| |
| NAME |
| bzip2, bunzip2 - a block-sorting file compressor, v1.0.4 |
| bzcat - decompresses files to stdout |
| bzip2recover - recovers data from damaged bzip2 files |
| |
| |
| SYNOPSIS |
| bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ] |
| bunzip2 [ -fkvsVL ] [ filenames ... ] |
| bzcat [ -s ] [ filenames ... ] |
| bzip2recover filename |
| |
| |
| DESCRIPTION |
| bzip2 compresses files using the Burrows-Wheeler block |
| sorting text compression 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 sta- |
| tistical compressors. |
| |
| The command-line options are deliberately very similar to |
| those of GNU gzip, but they are not identical. |
| |
| bzip2 expects a list of file names to accompany the com- |
| mand-line flags. Each file is replaced by a compressed |
| version of itself, with the name "original_name.bz2". |
| Each compressed file has the same modification date, per- |
| missions, and, when possible, ownership as the correspond- |
| ing 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 preserv- |
| ing original file names, permissions, ownerships or dates |
| in filesystems which lack these concepts, or have serious |
| file name length restrictions, such as MS-DOS. |
| |
| bzip2 and bunzip2 will by default not overwrite existing |
| files. If you want this to happen, specify the -f flag. |
| |
| If no file names are specified, bzip2 compresses from |
| standard input to standard output. In this case, bzip2 |
| will decline to write compressed output to a terminal, as |
| this would be entirely incomprehensible and therefore |
| pointless. |
| |
| bunzip2 (or bzip2 -d) decompresses all specified files. |
| Files which were not created by bzip2 will be detected and |
| ignored, and a warning issued. bzip2 attempts to guess |
| the filename for the decompressed file from that of the |
| compressed 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, |
| .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it cannot |
| guess the name of the original file, and uses the original |
| name with .out appended. |
| |
| As with compression, supplying no filenames causes decom- |
| pression from standard input to standard output. |
| |
| bunzip2 will correctly decompress a file which is the con- |
| catenation 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 com- |
| pressed and decompressed like this. The resulting outputs |
| are fed sequentially to stdout. Compression of multiple |
| files in this manner generates a stream containing multi- |
| ple compressed file representations. Such a stream can be |
| decompressed correctly only by bzip2 version 0.9.0 or |
| later. Earlier versions of bzip2 will stop after decom- |
| pressing the first file in the stream. |
| |
| bzcat (or bzip2 -dc) decompresses all specified files to |
| the standard output. |
| |
| bzip2 will read arguments from the environment variables |
| BZIP2 and BZIP, 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 |
| region 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, bzip2 uses 32-bit |
| CRCs to make sure that the decompressed version of a file |
| is identical to the original. This guards against corrup- |
| tion of the compressed data, and against undetected bugs |
| in bzip2 (hopefully very unlikely). The chances of data |
| corruption going undetected is microscopic, about one |
| chance in four billion for each file processed. 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 |
| bzip2recover to try to recover data from damaged 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 bzip2 to panic. |
| |
| |
| OPTIONS |
| -c --stdout |
| Compress or decompress to standard output. |
| |
| -d --decompress |
| Force decompression. bzip2, bunzip2 and bzcat are |
| really the same program, 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 bzip2 to decompress. |
| |
| -z --compress |
| The complement to -d: forces compression, |
| regardless of the invocation name. |
| |
| -t --test |
| Check integrity of the specified file(s), but don't |
| decompress them. This really performs a trial |
| decompression and throws away the result. |
| |
| -f --force |
| Force overwrite of output files. Normally, bzip2 |
| will not overwrite existing output files. Also |
| forces bzip2 to break hard links to files, which it |
| otherwise wouldn't do. |
| |
| bzip2 normally declines to decompress files which |
| don't have the correct magic header bytes. If |
| forced (-f), however, it will pass such files |
| through unmodified. This is how GNU gzip behaves. |
| |
| -k --keep |
| Keep (don't delete) input files during compression |
| or decompression. |
| |
| -s --small |
| Reduce memory usage, for compression, decompression |
| and testing. Files are decompressed and tested |
| using a modified algorithm which only requires 2.5 |
| bytes per block byte. This means any file can be |
| decompressed in 2300k of memory, albeit at about |
| half the normal speed. |
| |
| During compression, -s selects a block size of |
| 200k, which limits memory 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. |
| |
| -q --quiet |
| Suppress non-essential warning messages. Messages |
| pertaining to I/O errors and other critical events |
| will not be suppressed. |
| |
| -v --verbose |
| Verbose mode -- show the compression ratio for each |
| file processed. Further -v's increase the ver- |
| bosity level, spewing out lots of information which |
| is primarily of interest for diagnostic purposes. |
| |
| -L --license -V --version |
| Display the software version, license terms and |
| conditions. |
| |
| -1 (or --fast) to -9 (or --best) |
| 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 particular, --fast doesn't make things signifi- |
