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<!-- This HTML file has been created by texi2html 1.54
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from manual.texi on 23 March 2000 -->
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<TITLE>bzip2 and libbzip2 - How to use bzip2</TITLE>
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<link href="manual_3.html" rel=Next>
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<link href="manual_1.html" rel=Previous>
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<link href="manual_toc.html" rel=ToC>
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</HEAD>
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<BODY>
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<p>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>.
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<P><HR><P>
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<H1><A NAME="SEC2" HREF="manual_toc.html#TOC2">How to use <CODE>bzip2</CODE></A></H1>
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<P>
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This chapter contains a copy of the <CODE>bzip2</CODE> man page,
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and nothing else.
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</P>
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<BLOCKQUOTE>
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<H4><A NAME="SEC3" HREF="manual_toc.html#TOC3">NAME</A></H4>
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<UL>
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<LI><CODE>bzip2</CODE>, <CODE>bunzip2</CODE>
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- a block-sorting file compressor, v1.0
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<LI><CODE>bzcat</CODE>
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- decompresses files to stdout
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<LI><CODE>bzip2recover</CODE>
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- recovers data from damaged bzip2 files
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</UL>
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<H4><A NAME="SEC4" HREF="manual_toc.html#TOC4">SYNOPSIS</A></H4>
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<UL>
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<LI><CODE>bzip2</CODE> [ -cdfkqstvzVL123456789 ] [ filenames ... ]
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<LI><CODE>bunzip2</CODE> [ -fkvsVL ] [ filenames ... ]
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<LI><CODE>bzcat</CODE> [ -s ] [ filenames ... ]
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<LI><CODE>bzip2recover</CODE> filename
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</UL>
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<H4><A NAME="SEC5" HREF="manual_toc.html#TOC5">DESCRIPTION</A></H4>
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<P>
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<CODE>bzip2</CODE> compresses files using the Burrows-Wheeler block sorting
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text compression algorithm, and Huffman coding. Compression is
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generally considerably better than that achieved by more conventional
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LZ77/LZ78-based compressors, and approaches the performance of the PPM
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family of statistical compressors.
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</P>
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<P>
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The command-line options are deliberately very similar to those of GNU
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<CODE>gzip</CODE>, but they are not identical.
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</P>
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<P>
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<CODE>bzip2</CODE> expects a list of file names to accompany the command-line
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flags. Each file is replaced by a compressed version of itself, with
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the name <CODE>original_name.bz2</CODE>. Each compressed file has the same
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modification date, permissions, and, when possible, ownership as the
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corresponding original, so that these properties can be correctly
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restored at decompression time. File name handling is naive in the
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sense that there is no mechanism for preserving original file names,
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permissions, ownerships or dates in filesystems which lack these
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concepts, or have serious file name length restrictions, such as MS-DOS.
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</P>
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<P>
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<CODE>bzip2</CODE> and <CODE>bunzip2</CODE> will by default not overwrite existing
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files. If you want this to happen, specify the <CODE>-f</CODE> flag.
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</P>
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<P>
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If no file names are specified, <CODE>bzip2</CODE> compresses from standard
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input to standard output. In this case, <CODE>bzip2</CODE> will decline to
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write compressed output to a terminal, as this would be entirely
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incomprehensible and therefore pointless.
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</P>
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<P>
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<CODE>bunzip2</CODE> (or <CODE>bzip2 -d</CODE>) decompresses all
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specified files. Files which were not created by <CODE>bzip2</CODE>
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will be detected and ignored, and a warning issued.
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<CODE>bzip2</CODE> attempts to guess the filename for the decompressed file
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from that of the compressed file as follows:
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<UL>
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<LI><CODE>filename.bz2 </CODE> becomes <CODE>filename</CODE>
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<LI><CODE>filename.bz </CODE> becomes <CODE>filename</CODE>
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<LI><CODE>filename.tbz2</CODE> becomes <CODE>filename.tar</CODE>
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<LI><CODE>filename.tbz </CODE> becomes <CODE>filename.tar</CODE>
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<LI><CODE>anyothername </CODE> becomes <CODE>anyothername.out</CODE>
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</UL>
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<P>
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If the file does not end in one of the recognised endings,
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<CODE>.bz2</CODE>, <CODE>.bz</CODE>,
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<CODE>.tbz2</CODE> or <CODE>.tbz</CODE>, <CODE>bzip2</CODE> complains that it cannot
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guess the name of the original file, and uses the original name
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with <CODE>.out</CODE> appended.
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</P>
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<P>
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As with compression, supplying no
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filenames causes decompression from standard input to standard output.
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</P>
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<P>
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<CODE>bunzip2</CODE> will correctly decompress a file which is the
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concatenation of two or more compressed files. The result is the
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concatenation of the corresponding uncompressed files. Integrity
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testing (<CODE>-t</CODE>) of concatenated compressed files is also supported.
