+++ /dev/null
-
-
-NAME
- bzip2, bunzip2 - a block-sorting file compressor, v1.0
- bzcat - decompresses files to stdout
- bzip2recover - recovers data from damaged bzip2 files
-
-
-SYNOPSIS
- bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
- bunzip2 [ -fkvsVL ] [ filenames ... ]
- bzcat [ -s ] [ filenames ... ]
- bzip2recover filename
-
-
-DESCRIPTION
- bzip2 compresses files using the Burrows-Wheeler block
- sorting text compression algorithm, and Huffman coding.
- Compression is generally considerably better than that
- achieved by more conventional LZ77/LZ78-based compressors,
- and approaches the performance of the PPM family of sta-
- tistical compressors.
-
- The command-line options are deliberately very similar to
- those of GNU gzip, but they are not identical.
-
- bzip2 expects a list of file names to accompany the com-
- mand-line flags. Each file is replaced by a compressed
- version of itself, with the name "original_name.bz2".
- Each compressed file has the same modification date, per-
- missions, and, when possible, ownership as the correspond-
- ing original, so that these properties can be correctly
- restored at decompression time. File name handling is
- naive in the sense that there is no mechanism for preserv-
- ing original file names, permissions, ownerships or dates
- in filesystems which lack these concepts, or have serious
- file name length restrictions, such as MS-DOS.
-
- bzip2 and bunzip2 will by default not overwrite existing
- files. If you want this to happen, specify the -f flag.
-
- If no file names are specified, bzip2 compresses from
- standard input to standard output. In this case, bzip2
- will decline to write compressed output to a terminal, as
- this would be entirely incomprehensible and therefore
- pointless.
-
- bunzip2 (or bzip2 -d) decompresses all specified files.
- Files which were not created by bzip2 will be detected and
- ignored, and a warning issued. bzip2 attempts to guess
- the filename for the decompressed file from that of the
- compressed file as follows:
-
- filename.bz2 becomes filename
- filename.bz becomes filename
- filename.tbz2 becomes filename.tar
- filename.tbz becomes filename.tar
- anyothername becomes anyothername.out
-
- If the file does not end in one of the recognised endings,
- .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it cannot
- guess the name of the original file, and uses the original
- name with .out appended.
-
- As with compression, supplying no filenames causes decom-
- pression from standard input to standard output.
-
- bunzip2 will correctly decompress a file which is the con-
- catenation of two or more compressed files. The result is
- the concatenation of the corresponding uncompressed files.
- Integrity testing (-t) of concatenated compressed files is
- also supported.
-
- You can also compress or decompress files to the standard
- output by giving the -c flag. Multiple files may be com-
- pressed and decompressed like this. The resulting outputs
- are fed sequentially to stdout. Compression of multiple
- files in this manner generates a stream containing multi-
- ple compressed file representations. Such a stream can be
- decompressed correctly only by bzip2 version 0.9.0 or
- later. Earlier versions of bzip2 will stop after decom-
- pressing the first file in the stream.
-
- bzcat (or bzip2 -dc) decompresses all specified files to
- the standard output.
-
- bzip2 will read arguments from the environment variables
- BZIP2 and BZIP, in that order, and will process them
- before any arguments read from the command line. This
- gives a convenient way to supply default arguments.
-
- Compression is always performed, even if the compressed
- file is slightly larger than the original. Files of less
- than about one hundred bytes tend to get larger, since the
- compression mechanism has a constant overhead in the
- region of 50 bytes. Random data (including the output of
- most file compressors) is coded at about 8.05 bits per
- byte, giving an expansion of around 0.5%.
-
- As a self-check for your protection, bzip2 uses 32-bit
- CRCs to make sure that the decompressed version of a file
- is identical to the original. This guards against corrup-
- tion of the compressed data, and against undetected bugs
- in bzip2 (hopefully very unlikely). The chances of data
- corruption going undetected is microscopic, about one
- chance in four billion for each file processed. Be aware,
- though, that the check occurs upon decompression, so it
- can only tell you that something is wrong. It can't help
- you recover the original uncompressed data. You can use
- bzip2recover to try to recover data from damaged files.
-
- Return values: 0 for a normal exit, 1 for environmental
- problems (file not found, invalid flags, I/O errors, &c),
- 2 to indicate a corrupt compressed file, 3 for an internal
- consistency error (eg, bug) which caused bzip2 to panic.
-
-
-OPTIONS
- -c --stdout
- Compress or decompress to standard output.
-
- -d --decompress
- Force decompression. bzip2, bunzip2 and bzcat are
- really the same program, and the decision about
- what actions to take is done on the basis of which
- name is used. This flag overrides that mechanism,
- and forces bzip2 to decompress.
