bzip2(1) General Commands Manual bzip2(1) NNAAMMEE bzip2, bunzip2 - a block-sorting file compressor, v1.0.8 bzcat - decompresses files to stdout bzip2recover - recovers data from damaged bzip2 files SSYYNNOOPPSSIISS bbzziipp22 [ --ccddffkkqqssttvvzzVVLL112233445566778899 ] [ _f_i_l_e_n_a_m_e_s _._._. ] bbzziipp22 [ --hh||----hheellpp ] bbuunnzziipp22 [ --ffkkvvssVVLL ] [ _f_i_l_e_n_a_m_e_s _._._. ] bbuunnzziipp22 [ --hh||----hheellpp ] bbzzccaatt [ --ss ] [ _f_i_l_e_n_a_m_e_s _._._. ] bbzzccaatt [ --hh||----hheellpp ] bbzziipp22rreeccoovveerr _f_i_l_e_n_a_m_e DDEESSCCRRIIPPTTIIOONN _b_z_i_p_2 compresses files using the Burrows-Wheeler block sorting text compres‐ sion algorithm, and Huffman coding. Compression is generally considerably better than that achieved by more conventional LZ77/LZ78-based compressors, and approaches the performance of the PPM family of statistical compressors. The command-line options are deliberately very similar to those of _G_N_U _g_z_i_p_, but they are not identical. _b_z_i_p_2 expects a list of file names to accompany the command-line flags. Each file is replaced by a compressed version of itself, with the name "origi‐ nal_name.bz2". Each compressed file has the same modification date, permis‐ sions, and, when possible, ownership as the corresponding original, so that these properties can be correctly restored at decompression time. File name handling is naive in the sense that there is no mechanism for preserving orig‐ inal file names, permissions, ownerships or dates in filesystems which lack these concepts, or have serious file name length restrictions, such as MS-DOS. _b_z_i_p_2 and _b_u_n_z_i_p_2 will by default not overwrite existing files. If you want this to happen, specify the -f flag. If no file names are specified, _b_z_i_p_2 compresses from standard input to stan‐ dard output. In this case, _b_z_i_p_2 will decline to write compressed output to a terminal, as this would be entirely incomprehensible and therefore pointless. _b_u_n_z_i_p_2 (or _b_z_i_p_2 _-_d_) decompresses all specified files. Files which were not created by _b_z_i_p_2 will be detected and ignored, and a warning issued. _b_z_i_p_2 attempts to guess the filename for the decompressed file from that of the com‐ pressed file as follows: filename.bz2 becomes filename filename.bz becomes filename filename.tbz2 becomes filename.tar filename.tbz becomes filename.tar anyothername becomes anyothername.out If the file does not end in one of the recognised endings, _._b_z_2_, _._b_z_, _._t_b_z_2 or _._t_b_z_, _b_z_i_p_2 complains that it cannot guess the name of the original file, and uses the original name with _._o_u_t appended. As with compression, supplying no filenames causes decompression from standard input to standard output. _b_u_n_z_i_p_2 will correctly decompress a file which is the concatenation of two or more compressed files. The result is the concatenation of the corresponding uncompressed files. Integrity testing (-t) of concatenated compressed files is also supported. You can also compress or decompress files to the standard output by giving the -c flag. Multiple files may be compressed and decompressed like this. The resulting outputs are fed sequentially to stdout. Compression of multiple files in this manner generates a stream containing multiple compressed file representations. Such a stream can be decompressed correctly only by _b_z_i_p_2 version 0.9.0 or later. Earlier versions of _b_z_i_p_2 will stop after decompress‐ ing the first file in the stream. _b_z_c_a_t (or _b_z_i_p_2 _-_d_c_) decompresses all specified files to the standard output. _b_z_i_p_2 will read arguments from the environment variables _B_Z_I_P_2 and _B_Z_I_P_, in that order, and will process them before any arguments read from the command line. This gives a convenient way to supply default arguments. Compression is always performed, even if the compressed file is slightly larger than the original. Files of less than about one hundred bytes tend to get larger, since the compression mechanism has a constant overhead in the re‐ gion of 50 bytes. Random data (including the output of most file compressors) is coded at about 8.05 bits per byte, giving an expansion of around 0.5%. As a self-check for your protection, _b_z_i_p_2 uses 32-bit CRCs to make sure that the decompressed version of a file is identical to the original. This guards against corruption of the compressed data, and against undetected bugs in _b_z_i_p_2 (hopefully very unlikely). The chances of data corruption going unde‐ tected is microscopic, about one chance in four billion for each file pro‐ cessed. Be aware, though, that the check occurs upon decompression, so it can only tell you that something is wrong. It can't help you recover the original uncompressed data. You can use _b_z_i_p_2_r_e_c_o_v_e_r to try to recover data from dam‐ aged files. Return values: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors, &c), 2 to indicate a corrupt compressed file, 3 for an internal consistency error (eg, bug) which caused _b_z_i_p_2 to panic. OOPPTTIIOONNSS --cc ----ssttddoouutt Compress or decompress to standard output. --dd ----ddeeccoommpprreessss Force decompression. _b_z_i_p_2_, _b_u_n_z_i_p_2 and _b_z_c_a_t are really the same pro‐ gram, and the decision about what actions to take is done on the basis of which name is used. This flag overrides that