+++ /dev/null
-.PU
-.TH bzip2 1
-.SH NAME
-bzip2, bunzip2 \- a block-sorting file compressor, v1.0
-.br
-bzcat \- decompresses files to stdout
-.br
-bzip2recover \- recovers data from damaged bzip2 files
-
-.SH SYNOPSIS
-.ll +8
-.B bzip2
-.RB [ " \-cdfkqstvzVL123456789 " ]
-[
-.I "filenames \&..."
-]
-.ll -8
-.br
-.B bunzip2
-.RB [ " \-fkvsVL " ]
-[
-.I "filenames \&..."
-]
-.br
-.B bzcat
-.RB [ " \-s " ]
-[
-.I "filenames \&..."
-]
-.br
-.B bzip2recover
-.I "filename"
-
-.SH DESCRIPTION
-.I 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 statistical compressors.
-
-The command-line options are deliberately very similar to
-those of
-.I GNU gzip,
-but they are not identical.
-
-.I bzip2
-expects a list of file names to accompany the
-command-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, permissions, and, when possible,
-ownership as the corresponding 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 preserving original
-file names, permissions, ownerships or dates in filesystems which lack
-these concepts, or have serious file name length restrictions, such as
-MS-DOS.
-
-.I bzip2
-and
-.I bunzip2
-will by default not overwrite existing
-files. If you want this to happen, specify the \-f flag.
-
-If no file names are specified,
-.I bzip2
-compresses from standard
-input to standard output. In this case,
-.I bzip2
-will decline to
-write compressed output to a terminal, as this would be entirely
-incomprehensible and therefore pointless.
-
-.I bunzip2
-(or
-.I bzip2 \-d)
-decompresses all
-specified files. Files which were not created by
-.I bzip2
-will be detected and ignored, and a warning issued.
-.I 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,
-.I .bz2,
-.I .bz,
-.I .tbz2
-or
-.I .tbz,
-.I bzip2
-complains that it cannot
-guess the name of the original file, and uses the original name
-with
-.I .out
-appended.
-
-As with compression, supplying no
-filenames causes decompression from
-standard input to standard output.
-
-.I bunzip2
-will correctly decompress a file which is the
-concatenation 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 compressed and
-decompressed like this. The resulting outputs are fed sequentially to
-stdout. Compression of multiple files
-in this manner generates a stream
-containing multiple compressed file representations. Such a stream
-can be decompressed correctly only by
-.I bzip2
-version 0.9.0 or
-later. Earlier versions of
-.I bzip2
-will stop after decompressing
-the first file in the stream.
-
-.I bzcat
-(or
-.I bzip2 -dc)
-decompresses all specified files to
-the standard output.
-
-.I bzip2
-will read arguments from the environment variables
-.I BZIP2
-and
-.I 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,
-.I
-bzip2
-uses 32-bit CRCs to
-make sure that the decompressed version of a file is identical to the
-original. This guards against corruption of the compressed data, and
-against undetected bugs in
-.I 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
-.I 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
-.I bzip2
-to panic.
-
-.SH OPTIONS
-.TP
-.B \-c --stdout
-Compress or decompress to standard output.
-.TP
-.B \-d --decompress
-Force decompression.
-.I bzip2,
-.I bunzip2
-and
-.I 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
-.I bzip2
-to decompress.
-.TP
-.B \-z --compress
-The complement to \-d: forces compression, regardless of the
-invokation name.
-.TP
-.B \-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.
-.TP
-.B \-f --force
-Force overwrite of output files. Normally,
-.I bzip2
-will not overwrite
-existing output files. Also forces
-.I bzip2
-to break hard links
-to files, which it otherwise wouldn't do.
-.TP
-.B \-k --keep
-Keep (don't delete) input files during compression
-or decompression.
-.TP
-.B \-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.
-.TP
-.B \-q --quiet
-Suppress non-essential warning messages. Messages pertaining to
-I/O errors and other critical events will not be suppressed.
-.TP
-.B \-v --verbose
-Verbose mode -- show the compression ratio for each file processed.
-Further \-v's increase the verbosity level, spewing out lots of
-information which is primarily of interest for diagnostic purposes.
-.TP
-.B \-L --license -V --version
-Display the software version, license terms and conditions.
-.TP
-.B \-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.
-.TP
-.B \--
-Treats all subsequent arguments as file names, even if they start
-with a dash. This is so you can handle files with names beginning
-with a dash, for example: bzip2 \-- \-myfilename.
-.TP
-.B \--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 versions, which was sometimes useful. 0.9.5 and above have an
-improved algorithm which renders these flags irrelevant.
-
-.SH MEMORY MANAGEMENT
-.I 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) respectively. At decompression time, the block size used for
-compression is read from the header of the compressed file, and
-.I 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
-.I 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,
-.I bunzip2
-will require about 3700 kbytes to decompress. To support decompression
-of any file on a 4 megabyte machine,
-.I bunzip2
-has an option to
-decompress using approximately half this amount of memory, about 2300
-kbytes. Decompression 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 constraints allow,
-since that maximises the compression achieved. Compression and
-decompression speed are virtually 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 Compression 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 dominated 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
-
-.SH RECOVERING DATA FROM DAMAGED FILES
-.I bzip2
-compresses files in blocks, usually 900kbytes long. Each
-block is handled independently. If a media or transmission 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.
-
-.I 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
-.I bzip2
-\-t
-to test the
-integrity of the resulting files, and decompress those which are
-undamaged.
-
-.I bzip2recover
-takes a single argument, the name of the damaged 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.
-
-.I 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 minimise
-any potential data loss through media or transmission errors,
-you might consider compressing with a smaller
-block size.
-
-.SH 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 monitor progress in great detail, if you want.
-
-Decompression speed is unaffected by these phenomena.
-
-.I bzip2
-usually allocates several megabytes of memory to operate
-in, and then charges all over it in a fairly random fashion. This means
-that performance, both for compressing 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
-.I bzip2
-will perform best on machines with very large caches.
-
-.SH CAVEATS
-I/O error messages are not as helpful as they could be.
-.I 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
-.I bzip2.
-Compressed
-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
-decompress multiple concatenated compressed files. 0.1pl2 cannot do
-this; it will stop after decompressing just the first file in the
-stream.
-
-.I bzip2recover
-uses 32-bit integers to represent bit positions in
-compressed files, so it cannot handle compressed files more than 512
-megabytes long. This could easily be fixed.
-
-.SH AUTHOR
-Julian Seward, jseward@acm.org.
-
-http://sourceware.cygnus.com/bzip2
-http://www.muraroa.demon.co.uk
-
-The ideas embodied in
-.I bzip2
-are due to (at least) the following
-people: Michael Burrows and David Wheeler (for the block sorting
-transformation), David Wheeler (again, for the Huffman coder), Peter
-Fenwick (for the structured coding model in the original
-.I bzip,
-and many refinements), and Alistair Moffat, Radford Neal and Ian Witten
-(for the arithmetic coder in the original
-.I bzip).
-I am much
-indebted for their help, support and advice. See the manual 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 compression. 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