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<HTML><HEAD><!-- This HTML file has been created by texi2html 1.51 from manual.texi on 22 October 1998 --><TITLE>bzip2 and libbzip2 - How to use bzip2</TITLE></HEAD><BODY>Go to the <A HREF="manual_1.html">first</A>, <A HREF="manual_1.html">previous</A>, <A HREF="manual_3.html">next</A>, <A HREF="manual_4.html">last</A> section, <A HREF="manual_toc.html">table of contents</A>.<P><HR><P><H1><A NAME="SEC2" HREF="manual_toc.html#TOC2">How to use <CODE>bzip2</CODE></A></H1><P>This chapter contains a copy of the <CODE>bzip2</CODE> man page,and nothing else.<PRE>NAME bzip2, bunzip2 - a block-sorting file compressor, v0.9.0 bzcat - decompresses files to stdout bzip2recover - recovers data from damaged bzip2 filesSYNOPSIS bzip2 [ -cdfkstvzVL123456789 ] [ filenames ... ] bunzip2 [ -fkvsVL ] [ filenames ... ] bzcat [ -s ] [ filenames ... ] bzip2recover filenameDESCRIPTION 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 and permissions 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 original file names, per- missions and dates in filesystems which lack these con- cepts, 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 and restores all spec- ified files whose names end in ".bz2". Files without this suffix are ignored. Again, supplying no filenames causes decompression 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. 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 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.MEMORY MANAGEMENT Bzip2 compresses large files in blocks. The block size affects both the compression ratio achieved, and the amount of memory needed both for compression and decom- pression. 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 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 + ( 7 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 6700k of memory, but only touch 400k + 20000 * 7 = 540 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 1100k 500k 350k 914704 -2 1800k 900k 600k 877703 -3 2500k 1300k 850k 860338 -4 3200k 1700k 1100k 846899 -5 3900k 2100k 1350k 845160 -6 4600k 2500k 1600k 838626 -7 5400k 2900k 1850k 834096⌨️ 快捷键说明
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