1 .PU 2 .TH bzip2 1 3 .SH NAME 4 bzip2, bunzip2 \- a block-sorting file compressor, v1.0.6 5 .br 6 bzcat \- decompresses files to stdout 7 .br 8 bzip2recover \- recovers data from damaged bzip2 files 9 10 .SH SYNOPSIS 11 .ll +8 12 .B bzip2 13 .RB [ " \-cdfkqstvzVL123456789 " ] 14 [ 15 .I "filenames \&..." 16 ] 17 .ll -8 18 .br 19 .B bunzip2 20 .RB [ " \-fkvsVL " ] 21 [ 22 .I "filenames \&..." 23 ] 24 .br 25 .B bzcat 26 .RB [ " \-s " ] 27 [ 28 .I "filenames \&..." 29 ] 30 .br 31 .B bzip2recover 32 .I "filename" 33 34 .SH DESCRIPTION 35 .I bzip2 36 compresses files using the Burrows-Wheeler block sorting 37 text compression algorithm, and Huffman coding. Compression is 38 generally considerably better than that achieved by more conventional 39 LZ77/LZ78-based compressors, and approaches the performance of the PPM 40 family of statistical compressors. 41 42 The command-line options are deliberately very similar to 43 those of 44 .I GNU gzip, 45 but they are not identical. 46 47 .I bzip2 48 expects a list of file names to accompany the 49 command-line flags. Each file is replaced by a compressed version of 50 itself, with the name "original_name.bz2". 51 Each compressed file 52 has the same modification date, permissions, and, when possible, 53 ownership as the corresponding original, so that these properties can 54 be correctly restored at decompression time. File name handling is 55 naive in the sense that there is no mechanism for preserving original 56 file names, permissions, ownerships or dates in filesystems which lack 57 these concepts, or have serious file name length restrictions, such as 58 MS-DOS. 59 60 .I bzip2 61 and 62 .I bunzip2 63 will by default not overwrite existing 64 files. If you want this to happen, specify the \-f flag. 65 66 If no file names are specified, 67 .I bzip2 68 compresses from standard 69 input to standard output. In this case, 70 .I bzip2 71 will decline to 72 write compressed output to a terminal, as this would be entirely 73 incomprehensible and therefore pointless. 74 75 .I bunzip2 76 (or 77 .I bzip2 \-d) 78 decompresses all 79 specified files. Files which were not created by 80 .I bzip2 81 will be detected and ignored, and a warning issued. 82 .I bzip2 83 attempts to guess the filename for the decompressed file 84 from that of the compressed file as follows: 85 86 filename.bz2 becomes filename 87 filename.bz becomes filename 88 filename.tbz2 becomes filename.tar 89 filename.tbz becomes filename.tar 90 anyothername becomes anyothername.out 91 92 If the file does not end in one of the recognised endings, 93 .I .bz2, 94 .I .bz, 95 .I .tbz2 96 or 97 .I .tbz, 98 .I bzip2 99 complains that it cannot 100 guess the name of the original file, and uses the original name 101 with 102 .I .out 103 appended. 104 105 As with compression, supplying no 106 filenames causes decompression from 107 standard input to standard output. 108 109 .I bunzip2 110 will correctly decompress a file which is the 111 concatenation of two or more compressed files. The result is the 112 concatenation of the corresponding uncompressed files. Integrity 113 testing (\-t) 114 of concatenated 115 compressed files is also supported. 116 117 You can also compress or decompress files to the standard output by 118 giving the \-c flag. Multiple files may be compressed and 119 decompressed like this. The resulting outputs are fed sequentially to 120 stdout. Compression of multiple files 121 in this manner generates a stream 122 containing multiple compressed file representations. Such a stream 123 can be decompressed correctly only by 124 .I bzip2 125 version 0.9.0 or 126 later. Earlier versions of 127 .I bzip2 128 will stop after decompressing 129 the first file in the stream. 130 131 .I bzcat 132 (or 133 .I bzip2 -dc) 134 decompresses all specified files to 135 the standard output. 136 137 .I bzip2 138 will read arguments from the environment variables 139 .I BZIP2 140 and 141 .I BZIP, 142 in that order, and will process them 143 before any arguments read from the command line. This gives a 144 convenient way to supply default arguments. 145 146 Compression is always performed, even if the compressed 147 file is slightly 148 larger than the original. Files of less than about one hundred bytes 149 tend to get larger, since the compression mechanism has a constant 150 overhead in the region of 50 bytes. Random data (including the output 151 of most file compressors) is coded at about 8.05 bits per byte, giving 152 an expansion of around 0.5%. 