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A file archiver is a computer program that combines a number of files together into one
archive file In computing, an archive file is a computer file that is composed of one or more files along with metadata. Archive files are used to collect multiple data files together into a single file for easier portability and storage, or simply to compress ...
, or a series of archive files, for easier transportation or storage. File archivers may employ
lossless data compression Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statistic ...
in their
archive format In computing, an archive file is a computer file that is composed of one or more files along with metadata. Archive files are used to collect multiple data files together into a single file for easier portability and storage, or simply to compr ...
s to reduce the size of the archive. Basic archivers just take a list of files and concatenate their contents sequentially into archives. The archive files need to store metadata, at least the names and lengths of the original files, if proper reconstruction is possible. More advanced archivers store additional metadata, such as the original timestamps,
file attribute File attributes are a type of meta-data that describe and may modify how files and/or directories in a filesystem behave. Typical file attributes may, for example, indicate or specify whether a file is visible, modifiable, compressed, or encrypted ...
s or access control lists. 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''.


History

An early archiver was the
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of t ...
command ''archive'', descended from the CTSS command of the same name, which was a basic archiver and performed no compression. Multics also had a "tape_archiver" command, abbreviated ''ta'', which was perhaps the forerunner of the Unix command ''tar''.


Unix archivers

The
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
tools '' ar'', ''
tar Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black b ...
'', and ''
cpio cpio is a general file archiver utility and its associated file format. It is primarily installed on Unix-like computer operating systems. The software utility was originally intended as a tape archiving program as part of the Programmer's Work ...
'' act as archivers but not compressors. Users of the Unix tools use additional compression tools, such as
gzip gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and in ...
, bzip2, or xz, to compress the archive file after packing or remove compression before unpacking the archive file. The
filename extension A filename extension, file name extension or file extension is a suffix to the name of a computer file (e.g., .txt, .docx, .md). The extension indicates a characteristic of the file contents or its intended use. A filename extension is typically d ...
s are successively added at each step of this process. For example, archiving a collection of files with ''tar'' and then compressing the resulting archive file with ''gzip'' results a file with .tar.gz extension. This approach has two goals: # It follows the
Unix philosophy The Unix philosophy, originated by Ken Thompson, is a set of cultural norms and philosophical approaches to minimalist, modular software development. It is based on the experience of leading developers of the Unix operating system. Early Unix d ...
that each program should accomplish a single task to perfection, as opposed to attempting to accomplish everything with one tool. As compression technology progresses, users may use different compression programs without having to modify or abandon their archiver. # The archives use
solid compression In computing, solid compression is a method for data compression of multiple files, wherein all the uncompressed files are concatenated and treated as a single data block. Such an archive is called a solid archive. It is used natively in the 7z a ...
. When the files are combined, the compressor can exploit redundancy across several archived files and achieve better compression than a compressor that compresses each files individually. This approach, however, has disadvantages too: # Extracting or modifying one file is difficult. Extracting one file requires decompressing an entire archive, which can be time- and space-consuming. Modifying one means the file needs to be put back into archive and the archive recompressed again. This operation requires additional time and disk space. # The archive becomes damage-prone. If the area holding shared data for several files is damaged, all those files are lost. # It is impossible to take advantage of redundancy between files unless the compression window is larger than the size of an individual file. For example, gzip uses DEFLATE, which typically operates with a 32768-byte window, whereas bzip2 uses a
Burrows–Wheeler transform The Burrows–Wheeler transform (BWT, also called block-sorting compression) rearranges a character string into runs of similar characters. This is useful for compression, since it tends to be easy to compress a string that has runs of repeated c ...
roughly 27 times bigger. xz defaults to 8 MiB but supports significantly larger windows.


Windows archivers

The built-in archiver of
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for se ...
as well as third-party archiving software, such as
WinRAR WinRAR is a trialware file archiver utility for Windows, developed by Eugene Roshal of win.rar GmbH. It can create and view archives in RAR or ZIP file formats, and unpack numerous archive file formats. To enable the user to test the integrity ...
and 7-zip, often use a
graphical user interface The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows User (computing), users to Human–computer interaction, interact with electronic devices through graphical icon (comp ...
. They also offer an optional command-line interface, while Windows itself does not. Windows archivers perform both archiving and compression.
Solid compression In computing, solid compression is a method for data compression of multiple files, wherein all the uncompressed files are concatenated and treated as a single data block. Such an archive is called a solid archive. It is used natively in the 7z a ...
may or may not be offered, depending on the product: Windows itself does not support it; WinRAR and 7-zip offer it as an option that can be turned on or off.


See also

* Comparison of file archivers *
Archive format In computing, an archive file is a computer file that is composed of one or more files along with metadata. Archive files are used to collect multiple data files together into a single file for easier portability and storage, or simply to compr ...
*
List of archive formats This is a list of file formats used by archivers and compressors used to create archive files. Archiving only Compression only Archiving and compression Data recovery Comparison Containers and compression Notes While the original ...
* Comparison of archive formats


References


External links

* {{DEFAULTSORT:File Archiver Storage systems Computer file systems Computer archives Utility software types