mechanism, and forces _b_z_i_p_2 to decompress. --zz ----ccoommpprreessss The complement to -d: forces compression, regardless of the invocation name. --tt ----tteesstt Check integrity of the specified file(s), but don't decompress them. This really performs a trial decompression and throws away the result. --ff ----ffoorrccee Force overwrite of output files. Normally, _b_z_i_p_2 will not overwrite existing output files. Also forces _b_z_i_p_2 to break hard links to files, which it otherwise wouldn't do. bzip2 normally declines to decompress files which don't have the cor‐ rect magic header bytes. If forced (-f), however, it will pass such files through unmodified. This is how GNU gzip behaves. --kk ----kkeeeepp Keep (don't delete) input files during compression or decompression. --ss ----ssmmaallll Reduce memory usage, for compression, decompression and testing. Files are decompressed and tested using a modified algorithm which only re‐ quires 2.5 bytes per block byte. This means any file can be decom‐ pressed in 2300 k of memory, albeit at about half the normal speed. During compression, -s selects a block size of 200 k, which limits mem‐ ory use to around the same figure, at the expense of your compression ratio. In short, if your machine is low on memory (8 megabytes or less), use -s for everything. See MEMORY MANAGEMENT below. --qq ----qquuiieett Suppress non-essential warning messages. Messages pertaining to I/O errors and other critical events will not be suppressed. --vv ----vveerrbboossee Verbose mode -- show the compression ratio for each file processed. Further -v's increase the verbosity level, spewing out lots of informa‐ tion which is primarily of interest for diagnostic purposes. --hh ----hheellpp Print a help message and exit. --LL ----lliicceennssee --VV ----vveerrssiioonn Display the software version, license terms and conditions. --11 ((oorr ----ffaasstt)) ttoo --99 ((oorr ----bbeesstt)) Set the block size to 100 k, 200 k ... 900 k when compressing. Has no effect when decompressing. See MEMORY MANAGEMENT below. The --fast and --best aliases are primarily for GNU gzip compatibility. In par‐ ticular, --fast doesn't make things significantly faster. And --best merely selects the default behaviour. ---- Treats all subsequent arguments as file names, even if they start with a dash. This is so you can handle files with names beginning with a dash, for example: bzip2 -- -myfilename. ----rreeppeettiittiivvee--ffaasstt ----rreeppeettiittiivvee--bbeesstt These flags are redundant in versions 0.9.5 and above. They provided some coarse control over the behaviour of the sorting algorithm in ear‐ lier versions, which was sometimes useful. 0.9.5 and above have an im‐ proved algorithm which renders these flags irrelevant. MMEEMMOORRYY MMAANNAAGGEEMMEENNTT _b_z_i_p_2 compresses large files in blocks. The block size affects both the com‐ pression ratio achieved, and the amount of memory needed for compression and decompression. The flags -1 through -9 specify the block size to be 100,000 bytes through 900,000 bytes (the default) respectively. At decompression time, the block size used for compression is read from the header of the com‐ pressed file, and _b_u_n_z_i_p_2 then allocates itself just enough memory to decom‐ press the file. Since block sizes are stored in compressed files, it follows that the flags -1 to -9 are irrelevant to and so ignored during decompression. Compression and decompression requirements, in bytes, can be estimated as: Compression: 400 k + ( 8 x block size ) Decompression: 100 k + ( 4 x block size ), or 100 k + ( 2.5 x block size ) Larger block sizes give rapidly diminishing marginal returns. Most of the compression comes from the first two or three hundred k of block size, a fact worth bearing in mind when using _b_z_i_p_2 on small machines. It is also impor‐ tant to appreciate that the decompression memory requirement is set at com‐ pression time by the choice of block size. For files compressed with the default 900 k block size, _b_u_n_z_i_p_2 will require about 3700 kbytes to decompress. To support decompression of any file on a 4 megabyte machine, _b_u_n_z_i_p_2 has an option to decompress using approximately half this amount of memory, about 2300 kbytes. Decompression speed is also halved, so you should use this option only where necessary. The relevant flag is -s. In general, try and use the largest block size memory constraints allow, since that maximises the compression achieved. Compression and decompression speed are virtually unaffected by block size. Another significant point applies to files which fit in a single block -- that means most files you'd encounter using a large block size. The amount of real memory touched is proportional to the size of the file, since the file is smaller than a block. For example, compressing a file 20,000 bytes long with the flag -9 will cause the compressor to allocate around 7600 k of memory, but only touch 400 k + 20000 * 8 = 560 kbytes of it. Similarly, the decompressor will allocate 3700 k but only touch 100 k + 20000 * 4 = 180 kbytes. Here is a table which summarises the maximum memory usage for different block sizes. Also recorded is the total compressed size for 14 files of the Calgary Text Compression Corpus totalling 3,141,622 bytes. This column gives some feel for how compression varies with block size. These figures tend to