456 lines
16 KiB
Groff
456 lines
16 KiB
Groff
.\" $NetBSD: bzip2.1,v 1.1.1.2 2002/03/15 01:35:23 mjl Exp $
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.\"
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.PU
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.TH bzip2 1
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.SH NAME
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bzip2, bunzip2 \- a block-sorting file compressor, v1.0.2
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.br
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bzcat \- decompresses files to stdout
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.br
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bzip2recover \- recovers data from damaged bzip2 files
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.SH SYNOPSIS
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.ll +8
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.B bzip2
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.RB [ " \-cdfkqstvzVL123456789 " ]
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[
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.I "filenames \&..."
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]
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.ll -8
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.br
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.B bunzip2
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.RB [ " \-fkvsVL " ]
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[
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.I "filenames \&..."
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]
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.br
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.B bzcat
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.RB [ " \-s " ]
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[
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.I "filenames \&..."
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]
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.br
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.B bzip2recover
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.I "filename"
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.SH DESCRIPTION
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.I bzip2
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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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The command-line options are deliberately very similar to
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those of
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.I GNU gzip,
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but they are not identical.
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.I bzip2
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expects a list of file names to accompany the
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command-line flags. Each file is replaced by a compressed version of
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itself, with the name "original_name.bz2".
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Each compressed file
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has the same modification date, permissions, and, when possible,
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ownership as the corresponding original, so that these properties can
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be correctly restored at decompression time. File name handling is
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naive in the sense that there is no mechanism for preserving original
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file names, permissions, ownerships or dates in filesystems which lack
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these concepts, or have serious file name length restrictions, such as
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MS-DOS.
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.I bzip2
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and
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.I bunzip2
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will by default not overwrite existing
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files. If you want this to happen, specify the \-f flag.
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If no file names are specified,
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.I bzip2
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compresses from standard
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input to standard output. In this case,
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.I bzip2
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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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.I bunzip2
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(or
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.I bzip2 \-d)
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decompresses all
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specified files. Files which were not created by
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.I bzip2
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will be detected and ignored, and a warning issued.
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.I bzip2
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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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filename.bz2 becomes filename
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filename.bz becomes filename
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filename.tbz2 becomes filename.tar
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filename.tbz becomes filename.tar
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anyothername becomes anyothername.out
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If the file does not end in one of the recognised endings,
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.I .bz2,
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.I .bz,
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.I .tbz2
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or
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.I .tbz,
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.I bzip2
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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
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.I .out
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appended.
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As with compression, supplying no
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filenames causes decompression from
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standard input to standard output.
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.I bunzip2
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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 (\-t)
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of concatenated
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compressed files is also supported.
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You can also compress or decompress files to the standard output by
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giving the \-c 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
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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
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.I bzip2
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version 0.9.0 or
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later. Earlier versions of
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.I bzip2
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will stop after decompressing
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the first file in the stream.
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.I bzcat
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(or
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.I bzip2 -dc)
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decompresses all specified files to
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the standard output.
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.I bzip2
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will read arguments from the environment variables
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.I BZIP2
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and
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.I BZIP,
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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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Compression is always performed, even if the compressed
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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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As a self-check for your protection,
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.I
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bzip2
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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
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.I bzip2
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(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
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recover the original uncompressed
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data. You can use
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.I bzip2recover
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to try to recover data from
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damaged files.
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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
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.I bzip2
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to panic.
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.SH OPTIONS
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.TP
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.B \-c --stdout
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Compress or decompress to standard output.
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.TP
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.B \-d --decompress
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Force decompression.
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.I bzip2,
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.I bunzip2
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and
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.I bzcat
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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
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.I bzip2
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to decompress.
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.TP
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.B \-z --compress
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The complement to \-d: forces compression, regardless of the
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invocation name.
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.TP
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.B \-t --test
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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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.TP
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.B \-f --force
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Force overwrite of output files. Normally,
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.I bzip2
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will not overwrite
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existing output files. Also forces
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.I bzip2
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to break hard links
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to files, which it otherwise wouldn't do.
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bzip2 normally declines to decompress files which don't have the
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correct magic header bytes. If forced (-f), however, it will pass
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such files through unmodified. This is how GNU gzip behaves.
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.TP
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.B \-k --keep
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Keep (don't delete) input files during compression
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or decompression.
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.TP
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.B \-s --small
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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, \-s 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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.TP
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.B \-q --quiet
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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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.TP
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.B \-v --verbose
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Verbose mode -- show the compression ratio for each file processed.
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Further \-v'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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.TP
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.B \-L --license -V --version
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Display the software version, license terms and conditions.
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.TP
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.B \-1 (or \-\-fast) to \-9 (or \-\-best)
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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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The \-\-fast and \-\-best aliases are primarily for GNU gzip
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compatibility. In particular, \-\-fast doesn't make things
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significantly faster.
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And \-\-best merely selects the default behaviour.
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.TP
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.B \--
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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: bzip2 \-- \-myfilename.
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.TP
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.B \--repetitive-fast --repetitive-best
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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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.SH MEMORY MANAGEMENT
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.I bzip2
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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 \-1 through \-9
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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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.I bunzip2
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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 \-1 to \-9 are irrelevant to and so ignored
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during decompression.
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Compression and decompression requirements,
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in bytes, can be estimated as:
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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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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
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.I bzip2
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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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For files compressed with the default 900k block size,
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.I bunzip2
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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,
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.I bunzip2
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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 -s.
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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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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 -9 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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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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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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.SH RECOVERING DATA FROM DAMAGED FILES
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.I bzip2
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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 .bz2
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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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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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.I bzip2recover
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is a simple program whose purpose is to search for
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blocks in .bz2 files, and write each block out into its own .bz2
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file. You can then use
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.I bzip2
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\-t
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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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.I bzip2recover
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takes a single argument, the name of the damaged file,
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and writes a number of files "rec00001file.bz2",
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"rec00002file.bz2", 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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"bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in
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the correct order.
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.I bzip2recover
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should be of most use dealing with large .bz2
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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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.SH PERFORMANCE NOTES
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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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\-vvvv option to monitor progress in great detail, if you want.
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Decompression speed is unaffected by these phenomena.
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.I bzip2
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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
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.I bzip2
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will perform best on machines with very large caches.
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.SH CAVEATS
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I/O error messages are not as helpful as they could be.
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.I bzip2
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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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This manual page pertains to version 1.0.2 of
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.I bzip2.
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Compressed data created by this version is entirely forwards and
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backwards compatible with the previous public releases, versions
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0.1pl2, 0.9.0, 0.9.5, 1.0.0 and 1.0.1, but with the following
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exception: 0.9.0 and above can correctly decompress multiple
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concatenated compressed files. 0.1pl2 cannot do this; it will stop
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after decompressing just the first file in the stream.
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.I bzip2recover
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versions prior to this one, 1.0.2, used 32-bit integers to represent
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bit positions in compressed files, so it could not handle compressed
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files more than 512 megabytes long. Version 1.0.2 and above uses
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64-bit ints on some platforms which support them (GNU supported
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targets, and Windows). To establish whether or not bzip2recover was
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built with such a limitation, run it without arguments. In any event
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you can build yourself an unlimited version if you can recompile it
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with MaybeUInt64 set to be an unsigned 64-bit integer.
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.SH AUTHOR
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Julian Seward, jseward@acm.org.
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http://sources.redhat.com/bzip2
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The ideas embodied in
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.I bzip2
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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
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.I bzip,
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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
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.I bzip).
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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.
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The bz* scripts are derived from those of GNU gzip.
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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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