| blob | d2deb394911c9ea162c1428b2779507a2e3719ee |
2 NAME
3 bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
4 bzcat - decompresses files to stdout
5 bzip2recover - recovers data from damaged bzip2 files
8 SYNOPSIS
9 bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
10 bunzip2 [ -fkvsVL ] [ filenames ... ]
11 bzcat [ -s ] [ filenames ... ]
12 bzip2recover filename
15 DESCRIPTION
16 bzip2 compresses files using the Burrows-Wheeler block
17 sorting text compression algorithm, and Huffman coding.
18 Compression is generally considerably better than that
19 achieved by more conventional LZ77/LZ78-based compressors,
20 and approaches the performance of the PPM family of sta-
21 tistical compressors.
23 The command-line options are deliberately very similar to
24 those of GNU gzip, but they are not identical.
26 bzip2 expects a list of file names to accompany the com-
27 mand-line flags. Each file is replaced by a compressed
28 version of itself, with the name "original_name.bz2".
29 Each compressed file has the same modification date, per-
30 missions, and, when possible, ownership as the correspond-
31 ing original, so that these properties can be correctly
32 restored at decompression time. File name handling is
33 naive in the sense that there is no mechanism for preserv-
34 ing original file names, permissions, ownerships or dates
35 in filesystems which lack these concepts, or have serious
36 file name length restrictions, such as MS-DOS.
38 bzip2 and bunzip2 will by default not overwrite existing
39 files. If you want this to happen, specify the -f flag.
41 If no file names are specified, bzip2 compresses from
42 standard input to standard output. In this case, bzip2
43 will decline to write compressed output to a terminal, as
44 this would be entirely incomprehensible and therefore
45 pointless.
47 bunzip2 (or bzip2 -d) decompresses all specified files.
48 Files which were not created by bzip2 will be detected and
49 ignored, and a warning issued. bzip2 attempts to guess
50 the filename for the decompressed file from that of the
51 compressed file as follows:
53 filename.bz2 becomes filename
54 filename.bz becomes filename
55 filename.tbz2 becomes filename.tar
56 filename.tbz becomes filename.tar
57 anyothername becomes anyothername.out
59 If the file does not end in one of the recognised endings,
60 .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it cannot
61 guess the name of the original file, and uses the original
62 name with .out appended.
64 As with compression, supplying no filenames causes decom-
65 pression from standard input to standard output.
67 bunzip2 will correctly decompress a file which is the con-
68 catenation of two or more compressed files. The result is
69 the concatenation of the corresponding uncompressed files.
70 Integrity testing (-t) of concatenated compressed files is
71 also supported.
73 You can also compress or decompress files to the standard
74 output by giving the -c flag. Multiple files may be com-
75 pressed and decompressed like this. The resulting outputs
76 are fed sequentially to stdout. Compression of multiple
77 files in this manner generates a stream containing multi-
78 ple compressed file representations. Such a stream can be
79 decompressed correctly only by bzip2 version 0.9.0 or
80 later. Earlier versions of bzip2 will stop after decom-
81 pressing the first file in the stream.
83 bzcat (or bzip2 -dc) decompresses all specified files to
84 the standard output.
86 bzip2 will read arguments from the environment variables
87 BZIP2 and BZIP, in that order, and will process them
88 before any arguments read from the command line. This
89 gives a convenient way to supply default arguments.
91 Compression is always performed, even if the compressed
92 file is slightly larger than the original. Files of less
93 than about one hundred bytes tend to get larger, since the
94 compression mechanism has a constant overhead in the
95 region of 50 bytes. Random data (including the output of
96 most file compressors) is coded at about 8.05 bits per
97 byte, giving an expansion of around 0.5%.
99 As a self-check for your protection, bzip2 uses 32-bit
100 CRCs to make sure that the decompressed version of a file
101 is identical to the original. This guards against corrup-
102 tion of the compressed data, and against undetected bugs
103 in bzip2 (hopefully very unlikely). The chances of data
104 corruption going undetected is microscopic, about one
105 chance in four billion for each file processed. Be aware,
106 though, that the check occurs upon decompression, so it
107 can only tell you that something is wrong. It can't help
108 you recover the original uncompressed data. You can use
109 bzip2recover to try to recover data from damaged files.
111 Return values: 0 for a normal exit, 1 for environmental
112 problems (file not found, invalid flags, I/O errors, &c),
113 2 to indicate a corrupt compressed file, 3 for an internal
114 consistency error (eg, bug) which caused bzip2 to panic.
117 OPTIONS
118 -c --stdout
119 Compress or decompress to standard output.
121 -d --decompress
122 Force decompression. bzip2, bunzip2 and bzcat are
123 really the same program, and the decision about
124 what actions to take is done on the basis of which
125 name is used. This flag overrides that mechanism,
126 and forces bzip2 to decompress.
128 -z --compress
129 The complement to -d: forces compression,
130 regardless of the invocation name.
132 -t --test
133 Check integrity of the specified file(s), but don't
134 decompress them. This really performs a trial
135 decompression and throws away the result.
