of incompatible archive formats exist, including ace, arj, lha, rar, zip, and 7z.
Historical use of archiving
Archiving has been around since the days of the operating system Unix, where some of the first file archiving programs were developed.
Unix
The Unix tools ar, tar, cpio (for "archiver", "tape archiver, and "copy in/out", respectively) are examples of such a basic archiver.
Users of basic archivers may reach a similar effect through compressing the result of packing (and uncompressing before unpacking). In fact modern tar programs includes support for calling 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 eliminated. An archiver compressing each archived file in isolation cannot exploit these inter-file redundancy.