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1.PU 2.TH bzip2 1 3.SH NAME 4bzip2, bunzip2 \- a block-sorting file compressor, v1.0.4 5.br 6bzcat \- decompresses files to stdout 7.br 8bzip2recover \- 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 36compresses files using the Burrows-Wheeler block sorting 37text compression algorithm, and Huffman coding. Compression is 38generally considerably better than that achieved by more conventional 39LZ77/LZ78-based compressors, and approaches the performance of the PPM 40family of statistical compressors. 41 42The command-line options are deliberately very similar to 43those of 44.I GNU gzip, 45but they are not identical. 46 47.I bzip2 48expects a list of file names to accompany the 49command-line flags. Each file is replaced by a compressed version of 50itself, with the name "original_name.bz2". 51Each compressed file 52has the same modification date, permissions, and, when possible, 53ownership as the corresponding original, so that these properties can 54be correctly restored at decompression time. File name handling is 55naive in the sense that there is no mechanism for preserving original 56file names, permissions, ownerships or dates in filesystems which lack 57these concepts, or have serious file name length restrictions, such as 58MS-DOS. 59 60.I bzip2 61and 62.I bunzip2 63will by default not overwrite existing 64files. If you want this to happen, specify the \-f flag. 65 66If no file names are specified, 67.I bzip2 68compresses from standard 69input to standard output. In this case, 70.I bzip2 71will decline to 72write compressed output to a terminal, as this would be entirely 73incomprehensible and therefore pointless. 74 75.I bunzip2 76(or 77.I bzip2 \-d) 78decompresses all 79specified files. Files which were not created by 80.I bzip2 81will be detected and ignored, and a warning issued. 82.I bzip2 83attempts to guess the filename for the decompressed file 84from 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 92If the file does not end in one of the recognised endings, 93.I .bz2, 94.I .bz, 95.I .tbz2 96or 97.I .tbz, 98.I bzip2 99complains that it cannot 100guess the name of the original file, and uses the original name 101with 102.I .out 103appended. 104 105As with compression, supplying no 106filenames causes decompression from 107standard input to standard output. 108 109.I bunzip2 110will correctly decompress a file which is the 111concatenation of two or more compressed files. The result is the 112concatenation of the corresponding uncompressed files. Integrity 113testing (\-t) 114of concatenated 115compressed files is also supported. 116 117You can also compress or decompress files to the standard output by 118giving the \-c flag. Multiple files may be compressed and 119decompressed like this. The resulting outputs are fed sequentially to 120stdout. Compression of multiple files 121in this manner generates a stream 122containing multiple compressed file representations. Such a stream 123can be decompressed correctly only by 124.I bzip2 125version 0.9.0 or 126later. Earlier versions of 127.I bzip2 128will stop after decompressing 129the first file in the stream. 130 131.I bzcat 132(or 133.I bzip2 -dc) 134decompresses all specified files to 135the standard output. 136 137.I bzip2 138will read arguments from the environment variables 139.I BZIP2 140and 141.I BZIP, 142in that order, and will process them 143before any arguments read from the command line. This gives a 144convenient way to supply default arguments. 145 146Compression is always performed, even if the compressed 147file is slightly 148larger than the original. Files of less than about one hundred bytes 149tend to get larger, since the compression mechanism has a constant 150overhead in the region of 50 bytes. Random data (including the output 151of most file compressors) is coded at about 8.05 bits per byte, giving 152an expansion of around 0.5%. 153 154As a self-check for your protection, 155.I 156bzip2 157uses 32-bit CRCs to 158make sure that the decompressed version of a file is identical to the 159original. This guards against corruption of the compressed data, and 160against undetected bugs in 161.I bzip2 162(hopefully very unlikely). The 163chances of data corruption going undetected is microscopic, about one 164chance in four billion for each file processed. Be aware, though, that 165the check occurs upon decompression, so it can only tell you that 166something is wrong. It can't help you 167recover the original uncompressed 168data. You can use 169.I bzip2recover 170to try to recover data from 171damaged files. 172 173Return values: 0 for a normal exit, 1 for environmental problems (file 174not found, invalid flags, I/O errors, &c), 2 to indicate a corrupt 175compressed file, 3 for an internal consistency error (eg, bug) which 176caused 177.I bzip2 178to panic. 179 180.SH OPTIONS 181.TP 182.B \-c --stdout 183Compress or decompress to standard output. 184.TP 185.B \-d --decompress 186Force decompression. 187.I bzip2, 188.I bunzip2 189and 190.I bzcat 191are 192really the same program, and the decision about what actions to take is 193done on the basis of which name is used. This flag overrides that 194mechanism, and forces 195.I bzip2 196to decompress. 