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authorThe Android Open Source Project <initial-contribution@android.com>2008-12-17 18:04:08 -0800
committerThe Android Open Source Project <initial-contribution@android.com>2008-12-17 18:04:08 -0800
commitd78fa13a44d08622d61d0df71e731de77021dcf1 (patch)
tree528137ab7928f44991af00d35683fb5c85ae0880 /bzip2.1.preformatted
downloadbzip2-d78fa13a44d08622d61d0df71e731de77021dcf1.tar.gz
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+bzip2(1) bzip2(1)
+
+
+
+NNAAMMEE
+ bzip2, bunzip2 − a block‐sorting file compressor, v1.0.4
+ bzcat − decompresses files to stdout
+ bzip2recover − recovers data from damaged bzip2 files
+
+
+SSYYNNOOPPSSIISS
+ bbzziipp22 [ −−ccddffkkqqssttvvzzVVLL112233445566778899 ] [ _f_i_l_e_n_a_m_e_s _._._. ]
+ bbuunnzziipp22 [ −−ffkkvvssVVLL ] [ _f_i_l_e_n_a_m_e_s _._._. ]
+ bbzzccaatt [ −−ss ] [ _f_i_l_e_n_a_m_e_s _._._. ]
+ bbzziipp22rreeccoovveerr _f_i_l_e_n_a_m_e
+
+
+DDEESSCCRRIIPPTTIIOONN
+ _b_z_i_p_2 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 _G_N_U _g_z_i_p_, but they are not identical.
+
+ _b_z_i_p_2 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.
+
+ _b_z_i_p_2 and _b_u_n_z_i_p_2 will by default not overwrite existing
+ files. If you want this to happen, specify the −f flag.
+
+ If no file names are specified, _b_z_i_p_2 compresses from
+ standard input to standard output. In this case, _b_z_i_p_2
+ will decline to write compressed output to a terminal, as
+ this would be entirely incomprehensible and therefore
+ pointless.
+
+ _b_u_n_z_i_p_2 (or _b_z_i_p_2 _−_d_) decompresses all specified files.
+ Files which were not created by _b_z_i_p_2 will be detected and
+ ignored, and a warning issued. _b_z_i_p_2 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,
+ _._b_z_2_, _._b_z_, _._t_b_z_2 or _._t_b_z_, _b_z_i_p_2 complains that it cannot
+ guess the name of the original file, and uses the original
+ name with _._o_u_t appended.
+
+ As with compression, supplying no filenames causes decom­
+ pression from standard input to standard output.
+
+ _b_u_n_z_i_p_2 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 _b_z_i_p_2 version 0.9.0 or
+ later. Earlier versions of _b_z_i_p_2 will stop after decom­
+ pressing the first file in the stream.
+
+ _b_z_c_a_t (or _b_z_i_p_2 _‐_d_c_) decompresses all specified files to
+ the standard output.
+
+ _b_z_i_p_2 will read arguments from the environment variables
+ _B_Z_I_P_2 and _B_Z_I_P_, 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, _b_z_i_p_2 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 _b_z_i_p_2 (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
+ _b_z_i_p_2_r_e_c_o_v_e_r 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 _b_z_i_p_2 to panic.
+
+
+OOPPTTIIOONNSS
+ −−cc ‐‐‐‐ssttddoouutt
+ Compress or decompress to standard output.
+
+ −−dd ‐‐‐‐ddeeccoommpprreessss
+ Force decompression. _b_z_i_p_2_, _b_u_n_z_i_p_2 and _b_z_c_a_t 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 _b_z_i_p_2 to decompress.
+
+ −−zz ‐‐‐‐ccoommpprreessss
+ The complement to −d: forces compression,
+ regardless of the invocation name.
+
+ −−tt ‐‐‐‐tteesstt
+ Check integrity of the specified file(s), but don’t
+ decompress them. This really performs a trial
+ decompression and throws away the result.
+
+ −−ff ‐‐‐‐ffoorrccee
+ Force overwrite of output files. Normally, _b_z_i_p_2
+ will not overwrite existing output files. Also
+ forces _b_z_i_p_2 to break hard links to files, which it
+ otherwise wouldn’t do.
+
+ bzip2 normally declines to decompress files which
+ don’t have the correct magic header bytes. If
+ forced (‐f), however, it will pass such files
+ through unmodified. This is how GNU gzip behaves.