-
- -z --compress
- The complement to -d: forces compression, regard-
- less of the invokation name.
-
- -t --test
- Check integrity of the specified file(s), but don't
- decompress them. This really performs a trial
- decompression and throws away the result.
-
- -f --force
- Force overwrite of output files. Normally, bzip2
- will not overwrite existing output files. Also
- forces bzip2 to break hard links to files, which it
- otherwise wouldn't do.
-
- -k --keep
- Keep (don't delete) input files during compression
- or decompression.
-
- -s --small
- Reduce memory usage, for compression, decompression
- and testing. Files are decompressed and tested
- using a modified algorithm which only requires 2.5
- bytes per block byte. This means any file can be
- decompressed in 2300k of memory, albeit at about
- half the normal speed.
-
- During compression, -s selects a block size of
- 200k, which limits memory use to around the same
- figure, at the expense of your compression ratio.
- In short, if your machine is low on memory (8
- megabytes or less), use -s for everything. See
- MEMORY MANAGEMENT below.
-
- -q --quiet
- Suppress non-essential warning messages. Messages
- pertaining to I/O errors and other critical events
- will not be suppressed.
-
- -v --verbose
- Verbose mode -- show the compression ratio for each
- file processed. Further -v's increase the ver-
- bosity level, spewing out lots of information which
- is primarily of interest for diagnostic purposes.
-
- -L --license -V --version
- Display the software version, license terms and
- conditions.
-
- -1 to -9
- Set the block size to 100 k, 200 k .. 900 k when
- compressing. Has no effect when decompressing.
- See MEMORY MANAGEMENT below.
-
- -- Treats all subsequent arguments as file names, even
- if they start with a dash. This is so you can han-
- dle files with names beginning with a dash, for
- example: bzip2 -- -myfilename.
-
- --repetitive-fast --repetitive-best
- These flags are redundant in versions 0.9.5 and
- above. They provided some coarse control over the
- behaviour of the sorting algorithm in earlier ver-
- sions, which was sometimes useful. 0.9.5 and above
- have an improved algorithm which renders these
- flags irrelevant.
-
-
-MEMORY MANAGEMENT
- bzip2 compresses large files in blocks. The block size
- affects both the compression ratio achieved, and the
- amount of memory needed for compression and decompression.
- The flags -1 through -9 specify the block size to be
- 100,000 bytes through 900,000 bytes (the default) respec-
- tively. At decompression time, the block size used for
- compression is read from the header of the compressed
- file, and bunzip2 then allocates itself just enough memory
- to decompress the file. Since block sizes are stored in
- compressed files, it follows that the flags -1 to -9 are
- irrelevant to and so ignored during decompression.
-
- Compression and decompression requirements, in bytes, can
- be estimated as:
-
- Compression: 400k + ( 8 x block size )
-
- Decompression: 100k + ( 4 x block size ), or
- 100k + ( 2.5 x block size )
-
- Larger block sizes give rapidly diminishing marginal
- returns. Most of the compression comes from the first two
- or three hundred k of block size, a fact worth bearing in
- mind when using bzip2 on small machines. It is also
- important to appreciate that the decompression memory
- requirement is set at compression time by the choice of
- block size.
-
- For files compressed with the default 900k block size,
- bunzip2 will require about 3700 kbytes to decompress. To
- support decompression of any file on a 4 megabyte machine,
- bunzip2 has an option to decompress using approximately
- half this amount of memory, about 2300 kbytes. Decompres-
- sion speed is also halved, so you should use this option
- only where necessary. The relevant flag is -s.
-
- In general, try and use the largest block size memory con-
- straints allow, since that maximises the compression
- achieved. Compression and decompression speed are virtu-
- ally unaffected by block size.
-
- Another significant point applies to files which fit in a
- single block -- that means most files you'd encounter
- using a large block size. The amount of real memory
- touched is proportional to the size of the file, since the
- file is smaller than a block. For example, compressing a
- file 20,000 bytes long with the flag -9 will cause the
- compressor to allocate around 7600k of memory, but only
- touch 400k + 20000 * 8 = 560 kbytes of it. Similarly, the
- decompressor will allocate 3700k but only touch 100k +
- 20000 * 4 = 180 kbytes.
-
- Here is a table which summarises the maximum memory usage
- for different block sizes. Also recorded is the total
- compressed size for 14 files of the Calgary Text Compres-
- sion Corpus totalling 3,141,622 bytes. This column gives
- some feel for how compression varies with block size.
- These figures tend to understate the advantage of larger
- block sizes for larger files, since the Corpus is domi-
- nated by smaller files.
