1 bzip2(1) bzip2(1) 2 3 4 5 NNAAMMEE 6 bzip2, bunzip2 a blocksorting file compressor, v1.0.6 7 bzcat decompresses files to stdout 8 bzip2recover recovers data from damaged bzip2 files 9 10 11 SSYYNNOOPPSSIISS 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 18 DDEESSCCRRIIPPTTIIOONN 19 _b_z_i_p_2 compresses files using the BurrowsWheeler block 20 sorting text compression algorithm, and Huffman coding. 21 Compression is generally considerably better than that 22 achieved by more conventional LZ77/LZ78based compressors, 23 and approaches the performance of the PPM family of sta 24 tistical compressors. 25 26 The commandline 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 mandline 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 MSDOS. 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 selfcheck for your protection, _b_z_i_p_2 uses 32bit 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 cant 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. 118 119 120 OOPPTTIIOONNSS 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 dont 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 wouldnt do. 145 146 bzip2 normally declines to decompress files which 147 dont 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 (dont 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 nonessential 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 vs 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 doesnt 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 rreeppeettiittiivveeffaasstt rreeppeettiittiivveebbeesstt 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. 206 207 208 MMEEMMOORRYY 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 youd 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 282 283 284 RREECCOOVVEERRIINNGG 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 multiblock .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 48bit pattern, which makes it possible to find the 293 block boundaries with reasonable certainty. Each block 294 also carries its own 32bit 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 singleblock 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. 318 319 320 PPEERRFFOORRMMAANNCCEE 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 worstcase and averagecase 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. 343 344 345 CCAAVVEEAATTSS 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 1.0.6 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, 355 1.0.2 and above, but with the following exception: 0.9.0 356 and above can correctly decompress multiple concatenated 357 compressed files. 0.1pl2 cannot do this; it will stop 358 after decompressing 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 32bit 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 64bit 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 64bit integer. 370 371 372 373 374 AAUUTTHHOORR 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 worstcase 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) 400
