.TH BUNZIP2 1 "9 June 2003" .SH NAME bunzip2 \- a block-sorting file decompressor, v1.0.2gs1 .br bzcat \- decompresses files to stdout .br bzip2recover \- recovers data from damaged bzip2 files .SH SYNOPSIS .br .B bunzip2 .RB [ " \-fkvsVL " ] [ .I "filenames \&..." ] .br .B bzcat .RB [ " \-s " ] [ .I "filenames \&..." ] .br .B bzip2recover .I "filename" .SH DESCRIPTION .I bunzip2 decompresses files created by .I bzip2 using the Burrows-Wheeler block sorting text compression algorithm, and Huffman coding. .I bzip2 generally achieves considerably better compression than that achieved by more conventional LZ77/LZ78-based compressors, and approaches the performance of the PPM family of statistical compressors. .LP The command-line options are deliberately very similar to those of .I GNU .I gunzip, but they are not identical. .LP .I bunzip2 will by default not overwrite existing files. If you want this to happen, specify the \-f flag. .LP .I bunzip2 decompresses all specified files. Files which were not created by .I bzip2 will be detected and ignored, and a warning issued. .I bunzip2 attempts to guess the filename for the decompressed file from that of the compressed file as follows: .LP .nf filename.bz2 becomes filename filename.bz becomes filename filename.tbz2 becomes filename.tar filename.tbz becomes filename.tar anyothername becomes anyothername.out .fi .LP If the file does not end in one of the recognised endings, .I .bz2, .I .bz, .I .tbz2 or .I .tbz, .I bunzip2 complains that it cannot guess the name of the original file, and uses the original name with .I .out appended. .LP Supplying no filenames causes decompression from standard input to standard output. .LP File name handling is naive in the sense that there is no mechanism for preserving original file names, permissions, ownerships or dates in operating systems or filesystems which lack these concepts, or have serious file name length restrictions, such as MS-DOS or GS/OS. .LP .I bunzip2 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. .LP You can also decompress files to the standard output by giving the \-c flag. Multiple files may be decompressed like this. The resulting outputs are fed sequentially to stdout. .LP .I bzcat (or .I bunzip2 .I \-c) decompresses all specified files to the standard output. .LP .I bunzip2 will read arguments from the environment variables .I BZIP2 and .I 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. .LP As a self-check for your protection, .I bzip2 and .I bunzip2 use 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 .I bzip2 and .I bunzip2 (hopefully very unlikely). The chances of data corruption going undetected are 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 .I bzip2recover to try to recover data from damaged files. .LP This manual page pertains to version 1.0.2gs1 of .I bunzip2. It is fully campatible with compressed data created with all of the previous public releases of bzip2, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0 and 1.0.1, as well as version 1.0.2. .LP 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 .I bunzip2 to panic. .LP .SH OPTIONS .IP "\fB\-c\fP \fB\--stdout\fP" Decompress to standard output. .IP "\fB\-d\fP \fB\--decompress\fP" Force decompression. This flag is unnecessary on bunzip2 for GNO, since it always decompresses. .IP "\fB\-t\fP \fB\--test\fP" Check integrity of the specified file(s), but don't decompress them. This really performs a trial decompression and throws away the result. .IP "\fB\-f\fP \fB\--force\fP" Force overwrite of output files. Normally, .I bunzip2 will not overwrite existing output files. .sp .I bunzip2 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. .IP "\fB\-k\fP \fB\--keep\fP" Keep (don't delete) input files during decompression. .IP "\fB\-s\fP \fB\--small\fP" Reduce memory usage, for 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. .sp In short, if your machine is low on memory (5 megabytes or less), you will probably need to use \-s. See MEMORY MANAGEMENT below. .IP "\fB\-q\fP \fB\--quiet\fP" Suppress non-essential warning messages. Messages pertaining to I/O errors and other critical events will not be suppressed. .IP "\fB\-v\fP \fB\--verbose\fP" Verbose mode -- show the compression ratio for each file processed. Further \-v's increase the verbosity level, spewing out lots of information which is primarily of interest for diagnostic purposes. .IP "\fB\-L\fP \fB\--license\fP \fB\-V\fP \fB\--version\fP" Display the software