diff --git a/bzip2.1 b/bzip2.1 index ce3a78e..c34056c 100644 --- a/bzip2.1 +++ b/bzip2.1 @@ -1,4 +1,3 @@ -.PU .TH bzip2 1 .SH NAME bzip2, bunzip2 \- a block-sorting file compressor, v1.0.6 @@ -18,13 +17,13 @@ bzip2recover \- recovers data from damaged bzip2 files .br .B bunzip2 .RB [ " \-fkvsVL " ] -[ +[ .I "filenames \&..." ] .br .B bzcat .RB [ " \-s " ] -[ +[ .I "filenames \&..." ] .br @@ -39,15 +38,15 @@ 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 -.I GNU gzip, +The command-line options are deliberately very similar to +those of +.I GNU gzip, but they are not identical. .I bzip2 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 "original_name.bz2". +itself, with the name "original_name.bz2". Each compressed file has the same modification date, permissions, and, when possible, ownership as the corresponding original, so that these properties can @@ -74,13 +73,13 @@ incomprehensible and therefore pointless. .I bunzip2 (or -.I bzip2 \-d) +.I bzip2 \-d) decompresses all -specified files. Files which were not created by +specified files. Files which were not created by .I bzip2 -will be detected and ignored, and a warning issued. +will be detected and ignored, and a warning issued. .I bzip2 -attempts to guess the filename for the decompressed file +attempts to guess the filename for the decompressed file from that of the compressed file as follows: filename.bz2 becomes filename @@ -89,13 +88,13 @@ from that of the compressed file as follows: filename.tbz becomes filename.tar anyothername becomes anyothername.out -If the file does not end in one of the recognised endings, -.I .bz2, -.I .bz, +If the file does not end in one of the recognised endings, +.I .bz2, +.I .bz, .I .tbz2 or -.I .tbz, -.I bzip2 +.I .tbz, +.I bzip2 complains that it cannot guess the name of the original file, and uses the original name with @@ -103,25 +102,25 @@ with appended. As with compression, supplying no -filenames causes decompression from +filenames causes decompression from standard input to standard output. -.I bunzip2 +.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 +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 +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 -.I bzip2 +.I bzip2 version 0.9.0 or later. Earlier versions of .I bzip2 @@ -130,7 +129,7 @@ the first file in the stream. .I bzcat (or -.I bzip2 -dc) +.I bzip2 -dc) decompresses all specified files to the standard output. @@ -140,10 +139,10 @@ will read arguments from the environment variables and .I BZIP, in that order, and will process them -before any arguments read from the command line. This gives a +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 +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 @@ -151,9 +150,8 @@ 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, -.I -bzip2 +As a self-check for your protection, +.I bzip2 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 @@ -163,9 +161,9 @@ against undetected bugs in 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 +something is wrong. It can't help you recover the original uncompressed -data. You can use +data. You can use .I bzip2recover to try to recover data from damaged files. @@ -183,15 +181,15 @@ to panic. Compress or decompress to standard output. .TP .B \-d --decompress -Force decompression. -.I bzip2, -.I bunzip2 +Force decompression. +.I bzip2, +.I bunzip2 and -.I bzcat +.I 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 +mechanism, and forces .I bzip2 to decompress. .TP @@ -205,10 +203,10 @@ This really performs a trial decompression and throws away the result. .TP .B \-f --force Force overwrite of output files. Normally, -.I bzip2 +.I bzip2 will not overwrite -existing output files. Also forces -.I bzip2 +existing output files. Also forces +.I bzip2 to break hard links to files, which it otherwise wouldn't do. @@ -224,9 +222,9 @@ or decompression. 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. +decompressed in 2300\ k of memory, albeit at about half the normal speed. -During compression, \-s selects a block size of 200k, which limits +During compression, \-s selects a block size of 200\ k, 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. @@ -244,11 +242,11 @@ information which is primarily of interest for diagnostic purposes. Display the software version, license terms and conditions. .TP .B \-1 (or \-\-fast) to \-9 (or \-\-best) -Set the block size to 100 k, 200 k .. 900 k when compressing. Has no +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 +The \-\-fast and \-\-best aliases are primarily for GNU gzip compatibility. In particular, \-\-fast doesn't make things -significantly faster. +significantly faster. And \-\-best merely selects the default behaviour. .TP .B \-- @@ -263,7 +261,7 @@ earlier versions, which was sometimes useful. 0.9.5 and above have an improved algorithm which renders these flags irrelevant. .SH MEMORY MANAGEMENT -.I bzip2 +.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 flags \-1 through \-9 @@ -276,13 +274,13 @@ 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, +Compression and decompression requirements, in bytes, can be estimated as: - Compression: 400k + ( 8 x block size ) + Compression: 400\ k + ( 8 x block size ) - Decompression: 100k + ( 4 x block size ), or - 100k + ( 2.5 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 @@ -292,10 +290,10 @@ 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, +For files compressed with the default 900\ k block size, .I bunzip2 will require about 3700 kbytes to decompress. To support decompression -of any file on a 4 megabyte machine, +of any file on a 4 megabyte machine, .I bunzip2 has an option to decompress using approximately half this amount of memory, about 2300 @@ -311,9 +309,9 @@ Another significant point applies to files which fit in a single block 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. +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 @@ -337,7 +335,7 @@ larger files, since the Corpus is dominated by smaller files. .SH RECOVERING DATA FROM DAMAGED FILES .I bzip2 -compresses files in blocks, usually 900kbytes long. Each +compresses files in blocks, usually 900\ kbytes 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 @@ -350,36 +348,36 @@ damaged blocks can be distinguished from undamaged ones. .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 +blocks in .bz2 files, and write each block out into its own .bz2 file. You can then use -.I bzip2 +.I bzip2 \-t to test the integrity of the resulting files, and decompress those which are undamaged. .I bzip2recover -takes a single argument, the name of the damaged file, +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 -wildcards in subsequent processing -- for example, -"bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in +"rec00002file.bz2", etc., containing the extracted blocks. +The output filenames are designed so that the use of +wildcards in subsequent processing -- for example, +"bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in the correct order. .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, +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. .SH 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 +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. @@ -395,7 +393,7 @@ 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 disproportionately large performance improvements. -I imagine +I imagine .I bzip2 will perform best on machines with very large caches. @@ -406,7 +404,7 @@ 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 1.0.6 of -.I bzip2. +.I bzip2. Compressed 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 above, but with the following @@ -440,13 +438,13 @@ 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 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. +worst-case compression performance. Donna Robinson XMLised the documentation. The bz* scripts are derived from those of GNU gzip. Many people sent patches, helped