source: trunk/minix/commands/bzip2-1.0.3/bzip2.1.preformatted@ 12

Last change on this file since 12 was 9, checked in by Mattia Monga, 14 years ago

Minix 3.1.2a
File size: 20.4 KB
Line 
1bzip2(1) bzip2(1)
2
3
4
5NNAAMMEE
6 bzip2, bunzip2 − a block‐sorting file compressor, v1.0.3
7 bzcat − decompresses files to stdout
8 bzip2recover − recovers data from damaged bzip2 files
9
10
11SSYYNNOOPPSSIISS
12 bbzziipp22 [ −−ccddffkkqqssttvvzzVVLL112233445566778899 ] [ _f_i_l_e_n_a_m_e_s _._._. ]
13 bbuunnzziipp22 [ −−ffkkvvssVVLL ] [ _f_i_l_e_n_a_m_e_s _._._. ]
14 bbzzccaatt [ −−ss ] [ _f_i_l_e_n_a_m_e_s _._._. ]
15 bbzziipp22rreeccoovveerr _f_i_l_e_n_a_m_e
16
17
18DDEESSCCRRIIPPTTIIOONN
19 _b_z_i_p_2 compresses files using the Burrows‐Wheeler block
20 sorting text compression algorithm, and Huffman coding.
21 Compression is generally considerably better than that
22 achieved by more conventional LZ77/LZ78‐based compressors,
23 and approaches the performance of the PPM family of sta­
24 tistical compressors.
25
26 The command‐line options are deliberately very similar to
27 those of _G_N_U _g_z_i_p_, but they are not identical.
28
29 _b_z_i_p_2 expects a list of file names to accompany the com­
30 mand‐line flags. Each file is replaced by a compressed
31 version of itself, with the name "original_name.bz2".
32 Each compressed file has the same modification date, per­
33 missions, and, when possible, ownership as the correspond­
34 ing original, so that these properties can be correctly
35 restored at decompression time. File name handling is
36 naive in the sense that there is no mechanism for preserv­
37 ing original file names, permissions, ownerships or dates
38 in filesystems which lack these concepts, or have serious
39 file name length restrictions, such as MS‐DOS.
40
41 _b_z_i_p_2 and _b_u_n_z_i_p_2 will by default not overwrite existing
42 files. If you want this to happen, specify the −f flag.
43
44 If no file names are specified, _b_z_i_p_2 compresses from
45 standard input to standard output. In this case, _b_z_i_p_2
46 will decline to write compressed output to a terminal, as
47 this would be entirely incomprehensible and therefore
48 pointless.
49
50 _b_u_n_z_i_p_2 (or _b_z_i_p_2 _−_d_) decompresses all specified files.
51 Files which were not created by _b_z_i_p_2 will be detected and
52 ignored, and a warning issued. _b_z_i_p_2 attempts to guess
53 the filename for the decompressed file from that of the
54 compressed file as follows:
55
56 filename.bz2 becomes filename
57 filename.bz becomes filename
58 filename.tbz2 becomes filename.tar
59 filename.tbz becomes filename.tar
60 anyothername becomes anyothername.out
61
62 If the file does not end in one of the recognised endings,
63 _._b_z_2_, _._b_z_, _._t_b_z_2 or _._t_b_z_, _b_z_i_p_2 complains that it cannot
64 guess the name of the original file, and uses the original
65 name with _._o_u_t appended.
66
67 As with compression, supplying no filenames causes decom­
68 pression from standard input to standard output.
69
70 _b_u_n_z_i_p_2 will correctly decompress a file which is the con­
71 catenation of two or more compressed files. The result is
72 the concatenation of the corresponding uncompressed files.
73 Integrity testing (−t) of concatenated compressed files is
74 also supported.
75
76 You can also compress or decompress files to the standard
77 output by giving the −c flag. Multiple files may be com­
78 pressed and decompressed like this. The resulting outputs
79 are fed sequentially to stdout. Compression of multiple
80 files in this manner generates a stream containing multi­
81 ple compressed file representations. Such a stream can be
82 decompressed correctly only by _b_z_i_p_2 version 0.9.0 or
83 later. Earlier versions of _b_z_i_p_2 will stop after decom­
84 pressing the first file in the stream.
85
86 _b_z_c_a_t (or _b_z_i_p_2 _‐_d_c_) decompresses all specified files to
87 the standard output.
88
89 _b_z_i_p_2 will read arguments from the environment variables
90 _B_Z_I_P_2 and _B_Z_I_P_, in that order, and will process them
91 before any arguments read from the command line. This
92 gives a convenient way to supply default arguments.
