File extension BZ2 is found most commonly on UNIX-based operating systems, and is used to denote a file which has been compressed using bzip2, an open source lossless compression algorithm originally released in 1996 by developer Julian Seward.
Like most data compression algorithms developed primarily for use on UNIX-based operating systems, bzip2 contains no archiving facility and can only be used to compress single files. Instead, an archiving utility such as tar will be used to create an archive from a file and folder structure before a utility such as bzip2 or gzip is used to compress it. These are known as compressed archives, and are similar in nature to ZIP or RAR files in Windows.
BZ2 files contain a higher level of compression than other similar file formats such as GZIP, which is achieved by using a number of different compression techniques including Run Length Encoding (RLE), Burrows-Wheeler transform and Huffman coding, the latter of which is also used to compress data in File extension MP3 and File extension JPEG files. These processes are stacked on top of one another to improve data compression to within 10% to 15% of the maximum currently possible, and are carried out in a specific order to create compressed BZ2 files. Decompression of a file simply involves performing the processes in the stack in reverse order. The high compression ratio of BZ2 files mean that they are small but can be slow to create, though decompression is relatively quick making them ideal for distributing files, particularly over networks.
BZ2 files can store data of any kind, and it is not possible to know the contents of a compressed file without extracting it. Applications and files, particularly for the UNIX or Linux platform, are often distributed over the web in BZ2 format. However, BZ2 files should be opened with caution as even though they cannot run the contents on extraction, they could potentially contain malware such as an infected file or application.
BZ2 files can be opened on UNIX and Linux operating systems using unbzip2 from a shell prompt. BZ2 files can also be opened in Windows using an application such as WinRAR, WinZip, Bzip2 or Stuffit, the latter of which is also available on Mac.
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