1 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 6 7 8 SYNOPSIS 9 bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ] 10 bunzip2 [ -fkvsVL ] [ filenames ... ] 11 bzcat [ -s ] [ filenames ... ] 12 bzip2recover filename 13 14 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. 22 23 The command-line options are deliberately very similar to 24 those of GNU gzip, but they are not identical. 25 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. 37 38 bzip2 and bunzip2 will by default not overwrite existing 39 files. If you want this to happen, specify the -f flag. 40 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. 46 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: 52 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 58 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. 63 64 As with compression, supplying no filenames causes decom- 65 pression from standard input to standard output. 66 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. 72 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. 82 83 bzcat (or bzip2 -dc) decompresses all specified files to 84 the standard output. 85 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. 90 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%. 98 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. 110 111 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. 115 116 117 OPTIONS 118 -c --stdout 119 Compress or decompress to standard output. 120 121 -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. 127 128 -z --compress 129 The complement to -d: forces compression, 130 regardless of the invocation name. 131 132 -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. 136 137 -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. 142 143 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. 147 148 -k --keep 149 Keep (don't delete) input files during compression 150 or decompression. 151 152 -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. 159 160 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. 166 167 -q --quiet 168 Suppress non-essential warning messages. Messages 169 pertaining to I/O errors and other critical events 170 will not be suppressed. 171 172 -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. 177 178 -L --license -V --version 179 Display the software version, license terms and 180 conditions. 181 182 -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. 190 191 -- 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. 195 196 --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. 203 204 205 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. 217 218 Compression and decompression requirements, in bytes, can 219 be estimated as: 220 221 Compression: 400k + ( 8 x block size ) 222 223 Decompression: 100k + ( 4 x block size ), or 224 100k + ( 2.5 x block size ) 225 226 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. 233 234 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. 241 242 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. 246 247 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. 257 258 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. 266 267 Compress Decompress Decompress Corpus 268 Flag usage usage -s usage Size 269 270 -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 279 280 281 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. 287 288 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. 293 294 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. 299 300 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. 307 308 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. 315 316 317 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. 328 329 Decompression speed is unaffected by these phenomena. 330 331 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. 340 341 342 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. 347 348 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. 356 357 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. 367 368 369 AUTHOR 370 Julian Seward, jsewardbzip.org. 371 372 http://www.bzip.org 373 374 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. 391 392