+
+ −−kk ‐‐‐‐kkeeeepp
+ Keep (don’t delete) input files during compression
+ or decompression.
+
+ −−ss ‐‐‐‐ssmmaallll
+ 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.
+
+ −−qq ‐‐‐‐qquuiieett
+ Suppress non‐essential warning messages. Messages
+ pertaining to I/O errors and other critical events
+ will not be suppressed.
+
+ −−vv ‐‐‐‐vveerrbboossee
+ 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.
+
+ −−LL ‐‐‐‐lliicceennssee ‐‐VV ‐‐‐‐vveerrssiioonn
+ Display the software version, license terms and
+ conditions.
+
+ −−11 ((oorr −−−−ffaasstt)) ttoo −−99 ((oorr −−−−bbeesstt))
+ Set the block size to 100 k, 200 k .. 900 k when
+ compressing. Has no effect when decompressing.
+ See MEMORY MANAGEMENT below. The −−fast and −−best
+ aliases are primarily for GNU gzip compatibility.
+ In particular, −−fast doesn’t make things signifi­
+ cantly faster. And −−best merely selects the
+ default behaviour.
+
+ −−‐‐ 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.
+
+ −−‐‐rreeppeettiittiivvee‐‐ffaasstt ‐‐‐‐rreeppeettiittiivvee‐‐bbeesstt
+ 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.
+
+
+MMEEMMOORRYY MMAANNAAGGEEMMEENNTT
+ _b_z_i_p_2 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 _b_u_n_z_i_p_2 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 _b_z_i_p_2 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,
+ _b_u_n_z_i_p_2 will require about 3700 kbytes to decompress. To
+ support decompression of any file on a 4 megabyte machine,
+ _b_u_n_z_i_p_2 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
+
+
+RREECCOOVVEERRIINNGG DDAATTAA FFRROOMM DDAAMMAAGGEEDD FFIILLEESS
+ _b_z_i_p_2 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.
+
+ _b_z_i_p_2_r_e_c_o_v_e_r 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 _b_z_i_p_2 −t to test
+ the integrity of the resulting files, and decompress those
+ which are undamaged.
+
+ _b_z_i_p_2_r_e_c_o_v_e_r takes a single argument, the name of the dam­
+ aged file, and writes a number of files
+ "rec00001file.bz2", "rec00002file.bz2", etc, containing
+ the extracted blocks. The output filenames are
+ designed so that the use of wildcards in subsequent pro­
+ cessing ‐‐ for example, "bzip2 ‐dc rec*file.bz2 > recov­
+ ered_data" ‐‐ processes the files in the correct order.
+
+ _b_z_i_p_2_r_e_c_o_v_e_r 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.
+
+
+PPEERRFFOORRMMAANNCCEE NNOOTTEESS
+ 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.
+
+ _b_z_i_p_2 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 _b_z_i_p_2 will per­
+ form best on machines with very large caches.
+
+
+CCAAVVEEAATTSS
+ I/O error messages are not as helpful as they could be.
+ _b_z_i_p_2 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 1.0.4 of _b_z_i_p_2_. Com­
+ pressed data created by this version is entirely forwards
+ and backwards compatible with the previous public
+ releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1,
+ 1.0.2 and 1.0.3, 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.
+
+ _b_z_i_p_2_r_e_c_o_v_e_r versions prior to 1.0.2 used 32‐bit integers
+ to represent bit positions in compressed files, so they
+ could not handle compressed files more than 512 megabytes
+ long. Versions 1.0.2 and above use 64‐bit ints on some
+ platforms which support them (GNU supported targets, and
+ Windows). To establish whether or not bzip2recover was
+ built with such a limitation, run it without arguments.
+ In any event you can build yourself an unlimited version
+ if you can recompile it with MaybeUInt64 set to be an
+ unsigned 64‐bit integer.
+
+
+
+
+AAUUTTHHOORR
+ Julian Seward, jsewardbzip.org.
+
+ http://www.bzip.org
+
+ The ideas embodied in _b_z_i_p_2 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 _b_z_i_p_, and many refinements), and
+ Alistair Moffat, Radford Neal and Ian Witten (for the
+ arithmetic coder in the original _b_z_i_p_)_. 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. Donna Robinson XMLised the docu­
+ mentation. The bz* scripts are derived from those of GNU
+ gzip. Many people sent patches, helped with portability
+ problems, lent machines, gave advice and were generally
+ helpful.
+
+
+
+ bzip2(1)