93
94 Compression is always performed, even if the compressed
95 file is slightly larger than the original. Files of less
96 than about one hundred bytes tend to get larger, since the
97 compression mechanism has a constant overhead in the
98 region of 50 bytes. Random data (including the output of
99 most file compressors) is coded at about 8.05 bits per
100 byte, giving an expansion of around 0.5%.
101
102 As a self‐check for your protection, _b_z_i_p_2 uses 32‐bit
103 CRCs to make sure that the decompressed version of a file
104 is identical to the original. This guards against corrup­
105 tion of the compressed data, and against undetected bugs
106 in _b_z_i_p_2 (hopefully very unlikely). The chances of data
107 corruption going undetected is microscopic, about one
108 chance in four billion for each file processed. Be aware,
109 though, that the check occurs upon decompression, so it
110 can only tell you that something is wrong. It can’t help
111 you recover the original uncompressed data. You can use
112 _b_z_i_p_2_r_e_c_o_v_e_r to try to recover data from damaged files.
113
114 Return values: 0 for a normal exit, 1 for environmental
115 problems (file not found, invalid flags, I/O errors, &c),
116 2 to indicate a corrupt compressed file, 3 for an internal
117 consistency error (eg, bug) which caused _b_z_i_p_2 to panic.
118
119
120OOPPTTIIOONNSS
121 −−cc ‐‐‐‐ssttddoouutt
122 Compress or decompress to standard output.
123
124 −−dd ‐‐‐‐ddeeccoommpprreessss
125 Force decompression. _b_z_i_p_2_, _b_u_n_z_i_p_2 and _b_z_c_a_t are
126 really the same program, and the decision about
127 what actions to take is done on the basis of which
128 name is used. This flag overrides that mechanism,
129 and forces _b_z_i_p_2 to decompress.
130
131 −−zz ‐‐‐‐ccoommpprreessss
132 The complement to −d: forces compression,
133 regardless of the invocation name.
134
135 −−tt ‐‐‐‐tteesstt
136 Check integrity of the specified file(s), but don’t
137 decompress them. This really performs a trial
138 decompression and throws away the result.
139
140 −−ff ‐‐‐‐ffoorrccee
141 Force overwrite of output files. Normally, _b_z_i_p_2
142 will not overwrite existing output files. Also
143 forces _b_z_i_p_2 to break hard links to files, which it
144 otherwise wouldn’t do.
145
146 bzip2 normally declines to decompress files which
147 don’t have the correct magic header bytes. If
148 forced (‐f), however, it will pass such files
149 through unmodified. This is how GNU gzip behaves.
150
151 −−kk ‐‐‐‐kkeeeepp
152 Keep (don’t delete) input files during compression
153 or decompression.
154
155 −−ss ‐‐‐‐ssmmaallll
156 Reduce memory usage, for compression, decompression
157 and testing. Files are decompressed and tested
158 using a modified algorithm which only requires 2.5
159 bytes per block byte. This means any file can be
160 decompressed in 2300k of memory, albeit at about
161 half the normal speed.
162
163 During compression, −s selects a block size of
164 200k, which limits memory use to around the same
165 figure, at the expense of your compression ratio.
166 In short, if your machine is low on memory (8
167 megabytes or less), use −s for everything. See
168 MEMORY MANAGEMENT below.
169
170 −−qq ‐‐‐‐qquuiieett
171 Suppress non‐essential warning messages. Messages
172 pertaining to I/O errors and other critical events
173 will not be suppressed.
174
175 −−vv ‐‐‐‐vveerrbboossee
176 Verbose mode ‐‐ show the compression ratio for each
177 file processed. Further −v’s increase the ver­
178 bosity level, spewing out lots of information which
179 is primarily of interest for diagnostic purposes.
180
181 −−LL ‐‐‐‐lliicceennssee ‐‐VV ‐‐‐‐vveerrssiioonn
182 Display the software version, license terms and
183 conditions.
184
185 −−11 ((oorr −−−−ffaasstt)) ttoo −−99 ((oorr −−−−bbeesstt))
186 Set the block size to 100 k, 200 k .. 900 k when
187 compressing. Has no effect when decompressing.
188 See MEMORY MANAGEMENT below. The −−fast and −−best
189 aliases are primarily for GNU gzip compatibility.
190 In particular, −−fast doesn’t make things signifi­
191 cantly faster. And −−best merely selects the
192 default behaviour.
193
194 −−‐‐ Treats all subsequent arguments as file names, even
195 if they start with a dash. This is so you can han­
196 dle files with names beginning with a dash, for
197 example: bzip2 −‐ −myfilename.