137 -f --force
138 Force overwrite of output files. Normally, bzip2
139 will not overwrite existing output files. Also
140 forces bzip2 to break hard links to files, which it
141 otherwise wouldn't do.
143 bzip2 normally declines to decompress files which
144 don't have the correct magic header bytes. If
145 forced (-f), however, it will pass such files
146 through unmodified. This is how GNU gzip behaves.
148 -k --keep
149 Keep (don't delete) input files during compression
150 or decompression.
152 -s --small
153 Reduce memory usage, for compression, decompression
154 and testing. Files are decompressed and tested
155 using a modified algorithm which only requires 2.5
156 bytes per block byte. This means any file can be
157 decompressed in 2300k of memory, albeit at about
158 half the normal speed.
160 During compression, -s selects a block size of
161 200k, which limits memory use to around the same
162 figure, at the expense of your compression ratio.
163 In short, if your machine is low on memory (8
164 megabytes or less), use -s for everything. See
165 MEMORY MANAGEMENT below.
167 -q --quiet
168 Suppress non-essential warning messages. Messages
169 pertaining to I/O errors and other critical events
170 will not be suppressed.
172 -v --verbose
173 Verbose mode -- show the compression ratio for each
174 file processed. Further -v's increase the ver-
175 bosity level, spewing out lots of information which
176 is primarily of interest for diagnostic purposes.
178 -L --license -V --version
179 Display the software version, license terms and
180 conditions.
182 -1 (or --fast) to -9 (or --best)
183 Set the block size to 100 k, 200 k .. 900 k when
184 compressing. Has no effect when decompressing.
185 See MEMORY MANAGEMENT below. The --fast and --best
186 aliases are primarily for GNU gzip compatibility.
187 In particular, --fast doesn't make things signifi-
188 cantly faster. And --best merely selects the
189 default behaviour.
191 -- Treats all subsequent arguments as file names, even
192 if they start with a dash. This is so you can han-
193 dle files with names beginning with a dash, for
194 example: bzip2 -- -myfilename.
196 --repetitive-fast --repetitive-best
197 These flags are redundant in versions 0.9.5 and
198 above. They provided some coarse control over the
199 behaviour of the sorting algorithm in earlier ver-
200 sions, which was sometimes useful. 0.9.5 and above
201 have an improved algorithm which renders these
202 flags irrelevant.
205 MEMORY MANAGEMENT
206 bzip2 compresses large files in blocks. The block size
207 affects both the compression ratio achieved, and the
208 amount of memory needed for compression and decompression.
209 The flags -1 through -9 specify the block size to be
210 100,000 bytes through 900,000 bytes (the default) respec-
211 tively. At decompression time, the block size used for
212 compression is read from the header of the compressed
213 file, and bunzip2 then allocates itself just enough memory
214 to decompress the file. Since block sizes are stored in
215 compressed files, it follows that the flags -1 to -9 are
216 irrelevant to and so ignored during decompression.
218 Compression and decompression requirements, in bytes, can
219 be estimated as:
221 Compression: 400k + ( 8 x block size )
223 Decompression: 100k + ( 4 x block size ), or
224 100k + ( 2.5 x block size )
226 Larger block sizes give rapidly diminishing marginal
227 returns. Most of the compression comes from the first two
228 or three hundred k of block size, a fact worth bearing in
229 mind when using bzip2 on small machines. It is also
230 important to appreciate that the decompression memory
231 requirement is set at compression time by the choice of
232 block size.
234 For files compressed with the default 900k block size,
235 bunzip2 will require about 3700 kbytes to decompress. To
236 support decompression of any file on a 4 megabyte machine,
237 bunzip2 has an option to decompress using approximately
238 half this amount of memory, about 2300 kbytes. Decompres-
239 sion speed is also halved, so you should use this option
240 only where necessary. The relevant flag is -s.
242 In general, try and use the largest block size memory con-
243 straints allow, since that maximises the compression
244 achieved. Compression and decompression speed are virtu-
245 ally unaffected by block size.
247 Another significant point applies to files which fit in a
248 single block -- that means most files you'd encounter
249 using a large block size. The amount of real memory
250 touched is proportional to the size of the file, since the
251 file is smaller than a block. For example, compressing a
252 file 20,000 bytes long with the flag -9 will cause the
253 compressor to allocate around 7600k of memory, but only
254 touch 400k + 20000 * 8 = 560 kbytes of it. Similarly, the
255 decompressor will allocate 3700k but only touch 100k +
256 20000 * 4 = 180 kbytes.
258 Here is a table which summarises the maximum memory usage
259 for different block sizes. Also recorded is the total
260 compressed size for 14 files of the Calgary Text Compres-
261 sion Corpus totalling 3,141,622 bytes. This column gives
262 some feel for how compression varies with block size.
263 These figures tend to understate the advantage of larger
264 block sizes for larger files, since the Corpus is domi-
265 nated by smaller files.