| cantly 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 han- |
| dle files with names beginning with a dash, for |
| example: bzip2 -- -myfilename. |
| |
| --repetitive-fast --repetitive-best |
| These flags are redundant in versions 0.9.5 and |
| above. They provided some coarse control over the |
| behaviour of the sorting algorithm in earlier ver- |
| sions, which was sometimes useful. 0.9.5 and above |
| have an improved algorithm which renders these |
| flags irrelevant. |
| |
| |
| MEMORY MANAGEMENT |
| bzip2 compresses large files in blocks. The block size |
| affects both the compression 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) respec- |
| tively. At decompression time, the block size used for |
| compression is read from the header of the compressed |
| file, and bunzip2 then allocates itself just enough memory |
| to decompress 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: 400k + ( 8 x block size ) |
| |
| Decompression: 100k + ( 4 x block size ), or |
| 100k + ( 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 bzip2 on small machines. It is also |
| important to appreciate that the decompression memory |
| requirement is set at compression time by the choice of |
| block size. |
| |
| For files compressed with the default 900k block size, |
| bunzip2 will require about 3700 kbytes to decompress. To |
| support decompression of any file on a 4 megabyte machine, |
| bunzip2 has an option to decompress using approximately |
| half this amount of memory, about 2300 kbytes. Decompres- |
| sion 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 con- |
| straints allow, since that maximises the compression |
| achieved. Compression and decompression speed are virtu- |
| ally 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 7600k of memory, but only |
| touch 400k + 20000 * 8 = 560 kbytes of it. Similarly, the |
| decompressor will allocate 3700k but only touch 100k + |
| 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 Compres- |
| sion Corpus totalling 3,141,622 bytes. This column gives |
| some feel for how compression varies with block size. |
| These figures tend to understate the advantage of larger |
| block sizes for larger files, since the Corpus is domi- |
| nated 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 |
| |
| |
| RECOVERING DATA FROM DAMAGED FILES |
| bzip2 compresses files in blocks, usually 900kbytes long. |
| Each block is handled independently. If a media or trans- |
| mission error causes a multi-block .bz2 file to become |
| damaged, it may be possible to recover data from the |
| undamaged 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 certainty. Each block |
| also carries its own 32-bit CRC, so damaged blocks can be |
| distinguished from undamaged ones. |
| |
| bzip2recover 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 bzip2 -t to test |
| the integrity of the resulting files, and decompress those |
| which are undamaged. |
| |
| bzip2recover takes a single argument, the name of the dam- |
| aged 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 wildcards in subsequent pro- |
| cessing -- for example, "bzip2 -dc rec*file.bz2 > recov- |
| ered_data" -- processes the files in the correct order. |
| |
| bzip2recover 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 transmis- |
| sion errors, you might consider compressing with a smaller |
| block size. |
| |
| |
| PERFORMANCE NOTES |
| 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 mon- |
| itor progress in great detail, if you want. |
| |
| Decompression speed is unaffected by these phenomena. |
| |
| bzip2 usually allocates several megabytes of memory to |
| operate in, and then charges all over it in a fairly ran- |
| dom fashion. This means that performance, both for com- |
| pressing 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 disproportionately |
| large performance improvements. I imagine bzip2 will per- |
| form best on machines with very large caches. |
| |
| |
| CAVEATS |
| I/O error messages are not as helpful as they could be. |
| bzip2 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 1.0.4 of bzip2. Com- |
| pressed data created by this version is entirely forwards |
| and backwards compatible with the previous public |
| releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1, |
| 1.0.2 and 1.0.3, but with the following exception: 0.9.0 |
| and above can correctly decompress multiple concatenated |
| compressed files. 0.1pl2 cannot do this; it will stop |
| after decompressing just the first file in the stream. |
| |
| bzip2recover versions prior to 1.0.2 used 32-bit integers |
| to represent bit positions 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 establish 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. |
| |
| |
| AUTHOR |
| Julian Seward, jsewardbzip.org. |
| |
| http://www.bzip.org |
| |
| The ideas embodied in bzip2 are due to (at least) the fol- |
| lowing people: Michael Burrows and David Wheeler (for the |
| block sorting transformation), David Wheeler (again, for |
| the Huffman coder), Peter Fenwick (for the structured cod- |
| ing model in the original bzip, and many refinements), and |
| Alistair Moffat, Radford Neal and Ian Witten (for the |
| arithmetic coder in the original bzip). I am much |
| indebted for their help, support and advice. See the man- |
| ual in the source distribution for pointers to sources of |
| documentation. Christian von Roques encouraged me to look |
| for faster sorting algorithms, so as to speed up compres- |
| sion. Bela Lubkin encouraged me to improve the worst-case |
| compression performance. Donna Robinson XMLised the docu- |
| mentation. 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 |
| helpful. |
| |