197.TP 198.B \-z --compress 199The complement to \-d: forces compression, regardless of the 200invocation name. 201.TP 202.B \-t --test 203Check integrity of the specified file(s), but don't decompress them. 204This really performs a trial decompression and throws away the result. 205.TP 206.B \-f --force 207Force overwrite of output files. Normally, 208.I bzip2 209will not overwrite 210existing output files. Also forces 211.I bzip2 212to break hard links 213to files, which it otherwise wouldn't do. 214 215bzip2 normally declines to decompress files which don't have the 216correct magic header bytes. If forced (-f), however, it will pass 217such files through unmodified. This is how GNU gzip behaves. 218.TP 219.B \-k --keep 220Keep (don't delete) input files during compression 221or decompression. 222.TP 223.B \-s --small 224Reduce memory usage, for compression, decompression and testing. Files 225are decompressed and tested using a modified algorithm which only 226requires 2.5 bytes per block byte. This means any file can be 227decompressed in 2300k of memory, albeit at about half the normal speed. 228 229During compression, \-s selects a block size of 200k, which limits 230memory use to around the same figure, at the expense of your compression 231ratio. In short, if your machine is low on memory (8 megabytes or 232less), use \-s for everything. See MEMORY MANAGEMENT below. 233.TP 234.B \-q --quiet 235Suppress non-essential warning messages. Messages pertaining to 236I/O errors and other critical events will not be suppressed. 237.TP 238.B \-v --verbose 239Verbose mode -- show the compression ratio for each file processed. 240Further \-v's increase the verbosity level, spewing out lots of 241information which is primarily of interest for diagnostic purposes. 242.TP 243.B \-L --license -V --version 244Display the software version, license terms and conditions. 245.TP 246.B \-1 (or \-\-fast) to \-9 (or \-\-best) 247Set the block size to 100 k, 200 k .. 900 k when compressing. Has no 248effect when decompressing. See MEMORY MANAGEMENT below. 249The \-\-fast and \-\-best aliases are primarily for GNU gzip 250compatibility. In particular, \-\-fast doesn't make things 251significantly faster. 252And \-\-best merely selects the default behaviour. 253.TP 254.B \-- 255Treats all subsequent arguments as file names, even if they start 256with a dash. This is so you can handle files with names beginning 257with a dash, for example: bzip2 \-- \-myfilename. 258.TP 259.B \--repetitive-fast --repetitive-best 260These flags are redundant in versions 0.9.5 and above. They provided 261some coarse control over the behaviour of the sorting algorithm in 262earlier versions, which was sometimes useful. 0.9.5 and above have an 263improved algorithm which renders these flags irrelevant. 264 265.SH MEMORY MANAGEMENT 266.I bzip2 267compresses large files in blocks. The block size affects 268both the compression ratio achieved, and the amount of memory needed for 269compression and decompression. The flags \-1 through \-9 270specify the block size to be 100,000 bytes through 900,000 bytes (the 271default) respectively. At decompression time, the block size used for 272compression is read from the header of the compressed file, and 273.I bunzip2 274then allocates itself just enough memory to decompress 275the file. Since block sizes are stored in compressed files, it follows 276that the flags \-1 to \-9 are irrelevant to and so ignored 277during decompression. 278 279Compression and decompression requirements, 280in 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 287Larger block sizes give rapidly diminishing marginal returns. Most of 288the compression comes from the first two or three hundred k of block 289size, a fact worth bearing in mind when using 290.I bzip2 291on small machines. 292It is also important to appreciate that the decompression memory 293requirement is set at compression time by the choice of block size. 294 295For files compressed with the default 900k block size, 296.I bunzip2 297will require about 3700 kbytes to decompress. To support decompression 298of any file on a 4 megabyte machine, 299.I bunzip2 300has an option to 301decompress using approximately half this amount of memory, about 2300 302kbytes. Decompression speed is also halved, so you should use this 303option only where necessary. The relevant flag is -s. 304 305In general, try and use the largest block size memory constraints allow, 306since that maximises the compression achieved. Compression and 307decompression speed are virtually unaffected by block size. 308 309Another 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 311amount of real memory touched is proportional to the size of the file, 312since the file is smaller than a block. For example, compressing a file 31320,000 bytes long with the flag -9 will cause the compressor to 314allocate around 7600k of memory, but only touch 400k + 20000 * 8 = 560 315kbytes of it. Similarly, the decompressor will allocate 3700k but only 316touch 100k + 20000 * 4 = 180 kbytes. 