267 Compress Decompress Decompress Corpus
268 Flag usage usage -s usage Size
270 -1 1200k 500k 350k 914704
271 -2 2000k 900k 600k 877703
272 -3 2800k 1300k 850k 860338
273 -4 3600k 1700k 1100k 846899
274 -5 4400k 2100k 1350k 845160
275 -6 5200k 2500k 1600k 838626
276 -7 6100k 2900k 1850k 834096
277 -8 6800k 3300k 2100k 828642
278 -9 7600k 3700k 2350k 828642
281 RECOVERING DATA FROM DAMAGED FILES
282 bzip2 compresses files in blocks, usually 900kbytes long.
283 Each block is handled independently. If a media or trans-
284 mission error causes a multi-block .bz2 file to become
285 damaged, it may be possible to recover data from the
286 undamaged blocks in the file.
288 The compressed representation of each block is delimited
289 by a 48-bit pattern, which makes it possible to find the
290 block boundaries with reasonable certainty. Each block
291 also carries its own 32-bit CRC, so damaged blocks can be
292 distinguished from undamaged ones.
294 bzip2recover is a simple program whose purpose is to
295 search for blocks in .bz2 files, and write each block out
296 into its own .bz2 file. You can then use bzip2 -t to test
297 the integrity of the resulting files, and decompress those
298 which are undamaged.
300 bzip2recover takes a single argument, the name of the dam-
301 aged file, and writes a number of files
302 "rec00001file.bz2", "rec00002file.bz2", etc, containing
303 the extracted blocks. The output filenames are
304 designed so that the use of wildcards in subsequent pro-
305 cessing -- for example, "bzip2 -dc rec*file.bz2 > recov-
306 ered_data" -- processes the files in the correct order.
308 bzip2recover should be of most use dealing with large .bz2
309 files, as these will contain many blocks. It is clearly
310 futile to use it on damaged single-block files, since a
311 damaged block cannot be recovered. If you wish to min-
312 imise any potential data loss through media or transmis-
313 sion errors, you might consider compressing with a smaller
314 block size.
317 PERFORMANCE NOTES
318 The sorting phase of compression gathers together similar
319 strings in the file. Because of this, files containing
320 very long runs of repeated symbols, like "aabaabaabaab
321 ..." (repeated several hundred times) may compress more
322 slowly than normal. Versions 0.9.5 and above fare much
323 better than previous versions in this respect. The ratio
324 between worst-case and average-case compression time is in
325 the region of 10:1. For previous versions, this figure
326 was more like 100:1. You can use the -vvvv option to mon-
327 itor progress in great detail, if you want.
329 Decompression speed is unaffected by these phenomena.
331 bzip2 usually allocates several megabytes of memory to
332 operate in, and then charges all over it in a fairly ran-
333 dom fashion. This means that performance, both for com-
334 pressing and decompressing, is largely determined by the
335 speed at which your machine can service cache misses.
336 Because of this, small changes to the code to reduce the
337 miss rate have been observed to give disproportionately
338 large performance improvements. I imagine bzip2 will per-
339 form best on machines with very large caches.
342 CAVEATS
343 I/O error messages are not as helpful as they could be.
344 bzip2 tries hard to detect I/O errors and exit cleanly,
345 but the details of what the problem is sometimes seem
346 rather misleading.
348 This manual page pertains to version 1.0.6 of bzip2. Com-
349 pressed data created by this version is entirely forwards
350 and backwards compatible with the previous public
351 releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1,
352 1.0.2 and above, but with the following exception: 0.9.0
353 and above can correctly decompress multiple concatenated
354 compressed files. 0.1pl2 cannot do this; it will stop
355 after decompressing just the first file in the stream.
357 bzip2recover versions prior to 1.0.2 used 32-bit integers
358 to represent bit positions in compressed files, so they
359 could not handle compressed files more than 512 megabytes
360 long. Versions 1.0.2 and above use 64-bit ints on some
361 platforms which support them (GNU supported targets, and
362 Windows). To establish whether or not bzip2recover was
363 built with such a limitation, run it without arguments.
364 In any event you can build yourself an unlimited version
365 if you can recompile it with MaybeUInt64 set to be an
366 unsigned 64-bit integer.
369 AUTHOR
370 Julian Seward, jsewardbzip.org.
372 http://www.bzip.org
374 The ideas embodied in bzip2 are due to (at least) the fol-
375 lowing people: Michael Burrows and David Wheeler (for the
376 block sorting transformation), David Wheeler (again, for
377 the Huffman coder), Peter Fenwick (for the structured cod-
378 ing model in the original bzip, and many refinements), and
379 Alistair Moffat, Radford Neal and Ian Witten (for the
380 arithmetic coder in the original bzip). I am much
381 indebted for their help, support and advice. See the man-
382 ual in the source distribution for pointers to sources of
383 documentation. Christian von Roques encouraged me to look
384 for faster sorting algorithms, so as to speed up compres-
385 sion. Bela Lubkin encouraged me to improve the worst-case
386 compression performance. Donna Robinson XMLised the docu-
387 mentation. The bz* scripts are derived from those of GNU
388 gzip. Many people sent patches, helped with portability
389 problems, lent machines, gave advice and were generally
390 helpful.