153 154 As a self-check for your protection, 155 .I 156 bzip2 157 uses 32-bit CRCs to 158 make sure that the decompressed version of a file is identical to the 159 original. This guards against corruption of the compressed data, and 160 against undetected bugs in 161 .I bzip2 162 (hopefully very unlikely). The 163 chances of data corruption going undetected is microscopic, about one 164 chance in four billion for each file processed. Be aware, though, that 165 the check occurs upon decompression, so it can only tell you that 166 something is wrong. It can't help you 167 recover the original uncompressed 168 data. You can use 169 .I bzip2recover 170 to try to recover data from 171 damaged files. 172 173 Return values: 0 for a normal exit, 1 for environmental problems (file 174 not found, invalid flags, I/O errors, &c), 2 to indicate a corrupt 175 compressed file, 3 for an internal consistency error (eg, bug) which 176 caused 177 .I bzip2 178 to panic. 179 180 .SH OPTIONS 181 .TP 182 .B \-c --stdout 183 Compress or decompress to standard output. 184 .TP 185 .B \-d --decompress 186 Force decompression. 187 .I bzip2, 188 .I bunzip2 189 and 190 .I bzcat 191 are 192 really the same program, and the decision about what actions to take is 193 done on the basis of which name is used. This flag overrides that 194 mechanism, and forces 195 .I bzip2 196 to decompress. 197 .TP 198 .B \-z --compress 199 The complement to \-d: forces compression, regardless of the 200 invocation name. 201 .TP 202 .B \-t --test 203 Check integrity of the specified file(s), but don't decompress them. 204 This really performs a trial decompression and throws away the result. 205 .TP 206 .B \-f --force 207 Force overwrite of output files. Normally, 208 .I bzip2 209 will not overwrite 210 existing output files. Also forces 211 .I bzip2 212 to break hard links 213 to files, which it otherwise wouldn't do. 214 215 bzip2 normally declines to decompress files which don't have the 216 correct magic header bytes. If forced (-f), however, it will pass 217 such files through unmodified. This is how GNU gzip behaves. 218 .TP 219 .B \-k --keep 220 Keep (don't delete) input files during compression 221 or decompression. 222 .TP 223 .B \-s --small 224 Reduce memory usage, for compression, decompression and testing. Files 225 are decompressed and tested using a modified algorithm which only 226 requires 2.5 bytes per block byte. This means any file can be 227 decompressed in 2300k of memory, albeit at about half the normal speed. 228 229 During compression, \-s selects a block size of 200k, which limits 230 memory use to around the same figure, at the expense of your compression 231 ratio. In short, if your machine is low on memory (8 megabytes or 232 less), use \-s for everything. See MEMORY MANAGEMENT below. 233 .TP 234 .B \-q --quiet 235 Suppress non-essential warning messages. Messages pertaining to 236 I/O errors and other critical events will not be suppressed. 237 .TP 238 .B \-v --verbose 239 Verbose mode -- show the compression ratio for each file processed. 240 Further \-v's increase the verbosity level, spewing out lots of 241 information which is primarily of interest for diagnostic purposes. 242 .TP 243 .B \-L --license -V --version 244 Display the software version, license terms and conditions. 245 .TP 246 .B \-1 (or \-\-fast) to \-9 (or \-\-best) 247 Set the block size to 100 k, 200 k .. 900 k when compressing. Has no 248 effect when decompressing. See MEMORY MANAGEMENT below. 249 The \-\-fast and \-\-best aliases are primarily for GNU gzip 250 compatibility. In particular, \-\-fast doesn't make things 251 significantly faster. 252 And \-\-best merely selects the default behaviour. 253 .TP 254 .B \-- 255 Treats all subsequent arguments as file names, even if they start 256 with a dash. This is so you can handle files with names beginning 257 with a dash, for example: bzip2 \-- \-myfilename. 258 .TP 259 .B \--repetitive-fast --repetitive-best 260 These flags are redundant in versions 0.9.5 and above. They provided 261 some coarse control over the behaviour of the sorting algorithm in 262 earlier versions, which was sometimes useful. 