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</P>
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<P>
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You can also compress or decompress files to the standard output by
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giving the <CODE>-c</CODE> flag. Multiple files may be compressed and
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decompressed like this. The resulting outputs are fed sequentially to
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stdout. Compression of multiple files in this manner generates a stream
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containing multiple compressed file representations. Such a stream
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can be decompressed correctly only by <CODE>bzip2</CODE> version 0.9.0 or
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later. Earlier versions of <CODE>bzip2</CODE> will stop after decompressing
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the first file in the stream.
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</P>
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<P>
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<CODE>bzcat</CODE> (or <CODE>bzip2 -dc</CODE>) decompresses all specified files to
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the standard output.
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</P>
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<P>
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<CODE>bzip2</CODE> will read arguments from the environment variables
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<CODE>BZIP2</CODE> and <CODE>BZIP</CODE>, in that order, and will process them
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before any arguments read from the command line. This gives a
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convenient way to supply default arguments.
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</P>
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<P>
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Compression is always performed, even if the compressed file is slightly
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larger than the original. Files of less than about one hundred bytes
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tend to get larger, since the compression mechanism has a constant
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overhead in the region of 50 bytes. Random data (including the output
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of most file compressors) is coded at about 8.05 bits per byte, giving
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an expansion of around 0.5%.
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</P>
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<P>
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As a self-check for your protection, <CODE>bzip2</CODE> uses 32-bit CRCs to
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make sure that the decompressed version of a file is identical to the
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original. This guards against corruption of the compressed data, and
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against undetected bugs in <CODE>bzip2</CODE> (hopefully very unlikely). The
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chances of data corruption going undetected is microscopic, about one
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chance in four billion for each file processed. Be aware, though, that
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the check occurs upon decompression, so it can only tell you that
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something is wrong. It can't help you recover the original uncompressed
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data. You can use <CODE>bzip2recover</CODE> to try to recover data from
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damaged files.
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</P>
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<P>
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Return values: 0 for a normal exit, 1 for environmental problems (file
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not found, invalid flags, I/O errors, &c), 2 to indicate a corrupt
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compressed file, 3 for an internal consistency error (eg, bug) which
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caused <CODE>bzip2</CODE> to panic.
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</P>
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<H4><A NAME="SEC6" HREF="manual_toc.html#TOC6">OPTIONS</A></H4>
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<DL COMPACT>
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<DT><CODE>-c --stdout</CODE>
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<DD>
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Compress or decompress to standard output.
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<DT><CODE>-d --decompress</CODE>
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<DD>
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Force decompression. <CODE>bzip2</CODE>, <CODE>bunzip2</CODE> and <CODE>bzcat</CODE> are
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really the same program, and the decision about what actions to take is
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done on the basis of which name is used. This flag overrides that
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mechanism, and forces bzip2 to decompress.
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<DT><CODE>-z --compress</CODE>
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<DD>
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The complement to <CODE>-d</CODE>: forces compression, regardless of the
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invokation name.
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<DT><CODE>-t --test</CODE>
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<DD>
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Check integrity of the specified file(s), but don't decompress them.
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This really performs a trial decompression and throws away the result.
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<DT><CODE>-f --force</CODE>
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<DD>
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Force overwrite of output files. Normally, <CODE>bzip2</CODE> will not overwrite
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existing output files. Also forces <CODE>bzip2</CODE> to break hard links
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to files, which it otherwise wouldn't do.
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<DT><CODE>-k --keep</CODE>
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<DD>
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Keep (don't delete) input files during compression
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or decompression.
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<DT><CODE>-s --small</CODE>
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<DD>
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Reduce memory usage, for compression, decompression and testing. Files
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are decompressed and tested using a modified algorithm which only
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requires 2.5 bytes per block byte. This means any file can be
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decompressed in 2300k of memory, albeit at about half the normal speed.
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During compression, <CODE>-s</CODE> selects a block size of 200k, which limits
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memory use to around the same figure, at the expense of your compression
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ratio. In short, if your machine is low on memory (8 megabytes or
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less), use -s for everything. See MEMORY MANAGEMENT below.
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<DT><CODE>-q --quiet</CODE>
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<DD>
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Suppress non-essential warning messages. Messages pertaining to
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I/O errors and other critical events will not be suppressed.
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<DT><CODE>-v --verbose</CODE>
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<DD>
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Verbose mode -- show the compression ratio for each file processed.
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Further <CODE>-v</CODE>'s increase the verbosity level, spewing out lots of
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information which is primarily of interest for diagnostic purposes.
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<DT><CODE>-L --license -V --version</CODE>
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<DD>
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Display the software version, license terms and conditions.