under‐ state the advantage of larger block sizes for larger files, since the Corpus is dominated by smaller files. Compress Decompress Decompress Corpus Flag usage usage -s usage Size -1 1200k 500k 350k 914704 -2 2000k 900k 600k 877703 -3 2800k 1300k 850k 860338 -4 3600k 1700k 1100k 846899 -5 4400k 2100k 1350k 845160 -6 5200k 2500k 1600k 838626 -7 6100k 2900k 1850k 834096 -8 6800k 3300k 2100k 828642 -9 7600k 3700k 2350k 828642 RREECCOOVVEERRIINNGG DDAATTAA FFRROOMM DDAAMMAAGGEEDD FFIILLEESS _b_z_i_p_2 compresses files in blocks, usually 900 kbytes long. Each block is han‐ dled independently. If a media or transmission error causes a multi-block .bz2 file to become damaged, it may be possible to recover data from the un‐ damaged blocks in the file. The compressed representation of each block is delimited by a 48-bit pattern, which makes it possible to find the block boundaries with reasonable cer‐ tainty. Each block also carries its own 32-bit CRC, so damaged blocks can be distinguished from undamaged ones. _b_z_i_p_2_r_e_c_o_v_e_r is a simple program whose purpose is to search for blocks in .bz2 files, and write each block out into its own .bz2 file. You can then use _b_z_i_p_2 -t to test the integrity of the resulting files, and decompress those which are undamaged. _b_z_i_p_2_r_e_c_o_v_e_r takes a single argument, the name of the damaged file, and writes a number of files "rec00001file.bz2", "rec00002file.bz2", etc., containing the extracted blocks. The output filenames are designed so that the use of wild‐ cards in subsequent processing -- for example, "bzip2 -dc rec*file.bz2 > re‐ covered_data" -- processes the files in the correct order. _b_z_i_p_2_r_e_c_o_v_e_r should be of most use dealing with large .bz2 files, as these will contain many blocks. It is clearly futile to use it on damaged single- block files, since a damaged block cannot be recovered. If you wish to min‐ imise any potential data loss through media or transmission errors, you might consider compressing with a smaller block size. PPEERRFFOORRMMAANNCCEE NNOOTTEESS The sorting phase of compression gathers together similar strings in the file. Because of this, files containing very long runs of repeated symbols, like "aabaabaabaab ..." (repeated several hundred times) may compress more slowly than normal. Versions 0.9.5 and above fare much better than previous versions in this respect. The ratio between worst-case and average-case compression time is in the region of 10:1. For previous versions, this figure was more like 100:1. You can use the -vvvv option to monitor progress in great detail, if you want. Decompression speed is unaffected by these phenomena. _b_z_i_p_2 usually allocates several megabytes of memory to operate in, and then charges all over it in a fairly random fashion. This means that performance, both for compressing and decompressing, is largely determined by the speed at which your machine can service cache misses. Because of this, small changes to the code to reduce the miss rate have been observed to give disproportion‐ ately large performance improvements. I imagine _b_z_i_p_2 will perform best on machines with very large caches. CCAAVVEEAATTSS I/O error messages are not as helpful as they could be. _b_z_i_p_2 tries hard to detect I/O errors and exit cleanly, but the details of what the problem is sometimes seem rather misleading. This manual page pertains to version 1.0.8 of _b_z_i_p_2_. Compressed data created by this version is entirely forwards and backwards compatible with the previ‐ ous public releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1, 1.0.2 and above, but with the following exception: 0.9.0 and above can correctly decom‐ press multiple concatenated compressed files. 0.1pl2 cannot do this; it will stop after decompressing just the first file in the stream. _b_z_i_p_2_r_e_c_o_v_e_r versions prior to 1.0.2 used 32-bit integers to represent bit po‐ sitions in compressed files, so they could not handle compressed files more than 512 megabytes long. Versions 1.0.2 and above use 64-bit ints on some platforms which support them (GNU supported targets, and Windows). To estab‐ lish whether or not bzip2recover was built with such a limitation, run it without arguments. In any event you can build yourself an unlimited version if you can recompile it with MaybeUInt64 set to be an unsigned 64-bit integer. AAUUTTHHOORR Julian Seward, jseward@acm.org. https://sourceware.org/bzip2/ The ideas embodied in _b_z_i_p_2 are due to (at least) the following people: Michael Burrows and David Wheeler (for the block sorting transformation), David Wheeler (again, for the Huffman coder), Peter Fenwick (for the struc‐ tured coding model in the original _b_z_i_p_, and many refinements), and Alistair Moffat, Radford Neal and Ian Witten (for the arithmetic coder in the original _b_z_i_p_)_. I am much indebted for their help, support and advice. See the manual in the source distribution for pointers to sources of documentation. Chris‐ tian von Roques encouraged me to look for faster sorting algorithms, so as to speed up compression. Bela Lubkin encouraged me to improve the worst-case compression performance. Donna Robinson XMLised the documentation. The bz* scripts are derived from those of GNU gzip. Many people sent patches, helped with portability problems, lent machines, gave advice and were generally help‐ ful. bzip2(1)