198
199 −−‐‐rreeppeettiittiivvee‐‐ffaasstt ‐‐‐‐rreeppeettiittiivvee‐‐bbeesstt
200 These flags are redundant in versions 0.9.5 and
201 above. They provided some coarse control over the
202 behaviour of the sorting algorithm in earlier ver­
203 sions, which was sometimes useful. 0.9.5 and above
204 have an improved algorithm which renders these
205 flags irrelevant.
206
207
208MMEEMMOORRYY MMAANNAAGGEEMMEENNTT
209 _b_z_i_p_2 compresses large files in blocks. The block size
210 affects both the compression ratio achieved, and the
211 amount of memory needed for compression and decompression.
212 The flags −1 through −9 specify the block size to be
213 100,000 bytes through 900,000 bytes (the default) respec­
214 tively. At decompression time, the block size used for
215 compression is read from the header of the compressed
216 file, and _b_u_n_z_i_p_2 then allocates itself just enough memory
217 to decompress the file. Since block sizes are stored in
218 compressed files, it follows that the flags −1 to −9 are
219 irrelevant to and so ignored during decompression.
220
221 Compression and decompression requirements, in bytes, can
222 be estimated as:
223
224 Compression: 400k + ( 8 x block size )
225
226 Decompression: 100k + ( 4 x block size ), or
227 100k + ( 2.5 x block size )
228
229 Larger block sizes give rapidly diminishing marginal
230 returns. Most of the compression comes from the first two
231 or three hundred k of block size, a fact worth bearing in
232 mind when using _b_z_i_p_2 on small machines. It is also
233 important to appreciate that the decompression memory
234 requirement is set at compression time by the choice of
235 block size.
236
237 For files compressed with the default 900k block size,
238 _b_u_n_z_i_p_2 will require about 3700 kbytes to decompress. To
239 support decompression of any file on a 4 megabyte machine,
240 _b_u_n_z_i_p_2 has an option to decompress using approximately
241 half this amount of memory, about 2300 kbytes. Decompres­
242 sion speed is also halved, so you should use this option
243 only where necessary. The relevant flag is ‐s.
244
245 In general, try and use the largest block size memory con­
246 straints allow, since that maximises the compression
247 achieved. Compression and decompression speed are virtu­
248 ally unaffected by block size.
249
250 Another significant point applies to files which fit in a
251 single block ‐‐ that means most files you’d encounter
252 using a large block size. The amount of real memory
253 touched is proportional to the size of the file, since the
254 file is smaller than a block. For example, compressing a
255 file 20,000 bytes long with the flag ‐9 will cause the
256 compressor to allocate around 7600k of memory, but only
257 touch 400k + 20000 * 8 = 560 kbytes of it. Similarly, the
258 decompressor will allocate 3700k but only touch 100k +
259 20000 * 4 = 180 kbytes.
260
261 Here is a table which summarises the maximum memory usage
262 for different block sizes. Also recorded is the total
263 compressed size for 14 files of the Calgary Text Compres­
264 sion Corpus totalling 3,141,622 bytes. This column gives
265 some feel for how compression varies with block size.
266 These figures tend to understate the advantage of larger
267 block sizes for larger files, since the Corpus is domi­
268 nated by smaller files.
269
270 Compress Decompress Decompress Corpus
271 Flag usage usage ‐s usage Size
272
273 ‐1 1200k 500k 350k 914704
274 ‐2 2000k 900k 600k 877703
275 ‐3 2800k 1300k 850k 860338
276 ‐4 3600k 1700k 1100k 846899
277 ‐5 4400k 2100k 1350k 845160
278 ‐6 5200k 2500k 1600k 838626
279 ‐7 6100k 2900k 1850k 834096
280 ‐8 6800k 3300k 2100k 828642
281 ‐9 7600k 3700k 2350k 828642
282
283
284RREECCOOVVEERRIINNGG DDAATTAA FFRROOMM DDAAMMAAGGEEDD FFIILLEESS
285 _b_z_i_p_2 compresses files in blocks, usually 900kbytes long.
286 Each block is handled independently. If a media or trans­
287 mission error causes a multi‐block .bz2 file to become
288 damaged, it may be possible to recover data from the
289 undamaged blocks in the file.
290
291 The compressed representation of each block is delimited
292 by a 48‐bit pattern, which makes it possible to find the
293 block boundaries with reasonable certainty. Each block
294 also carries its own 32‐bit CRC, so damaged blocks can be
295 distinguished from undamaged ones.