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<DT><CODE>-1 to -9</CODE>
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<DD>
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Set the block size to 100 k, 200 k .. 900 k when compressing. Has no
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effect when decompressing. See MEMORY MANAGEMENT below.
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<DT><CODE>--</CODE>
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<DD>
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Treats all subsequent arguments as file names, even if they start
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with a dash. This is so you can handle files with names beginning
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with a dash, for example: <CODE>bzip2 -- -myfilename</CODE>.
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<DT><CODE>--repetitive-fast</CODE>
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<DD>
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<DT><CODE>--repetitive-best</CODE>
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<DD>
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These flags are redundant in versions 0.9.5 and above. They provided
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some coarse control over the behaviour of the sorting algorithm in
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earlier versions, which was sometimes useful. 0.9.5 and above have an
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improved algorithm which renders these flags irrelevant.
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</DL>
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<H4><A NAME="SEC7" HREF="manual_toc.html#TOC7">MEMORY MANAGEMENT</A></H4>
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<P>
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<CODE>bzip2</CODE> compresses large files in blocks. The block size affects
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both the compression ratio achieved, and the amount of memory needed for
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compression and decompression. The flags <CODE>-1</CODE> through <CODE>-9</CODE>
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specify the block size to be 100,000 bytes through 900,000 bytes (the
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default) respectively. At decompression time, the block size used for
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compression is read from the header of the compressed file, and
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<CODE>bunzip2</CODE> then allocates itself just enough memory to decompress
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the file. Since block sizes are stored in compressed files, it follows
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that the flags <CODE>-1</CODE> to <CODE>-9</CODE> are irrelevant to and so ignored
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during decompression.
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</P>
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<P>
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Compression and decompression requirements, in bytes, can be estimated
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as:
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<PRE>
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Compression: 400k + ( 8 x block size )
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Decompression: 100k + ( 4 x block size ), or
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100k + ( 2.5 x block size )
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</PRE>
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<P>
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Larger block sizes give rapidly diminishing marginal returns. Most of
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the compression comes from the first two or three hundred k of block
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size, a fact worth bearing in mind when using <CODE>bzip2</CODE> on small machines.
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It is also important to appreciate that the decompression memory
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requirement is set at compression time by the choice of block size.
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</P>
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<P>
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For files compressed with the default 900k block size, <CODE>bunzip2</CODE>
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will require about 3700 kbytes to decompress. To support decompression
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of any file on a 4 megabyte machine, <CODE>bunzip2</CODE> has an option to
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decompress using approximately half this amount of memory, about 2300
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kbytes. Decompression speed is also halved, so you should use this
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option only where necessary. The relevant flag is <CODE>-s</CODE>.
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</P>
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<P>
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In general, try and use the largest block size memory constraints allow,
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since that maximises the compression achieved. Compression and
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decompression speed are virtually unaffected by block size.
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</P>
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<P>
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Another significant point applies to files which fit in a single block
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-- that means most files you'd encounter using a large block size. The
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amount of real memory touched is proportional to the size of the file,
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since the file is smaller than a block. For example, compressing a file
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20,000 bytes long with the flag <CODE>-9</CODE> will cause the compressor to
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allocate around 7600k of memory, but only touch 400k + 20000 * 8 = 560
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kbytes of it. Similarly, the decompressor will allocate 3700k but only
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touch 100k + 20000 * 4 = 180 kbytes.
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</P>
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<P>
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Here is a table which summarises the maximum memory usage for different
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block sizes. Also recorded is the total compressed size for 14 files of
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the Calgary Text Compression Corpus totalling 3,141,622 bytes. This
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column gives some feel for how compression varies with block size.
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These figures tend to understate the advantage of larger block sizes for
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larger files, since the Corpus is dominated by smaller files.
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<PRE>
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Compress Decompress Decompress Corpus
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Flag usage usage -s usage Size
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-1 1200k 500k 350k 914704
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-2 2000k 900k 600k 877703
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-3 2800k 1300k 850k 860338
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-4 3600k 1700k 1100k 846899
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-5 4400k 2100k 1350k 845160
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-6 5200k 2500k 1600k 838626
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-7 6100k 2900k 1850k 834096
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-8 6800k 3300k 2100k 828642
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-9 7600k 3700k 2350k 828642
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</PRE>
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<H4><A NAME="SEC8" HREF="manual_toc.html#TOC8">RECOVERING DATA FROM DAMAGED FILES</A></H4>
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<P>
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<CODE>bzip2</CODE> compresses files in blocks, usually 900kbytes long. Each
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block is handled independently. If a media or transmission error causes
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a multi-block <CODE>.bz2</CODE> file to become damaged, it may be possible to
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recover data from the undamaged blocks in the file.