0.9.5 and above have an 263 improved algorithm which renders these flags irrelevant. 264 265 .SH MEMORY MANAGEMENT 266 .I bzip2 267 compresses large files in blocks. The block size affects 268 both the compression ratio achieved, and the amount of memory needed for 269 compression and decompression. The flags \-1 through \-9 270 specify the block size to be 100,000 bytes through 900,000 bytes (the 271 default) respectively. At decompression time, the block size used for 272 compression is read from the header of the compressed file, and 273 .I bunzip2 274 then allocates itself just enough memory to decompress 275 the file. Since block sizes are stored in compressed files, it follows 276 that the flags \-1 to \-9 are irrelevant to and so ignored 277 during decompression. 278 279 Compression and decompression requirements, 280 in bytes, can be estimated as: 281 282 Compression: 400k + ( 8 x block size ) 283 284 Decompression: 100k + ( 4 x block size ), or 285 100k + ( 2.5 x block size ) 286 287 Larger block sizes give rapidly diminishing marginal returns. Most of 288 the compression comes from the first two or three hundred k of block 289 size, a fact worth bearing in mind when using 290 .I bzip2 291 on small machines. 292 It is also important to appreciate that the decompression memory 293 requirement is set at compression time by the choice of block size. 294 295 For files compressed with the default 900k block size, 296 .I bunzip2 297 will require about 3700 kbytes to decompress. To support decompression 298 of any file on a 4 megabyte machine, 299 .I bunzip2 300 has an option to 301 decompress using approximately half this amount of memory, about 2300 302 kbytes. Decompression speed is also halved, so you should use this 303 option only where necessary. The relevant flag is -s. 304 305 In general, try and use the largest block size memory constraints allow, 306 since that maximises the compression achieved. Compression and 307 decompression speed are virtually unaffected by block size. 308 309 Another significant point applies to files which fit in a single block 310 -- that means most files you'd encounter using a large block size. The 311 amount of real memory touched is proportional to the size of the file, 312 since the file is smaller than a block. For example, compressing a file 313 20,000 bytes long with the flag -9 will cause the compressor to 314 allocate around 7600k of memory, but only touch 400k + 20000 * 8 = 560 315 kbytes of it. Similarly, the decompressor will allocate 3700k but only 316 touch 100k + 20000 * 4 = 180 kbytes. 317 318 Here is a table which summarises the maximum memory usage for different 319 block sizes. Also recorded is the total compressed size for 14 files of 320 the Calgary Text Compression Corpus totalling 3,141,622 bytes. This 321 column gives some feel for how compression varies with block size. 322 These figures tend to understate the advantage of larger block sizes for 323 larger files, since the Corpus is dominated by smaller files. 324 325 Compress Decompress Decompress Corpus 326 Flag usage usage -s usage Size 327 328 -1 1200k 500k 350k 914704 329 -2 2000k 900k 600k 877703 330 -3 2800k 1300k 850k 860338 331 -4 3600k 1700k 1100k 846899 332 -5 4400k 2100k 1350k 845160 333 -6 5200k 2500k 1600k 838626 334 -7 6100k 2900k 1850k 834096 335 -8 6800k 3300k 2100k 828642 336 -9 7600k 3700k 2350k 828642 337 338 .SH RECOVERING DATA FROM DAMAGED FILES 339 .I bzip2 340 compresses files in blocks, usually 900kbytes long. Each 341 block is handled independently. If a media or transmission error causes 342 a multi-block .bz2 343 file to become damaged, it may be possible to 344 recover data from the undamaged blocks in the file. 345 346 The compressed representation of each block is delimited by a 48-bit 347 pattern, which makes it possible to find the block boundaries with 348 reasonable certainty. Each block also carries its own 32-bit CRC, so 349 damaged blocks can be distinguished from undamaged ones. 350 351 .I bzip2recover 352 is a simple program whose purpose is to search for 353 blocks in .bz2 files, and write each block out into its own .bz2 354 file. You can then use 355 .I bzip2 356 \-t 357 to test the 358 integrity of the resulting files, and decompress those which are 359 undamaged. 