RAR

Template: Infobox file format / Maintenance / default missing

Template: Infobox file format / Maintenance / Screenshot Format

5261.7221.1A07.00 hex Rar! \ X1A \ a \ 0 ( ASCII C notation )

RAR is an algorithm, software and a file format for data compression to reduce the storage requirements of files for archiving and transmission. RAR has a good compression rate depending on the file size and number of files. It was named after its inventor, the format, Yevgeny Lasarevich Roshal, the name stands for Roshal ARchive. The archive files usually carry the extension rar. ; the MIME type is application / x - rar -compressed.

Technical Details

Solid compression

RAR supports the technique of progressive or solid archiving, not all are compressed individually or together with the selected for archiving files, but. This corresponds approximately to a previous Generate a tarball with subsequent compression, for example, with ZIP. Before compression RAR sorts the data according to similar patterns or file type. The advantage of the progressive compression is that redundancies between multiple files can be eliminated, (eg source code) unfolds the greatest effect on many smaller similar files. Overall, a significantly better compression rate is achieved. This approach, however, has two disadvantages: It can be extracted from an archive, any individual files, without decoding the entire data stream, which also contains unwanted files. In addition, leads a damaged archive in the loss of all your archived files (not just the file in question, as there are no more individual files are in a compressed state ).

Recovery Records

To minimize the risk of data loss due to defective archives, RAR support - unlike other compression formats like ZIP, TAR or gzip - so-called Recovery Records. With additional error correction data (similar to Reed -Solomon codes ), faulty RAR archives are repaired to a limited extent. The amount of the correction data can be freely specified when compressing a percentage.

In addition, (reconstruction archives) can be created, each of which is as large as the largest single part of the archives for multipart archives in RAR hot multiple volumes, so-called image archives. If, for example, a ten -part archive and two mirrors before and archives were damaged any two part archives, they can be completely reconstructed with the help of two mirrors archives. This procedure works, even if a part of archive not only damaged, but is not present. However, must - as with RAID n - always at least as many reconstruction archives as corrupted or missing archives to be present for this method can be applied.

Encoding

RAR supports encryption of the compressed data using the Advanced Encryption Standards and encrypting the file name so that one can not infer the contents of an archive without knowing the password. It can also Alternate Data Streams of a file system are saved (for example, NTFS, HFS, HPFS ), what this format on according systems qualify as suitable backup.

If a large file, for example a movie, distributed on multiple archives, the file name in the archives are often the same and can, with the exception of the first part of archive or related parts first, not be unpacked individually. The joining of the total file automatically after unpacking the archive first, provided that all packages are available.

Dissemination

Since the developers have allowed the royalty-free acquisition of Entpackroutinen since time immemorial and provide for required routines in the source code, decompress RAR files is now supported by virtually every multi-format pack program and on a variety of operating systems. The creation of RAR archives, so the compression algorithm is, however, not released for adoption and not publicly documented. Officially therefore support only published by the manufacturers themselves RAR WinRAR, RAR for DOS, RAR for Linux, Macintosh, etc. UnRarX for this functionality. An earlier version of the RAR format, however, was reproduced by the authors of the programs SpeedCommander and Squeez, why this can also create RAR archives of the now-obsolete version 2. Guarantee that such created RAR files are compatible with each RAR implementation, but it can not give.

Although RAR has a large fan base, it was able to prevail only sporadically compared to the widely used ZIP format despite the higher packing ratio and superior functions (among native splitting into several files). One main reason is that RAR, unlike ZIP, a proprietary file format and only the decompression algorithm fully documented and is available as open source. In addition, although RAR compressed significantly better than ZIP, there are now but similarly powerful, but there are open source formats, such as 7z. The main reason of the lower distribution but is probably that the ZIP format had found widespread use before development of the RAR RAR Packers and therefore always had to fight against a large inventory of existing archives ( see path dependency).

History

The first version of RAR, RAR 0.1, was released in March 1993, with the notice published in the autumn version 1.3 only found significant distribution. The corresponding version 1.3 of the file format had no magic number. In early 2000, work began on a free decoder (Unique RAR File Library, unrarlib ) based on a version of the official unrar Rarlabs of which could be made available with the permission of Eugene Roshal also under the GPL. The result was a free unrar, the archives for all versions of RAR format can unzip to 2.x. With version 2.9 of WinRAR from late September 2001, a new generation (format version 3) of the RAR format has been introduced, which, among other things, the use of better compression method ( LZSS, PPMd ) allows and with the possibility of embedded error correction data (or restore parts in multipart archives ) previously used additional use of PAR is unnecessary. Since July 2010 exists with version 2.5 of The Unarchiver also free ( LGPL) software for unpacking archives for version 3 of the RAR format, which can also handle multi-part, self-extracting archives with encrypted as well as with.

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