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</P>
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<P>
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The compressed representation of each block is delimited by a 48-bit
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pattern, which makes it possible to find the block boundaries with
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reasonable certainty. Each block also carries its own 32-bit CRC, so
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damaged blocks can be distinguished from undamaged ones.
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</P>
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<P>
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<CODE>bzip2recover</CODE> is a simple program whose purpose is to search for
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blocks in <CODE>.bz2</CODE> files, and write each block out into its own
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<CODE>.bz2</CODE> file. You can then use <CODE>bzip2 -t</CODE> to test the
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integrity of the resulting files, and decompress those which are
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undamaged.
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</P>
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<P>
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<CODE>bzip2recover</CODE>
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takes a single argument, the name of the damaged file,
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and writes a number of files <CODE>rec0001file.bz2</CODE>,
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<CODE>rec0002file.bz2</CODE>, etc, containing the extracted blocks.
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The output filenames are designed so that the use of
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wildcards in subsequent processing -- for example,
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<CODE>bzip2 -dc rec*file.bz2 > recovered_data</CODE> -- lists the files in
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the correct order.
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</P>
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<P>
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<CODE>bzip2recover</CODE> should be of most use dealing with large <CODE>.bz2</CODE>
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files, as these will contain many blocks. It is clearly
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futile to use it on damaged single-block files, since a
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damaged block cannot be recovered. If you wish to minimise
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any potential data loss through media or transmission errors,
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you might consider compressing with a smaller
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block size.
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</P>
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<H4><A NAME="SEC9" HREF="manual_toc.html#TOC9">PERFORMANCE NOTES</A></H4>
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<P>
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The sorting phase of compression gathers together similar strings in the
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file. Because of this, files containing very long runs of repeated
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symbols, like "aabaabaabaab ..." (repeated several hundred times) may
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compress more slowly than normal. Versions 0.9.5 and above fare much
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better than previous versions in this respect. The ratio between
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worst-case and average-case compression time is in the region of 10:1.
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For previous versions, this figure was more like 100:1. You can use the
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<CODE>-vvvv</CODE> option to monitor progress in great detail, if you want.
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</P>
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<P>
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Decompression speed is unaffected by these phenomena.
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</P>
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<P>
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<CODE>bzip2</CODE> usually allocates several megabytes of memory to operate
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in, and then charges all over it in a fairly random fashion. This means
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that performance, both for compressing and decompressing, is largely
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determined by the speed at which your machine can service cache misses.
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Because of this, small changes to the code to reduce the miss rate have
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been observed to give disproportionately large performance improvements.
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I imagine <CODE>bzip2</CODE> will perform best on machines with very large
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caches.
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</P>
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<H4><A NAME="SEC10" HREF="manual_toc.html#TOC10">CAVEATS</A></H4>
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<P>
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I/O error messages are not as helpful as they could be. <CODE>bzip2</CODE>
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tries hard to detect I/O errors and exit cleanly, but the details of
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what the problem is sometimes seem rather misleading.
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</P>
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<P>
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This manual page pertains to version 1.0 of <CODE>bzip2</CODE>. Compressed
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data created by this version is entirely forwards and backwards
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compatible with the previous public releases, versions 0.1pl2, 0.9.0 and
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0.9.5, but with the following exception: 0.9.0 and above can correctly
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decompress multiple concatenated compressed files. 0.1pl2 cannot do
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this; it will stop after decompressing just the first file in the
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stream.
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</P>
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<P>
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<CODE>bzip2recover</CODE> uses 32-bit integers to represent bit positions in
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compressed files, so it cannot handle compressed files more than 512
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megabytes long. This could easily be fixed.
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</P>
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<H4><A NAME="SEC11" HREF="manual_toc.html#TOC11">AUTHOR</A></H4>
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<P>
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Julian Seward, <CODE>jseward@acm.org</CODE>.
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</P>
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<P>
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The ideas embodied in <CODE>bzip2</CODE> are due to (at least) the following
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people: Michael Burrows and David Wheeler (for the block sorting
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transformation), David Wheeler (again, for the Huffman coder), Peter
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Fenwick (for the structured coding model in the original <CODE>bzip</CODE>,
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and many refinements), and Alistair Moffat, Radford Neal and Ian Witten
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(for the arithmetic coder in the original <CODE>bzip</CODE>). I am much
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indebted for their help, support and advice. See the manual in the
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source distribution for pointers to sources of documentation. Christian
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von Roques encouraged me to look for faster sorting algorithms, so as to
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speed up compression. Bela Lubkin encouraged me to improve the
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worst-case compression performance. Many people sent patches, helped
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with portability problems, lent machines, gave advice and were generally
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helpful.
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</P>
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</BLOCKQUOTE>
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