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In computing, RAR is a proprietary file format for data compression and archiving.
The RAR file format was developed by Eugene Roshal (hence the name RAR: Roshal ARchive), who was born on March 10, 1972 in Russia and graduated from Chelyabinsk Technical University. He also developed programs for packing and unpacking RAR files, originally for DOS, and later ported to other platforms. The encoder (the main Windows version known as WinRAR) is distributed as shareware, but Roshal has released the decoder's source code under a license that allows free distribution and modification, on condition that it is not reverse-engineered to build a compatible encoder. The current developer is Alexander Roshal. The encoding method is held to be proprietary, but compatible programs for decompression are available for several platforms, such as the open-source 7-Zip.
The files extension is .rar. The MIME-Type is application/x-rar-compressed.
Comparison to other compression algorithms
As computing power has increased according to Moore's Law, users have responded by preferring compression algorithms which achieve a better rate of compression at the expense of computational time when compressing and decompressing. RAR compression operations are typically much slower than compressing the same data with early compression algorithms like ZIP and gzip, but a much better rate of compression is achieved whenever the data can still be compressed further. If the data is as tight as it can get, such as if it is already compressed, all archivers may produce a file which will turn out to be larger than the original, due to computational overhead. Bzip2, which came after ZIP and gzip, compresses more efficiently at a cost of computational time compared to them, but is in turn less efficient and computationally cheaper than RAR. 7z's LZMA algorithm is quite similar to RAR in providing extremely high compression efficiency at the cost of compute time to compress and decompress. Both provide among the highest compression efficiency of any popular scheme, with the question of which algorithm is the more efficient compression scheme strongly depending on the files being compressed. RAR is a proprietary algorithm, which could be counted against it if the comparison is not made directly on the grounds of effectiveness. Both WinRAR and 7-Zip are still being actively developed.
Archiver features
Apart from the excellent rate of compression, RAR has several other features not previously found in many other archive formats, which has contributed greatly to its popularity:
- Its handling of split volumes is very good. The most notable such format, before the advent of RAR, was ARJ. It is unnecessary to use split volumes for this purpose alone since just binary splitting the files will work fine, and they can be reassembled with cat or binary copy. Multi-volume files have wide use though, mainly because they're seen as easier to handle by novice users.
- RAR archives can be of a solid format, in which all of the compressed files are treated as a single data block. Most currently used compression formats (with the exception of the older ZIP) allow solid structuring.
- It features strong encryption capabilities. Older versions of the file format used a proprietary but strong encryption algorithm; newer versions use the AES encryption algorithm, which is considered very strong by today's standards. The only ways to recover an encrypted file are via dictionary or brute force attacks, which are usually infeasible, depending on the passphrase length.
- Extra redundancy can be added to an archive, making it more resistant to corruption. Even if part of an archive is damaged, it may be possible to fully recover the stored data if a large enough "recovery record" exists.
- RAR provides effective lossless compression of Linear PCM audio files (commonly stored as .wav or .au) where other generalised compression formats (zip, 7-zip) do not. The compression ratio is typically between 50%–90% dependent on the complexity of the source audio and the predictability of the incoming waveform. While RAR does not compress PCM audio as well as, say, WavPack, Monkey's Audio or FLAC, this feature is of considerable benefit to musicians and composers, as entire projects can be backed up and compressed properly without having to losslessly compress the audio files in advance.
- Versions of RAR up to 2.50 closely resemble earlier versions of Norton Commander.
- (Win)RAR also has the capability of storing NTFS streams and security information within the archive – information that is usually lost on compression.
- RAR files can be embedded in other file types, probably the most common being JPEG. Image handling programs, browsers, and other utilities usually ignore any additional data after the end of the image, while RAR ignores anything before the RAR header. The procedure to create such a file is to append a RAR file to a JPEG. (e.g.: in Windows: copy /b image1.jpg+something.rar image2.jpg, in UNIX: cat image1.jpg something.rar > image2.jpg).
- RAR features six levels (0..5) of compression unlike most other archive formats feature ten (0..9).
See also
External links
- Official RAR site, containing archivers for several platforms
- MacRAR
- UnRarX – UnRar for Mac OS X
- RAR add-ons at official site – UnRAR for various platforms, including Eugene Roshal's source code
- Unrarlib – a library for decoding RAR (version 2) archives based on the Roshal's code, is GPL compatible
- unrar – a simple free-software unrar utility using Unrarlib above
- 7-zip an open source windows file archiver that supports unpacking (but not packing) RAR files, as well as other formats.
- Dziobas Rar Player Play dvix/xvid movies from rar files.
Archive formats | Lossless compression algorithms
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"RAR (file format)"