version, license terms and conditions. .IP "\fB\--\fP" 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: bunzip2 \-- \-myfilename. .LP .SH MEMORY MANAGEMENT .I 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 block size can be specified to be 100,000 bytes through 900,000 bytes (the default). At decompression time, the block size used for compression is read from the header of the compressed file, and .I bunzip2 then allocates itself just enough memory to decompress the file. .LP Decompression requirements, in bytes, can be estimated as: .LP .nf 100k + ( 4 x block size ), or 100k + ( 2.5 x block size ) if using \-s .fi .LP For files compressed with the default 900k block size, .I bunzip2 will require about 3700 kbytes to decompress. To support decompression of any file on a 4 megabyte machine, .I bunzip2 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. .LP Decompression speeds are virtually unaffected by block size. .LP 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 a 900k block size will cause the decompressor to allocate 3700k but only touch 100k + 20000 * 4 = 180 kbytes when decompressing it. .LP 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 understate the advantage of larger block sizes for larger files, since the Corpus is dominated by smaller files. .LP .nf Block Decompress Decompress Corpus Size usage -s usage Size .fi .LP .nf 100k 500k 350k 914704 200k 900k 600k 877703 300k 1300k 850k 860338 400k 1700k 1100k 846899 500k 2100k 1350k 845160 600k 2500k 1600k 838626 700k 2900k 1850k 834096 800k 3300k 2100k 828642 900k 3700k 2350k 828642 .fi .LP .SH RECOVERING DATA FROM DAMAGED FILES .I bzip2 compresses files in blocks, usually 900kbytes long. Each block is handled 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 undamaged blocks in the file. .LP 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. .LP .I 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 .I bunzip2 \-t to test the integrity of the resulting files, and decompress those which are undamaged. .LP .I bzip2recover takes a single argument, the name of the damaged file, and writes a number of files named "rec0001file.bz2", "rec0002file.bz2", etc, containing the extracted blocks. The output filenames are designed so that the use of wildcards in subsequent processing -- for example, "bunzip2 -c rec*file.bz2 > recovered_data" -- processes the files in the correct order. .LP .I 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 minimise any potential data loss through media or transmission errors, you might consider compressing with a smaller block size. .LP .SH PERFORMANCE NOTES .I bunzip2 usually allocates several megabytes of memory to operate in, and then charges all over it in a fairly random fashion. This means that performance is largely determined by the speed at which your machine can access main memory or (if you have a caching accelerator) serve 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 that .I bunzip2 will perform best on machines with very large caches. .LP .SH CAVEATS I/O error messages are not as helpful as they could be. .I bunzip2 tries hard to detect I/O errors and exit cleanly, but the details of what the problem is sometimes seem rather misleading. .LP .I bzip2recover for GNO uses 32-bit integers to represent bit positions in compressed files, so it cannot handle compressed files more than 512 megabytes long. .LP .SH AUTHOR Julian Seward, jseward@acm.org. .LP http://sources.redhat.com/bzip2 .LP The ideas embodied in .I bzip2 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 structured coding model in the original .I bzip, and many refinements), and Alistair Moffat, Radford Neal and Ian Witten (for the arithmetic coder in the original .I bzip). I am much indebted for their help, support and advice. See the manual 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 compression. Bela Lubkin encouraged me to improve the worst-case compression performance. Many people sent patches, helped with portability problems, lent machines, gave advice and were generally helpful. .LP This version of .I bunzip2 for GNO has been ported by Stephen Heumann from Julian Seward's .I bzip2 version 1.0.2 for other platforms. .LP This program contains material from the ORCA/C Run-Time Libraries, copyright 1987-1996 by Byte Works, Inc. Used with permission. .LP It also incorporates a public domain stristr routine by Fred Cole, Bob Stout, and Greg Thayer, which was obtained from http://www.snippets.org .