-
- Compress Decompress Decompress Corpus
- Flag usage usage -s usage Size
-
- -1 1200k 500k 350k 914704
- -2 2000k 900k 600k 877703
- -3 2800k 1300k 850k 860338
- -4 3600k 1700k 1100k 846899
- -5 4400k 2100k 1350k 845160
- -6 5200k 2500k 1600k 838626
- -7 6100k 2900k 1850k 834096
- -8 6800k 3300k 2100k 828642
- -9 7600k 3700k 2350k 828642
-
-
-RECOVERING DATA FROM DAMAGED FILES
- bzip2 compresses files in blocks, usually 900kbytes long.
- Each block is handled independently. If a media or trans-
- mission error causes a multi-block .bz2 file to become
- damaged, it may be possible to recover data from the
- undamaged blocks in the file.
-
- The compressed representation of each block is delimited
- by a 48-bit pattern, which makes it possible to find the
- block boundaries with reasonable certainty. Each block
- also carries its own 32-bit CRC, so damaged blocks can be
- distinguished from undamaged ones.
-
- bzip2recover is a simple program whose purpose is to
- search for blocks in .bz2 files, and write each block out
- into its own .bz2 file. You can then use bzip2 -t to test
- the integrity of the resulting files, and decompress those
- which are undamaged.
-
- bzip2recover takes a single argument, the name of the dam-
- aged file, and writes a number of files "rec0001file.bz2",
- "rec0002file.bz2", etc, containing the extracted blocks.
- The output filenames are designed so that the use of
- wildcards in subsequent processing -- for example, "bzip2
- -dc rec*file.bz2 > recovered_data" -- lists the files in
- the correct order.
-
- bzip2recover should be of most use dealing with large .bz2
- files, as these will contain many blocks. It is clearly
- futile to use it on damaged single-block files, since a
- damaged block cannot be recovered. If you wish to min-
- imise any potential data loss through media or transmis-
- sion errors, you might consider compressing with a smaller
- block size.
-
-
-PERFORMANCE NOTES
- The sorting phase of compression gathers together similar
- strings in the file. Because of this, files containing
- very long runs of repeated symbols, like "aabaabaabaab
- ..." (repeated several hundred times) may compress more
- slowly than normal. Versions 0.9.5 and above fare much
- better than previous versions in this respect. The ratio
- between worst-case and average-case compression time is in
- the region of 10:1. For previous versions, this figure
- was more like 100:1. You can use the -vvvv option to mon-
- itor progress in great detail, if you want.
-
- Decompression speed is unaffected by these phenomena.
-
- bzip2 usually allocates several megabytes of memory to
- operate in, and then charges all over it in a fairly ran-
- dom fashion. This means that performance, both for com-
- pressing and decompressing, is largely determined by the
- speed at which your machine can service cache misses.
- Because of this, small changes to the code to reduce the
- miss rate have been observed to give disproportionately
- large performance improvements. I imagine bzip2 will per-
- form best on machines with very large caches.
-
-
-CAVEATS
- I/O error messages are not as helpful as they could be.
- bzip2 tries hard to detect I/O errors and exit cleanly,
- but the details of what the problem is sometimes seem
- rather misleading.
-
- This manual page pertains to version 1.0 of bzip2. Com-
- pressed data created by this version is entirely forwards
- and backwards compatible with the previous public
- releases, versions 0.1pl2, 0.9.0 and 0.9.5, but with the
- following exception: 0.9.0 and above can correctly decom-
- press multiple concatenated compressed files. 0.1pl2 can-
- not do this; it will stop after decompressing just the
- first file in the stream.
-
- bzip2recover uses 32-bit integers to represent bit posi-
- tions in compressed files, so it cannot handle compressed
- files more than 512 megabytes long. This could easily be
- fixed.
-
-
-AUTHOR
- Julian Seward, jseward@acm.org.
-
- http://sourceware.cygnus.com/bzip2
- http://www.muraroa.demon.co.uk
-
- The ideas embodied in bzip2 are due to (at least) the fol-
- lowing people: Michael Burrows and David Wheeler (for the
- block sorting transformation), David Wheeler (again, for
- the Huffman coder), Peter Fenwick (for the structured cod-
- ing model in the original bzip, and many refinements), and
- Alistair Moffat, Radford Neal and Ian Witten (for the
- arithmetic coder in the original bzip). I am much
- indebted for their help, support and advice. See the man-
- ual in the source distribution for pointers to sources of
- documentation. Christian von Roques encouraged me to look
- for faster sorting algorithms, so as to speed up compres-
- sion. Bela Lubkin encouraged me to improve the worst-case
- compression performance. Many people sent patches, helped
- with portability problems, lent machines, gave advice and
- were generally helpful.
-
projects / reactos.git / blobdiff