360 361 .I bzip2recover 362 takes a single argument, the name of the damaged file, 363 and writes a number of files "rec00001file.bz2", 364 "rec00002file.bz2", etc, containing the extracted blocks. 365 The output filenames are designed so that the use of 366 wildcards in subsequent processing -- for example, 367 "bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in 368 the correct order. 369 370 .I bzip2recover 371 should be of most use dealing with large .bz2 372 files, as these will contain many blocks. It is clearly 373 futile to use it on damaged single-block files, since a 374 damaged block cannot be recovered. If you wish to minimise 375 any potential data loss through media or transmission errors, 376 you might consider compressing with a smaller 377 block size. 378 379 .SH PERFORMANCE NOTES 380 The sorting phase of compression gathers together similar strings in the 381 file. Because of this, files containing very long runs of repeated 382 symbols, like "aabaabaabaab ..." (repeated several hundred times) may 383 compress more slowly than normal. Versions 0.9.5 and above fare much 384 better than previous versions in this respect. The ratio between 385 worst-case and average-case compression time is in the region of 10:1. 386 For previous versions, this figure was more like 100:1. You can use the 387 \-vvvv option to monitor progress in great detail, if you want. 388 389 Decompression speed is unaffected by these phenomena. 390 391 .I bzip2 392 usually allocates several megabytes of memory to operate 393 in, and then charges all over it in a fairly random fashion. This means 394 that performance, both for compressing and decompressing, is largely 395 determined by the speed at which your machine can service cache misses. 396 Because of this, small changes to the code to reduce the miss rate have 397 been observed to give disproportionately large performance improvements. 398 I imagine 399 .I bzip2 400 will perform best on machines with very large caches. 401 402 .SH CAVEATS 403 I/O error messages are not as helpful as they could be. 404 .I bzip2 405 tries hard to detect I/O errors and exit cleanly, but the details of 406 what the problem is sometimes seem rather misleading. 407 408 This manual page pertains to version 1.0.6 of 409 .I bzip2. 410 Compressed data created by this version is entirely forwards and 411 backwards compatible with the previous public releases, versions 412 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1, 1.0.2 and above, but with the following 413 exception: 0.9.0 and above can correctly decompress multiple 414 concatenated compressed files. 0.1pl2 cannot do this; it will stop 415 after decompressing just the first file in the stream. 416 417 .I bzip2recover 418 versions prior to 1.0.2 used 32-bit integers to represent 419 bit positions in compressed files, so they could not handle compressed 420 files more than 512 megabytes long. Versions 1.0.2 and above use 421 64-bit ints on some platforms which support them (GNU supported 422 targets, and Windows). To establish whether or not bzip2recover was 423 built with such a limitation, run it without arguments. In any event 424 you can build yourself an unlimited version if you can recompile it 425 with MaybeUInt64 set to be an unsigned 64-bit integer. 426 427 428 429 .SH AUTHOR 430 Julian Seward, jsewardbzip.org. 431 432 http://www.bzip.org 433 434 The ideas embodied in 435 .I bzip2 436 are due to (at least) the following 437 people: Michael Burrows and David Wheeler (for the block sorting 438 transformation), David Wheeler (again, for the Huffman coder), Peter 439 Fenwick (for the structured coding model in the original 440 .I bzip, 441 and many refinements), and Alistair Moffat, Radford Neal and Ian Witten 442 (for the arithmetic coder in the original 443 .I bzip). 444 I am much 445 indebted for their help, support and advice. See the manual in the 446 source distribution for pointers to sources of documentation. Christian 447 von Roques encouraged me to look for faster sorting algorithms, so as to 448 speed up compression. Bela Lubkin encouraged me to improve the 449 worst-case compression performance. 450 Donna Robinson XMLised the documentation. 451 The bz* scripts are derived from those of GNU gzip. 452 Many people sent patches, helped 453 with portability problems, lent machines, gave advice and were generally 454 helpful. 455