296
297 _b_z_i_p_2_r_e_c_o_v_e_r is a simple program whose purpose is to
298 search for blocks in .bz2 files, and write each block out
299 into its own .bz2 file. You can then use _b_z_i_p_2 −t to test
300 the integrity of the resulting files, and decompress those
301 which are undamaged.
302
303 _b_z_i_p_2_r_e_c_o_v_e_r takes a single argument, the name of the dam­
304 aged file, and writes a number of files
305 "rec00001file.bz2", "rec00002file.bz2", etc, containing
306 the extracted blocks. The output filenames are
307 designed so that the use of wildcards in subsequent pro­
308 cessing ‐‐ for example, "bzip2 ‐dc rec*file.bz2 > recov­
309 ered_data" ‐‐ processes the files in the correct order.
310
311 _b_z_i_p_2_r_e_c_o_v_e_r should be of most use dealing with large .bz2
312 files, as these will contain many blocks. It is clearly
313 futile to use it on damaged single‐block files, since a
314 damaged block cannot be recovered. If you wish to min­
315 imise any potential data loss through media or transmis­
316 sion errors, you might consider compressing with a smaller
317 block size.
318
319
320PPEERRFFOORRMMAANNCCEE NNOOTTEESS
321 The sorting phase of compression gathers together similar
322 strings in the file. Because of this, files containing
323 very long runs of repeated symbols, like "aabaabaabaab
324 ..." (repeated several hundred times) may compress more
325 slowly than normal. Versions 0.9.5 and above fare much
326 better than previous versions in this respect. The ratio
327 between worst‐case and average‐case compression time is in
328 the region of 10:1. For previous versions, this figure
329 was more like 100:1. You can use the −vvvv option to mon­
330 itor progress in great detail, if you want.
331
332 Decompression speed is unaffected by these phenomena.
333
334 _b_z_i_p_2 usually allocates several megabytes of memory to
335 operate in, and then charges all over it in a fairly ran­
336 dom fashion. This means that performance, both for com­
337 pressing and decompressing, is largely determined by the
338 speed at which your machine can service cache misses.
339 Because of this, small changes to the code to reduce the
340 miss rate have been observed to give disproportionately
341 large performance improvements. I imagine _b_z_i_p_2 will per­
342 form best on machines with very large caches.
343
344
345CCAAVVEEAATTSS
346 I/O error messages are not as helpful as they could be.
347 _b_z_i_p_2 tries hard to detect I/O errors and exit cleanly,
348 but the details of what the problem is sometimes seem
349 rather misleading.
350
351 This manual page pertains to version 1.0.3 of _b_z_i_p_2_. Com­
352 pressed data created by this version is entirely forwards
353 and backwards compatible with the previous public
354 releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1 and
355 1.0.2, but with the following exception: 0.9.0 and above
356 can correctly decompress multiple concatenated compressed
357 files. 0.1pl2 cannot do this; it will stop after decom­
358 pressing just the first file in the stream.
359
360 _b_z_i_p_2_r_e_c_o_v_e_r versions prior to 1.0.2 used 32‐bit integers
361 to represent bit positions in compressed files, so they
362 could not handle compressed files more than 512 megabytes
363 long. Versions 1.0.2 and above use 64‐bit ints on some
364 platforms which support them (GNU supported targets, and
365 Windows). To establish whether or not bzip2recover was
366 built with such a limitation, run it without arguments.
367 In any event you can build yourself an unlimited version
368 if you can recompile it with MaybeUInt64 set to be an
369 unsigned 64‐bit integer.
370
371
372
373
374AAUUTTHHOORR
375 Julian Seward, jsewardbzip.org.
376
377 http://www.bzip.org
378
379 The ideas embodied in _b_z_i_p_2 are due to (at least) the fol­
380 lowing people: Michael Burrows and David Wheeler (for the
381 block sorting transformation), David Wheeler (again, for
382 the Huffman coder), Peter Fenwick (for the structured cod­
383 ing model in the original _b_z_i_p_, and many refinements), and
384 Alistair Moffat, Radford Neal and Ian Witten (for the
385 arithmetic coder in the original _b_z_i_p_)_. I am much
386 indebted for their help, support and advice. See the man­
387 ual in the source distribution for pointers to sources of
388 documentation. Christian von Roques encouraged me to look
389 for faster sorting algorithms, so as to speed up compres­
390 sion. Bela Lubkin encouraged me to improve the worst‐case
391 compression performance. Donna Robinson XMLised the docu­
392 mentation. The bz* scripts are derived from those of GNU
393 gzip. Many people sent patches, helped with portability
394 problems, lent machines, gave advice and were generally
395 helpful.
396
397
398
399 bzip2(1)
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