317 318Here is a table which summarises the maximum memory usage for different 319block sizes. Also recorded is the total compressed size for 14 files of 320the Calgary Text Compression Corpus totalling 3,141,622 bytes. This 321column gives some feel for how compression varies with block size. 322These figures tend to understate the advantage of larger block sizes for 323larger 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 340compresses files in blocks, usually 900kbytes long. Each 341block is handled independently. If a media or transmission error causes 342a multi-block .bz2 343file to become damaged, it may be possible to 344recover data from the undamaged blocks in the file. 345 346The compressed representation of each block is delimited by a 48-bit 347pattern, which makes it possible to find the block boundaries with 348reasonable certainty. Each block also carries its own 32-bit CRC, so 349damaged blocks can be distinguished from undamaged ones. 350 351.I bzip2recover 352is a simple program whose purpose is to search for 353blocks in .bz2 files, and write each block out into its own .bz2 354file. You can then use 355.I bzip2 356\-t 357to test the 358integrity of the resulting files, and decompress those which are 359undamaged. 360 361.I bzip2recover 362takes a single argument, the name of the damaged file, 363and writes a number of files "rec00001file.bz2", 364"rec00002file.bz2", etc, containing the extracted blocks. 365The output filenames are designed so that the use of 366wildcards in subsequent processing -- for example, 367"bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in 368the correct order. 369 370.I bzip2recover 371should be of most use dealing with large .bz2 372files, as these will contain many blocks. It is clearly 373futile to use it on damaged single-block files, since a 374damaged block cannot be recovered. If you wish to minimise 375any potential data loss through media or transmission errors, 376you might consider compressing with a smaller 377block size. 378 379.SH PERFORMANCE NOTES 380The sorting phase of compression gathers together similar strings in the 381file. Because of this, files containing very long runs of repeated 382symbols, like "aabaabaabaab ..." (repeated several hundred times) may 383compress more slowly than normal. Versions 0.9.5 and above fare much 384better than previous versions in this respect. The ratio between 385worst-case and average-case compression time is in the region of 10:1. 386For 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 389Decompression speed is unaffected by these phenomena. 390 391.I bzip2 392usually allocates several megabytes of memory to operate 393in, and then charges all over it in a fairly random fashion. This means 394that performance, both for compressing and decompressing, is largely 395determined by the speed at which your machine can service cache misses. 396Because of this, small changes to the code to reduce the miss rate have 397been observed to give disproportionately large performance improvements. 398I imagine 399.I bzip2 400will perform best on machines with very large caches. 401 402.SH CAVEATS 403I/O error messages are not as helpful as they could be. 404.I bzip2 405tries hard to detect I/O errors and exit cleanly, but the details of 406what the problem is sometimes seem rather misleading. 407 408This manual page pertains to version 1.0.4 of 409.I bzip2. 410Compressed data created by this version is entirely forwards and 411backwards compatible with the previous public releases, versions 4120.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1, 1.0.2 and 1.0.3, but with the following 413exception: 0.9.0 and above can correctly decompress multiple 414concatenated compressed files. 0.1pl2 cannot do this; it will stop 415after decompressing just the first file in the stream. 416 417.I bzip2recover 418versions prior to 1.0.2 used 32-bit integers to represent 419bit positions in compressed files, so they could not handle compressed 420files more than 512 megabytes long. Versions 1.0.2 and above use 42164-bit ints on some platforms which support them (GNU supported 422targets, and Windows). To establish whether or not bzip2recover was 423built with such a limitation, run it without arguments. In any event 424you can build yourself an unlimited version if you can recompile it 425with MaybeUInt64 set to be an unsigned 64-bit integer. 426 427 428 429.SH AUTHOR 430Julian Seward, jsewardbzip.org. 431 432http://www.bzip.org 433 434The ideas embodied in 435.I bzip2 436are due to (at least) the following 437people: Michael Burrows and David Wheeler (for the block sorting 438transformation), David Wheeler (again, for the Huffman coder), Peter 439Fenwick (for the structured coding model in the original 440.I bzip, 441and many refinements), and Alistair Moffat, Radford Neal and Ian Witten 442(for the arithmetic coder in the original 443.I bzip). 444I am much 445indebted for their help, support and advice. See the manual in the 446source distribution for pointers to sources of documentation. Christian 447von Roques encouraged me to look for faster sorting algorithms, so as to 448speed up compression. Bela Lubkin encouraged me to improve the 449worst-case compression performance. 450Donna Robinson XMLised the documentation. 451The bz* scripts are derived from those of GNU gzip. 452Many people sent patches, helped 453with portability problems, lent machines, gave advice and were generally 454helpful.