bunzip2 man page on MirBSD

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

NAME
     bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
     bzcat - decompresses files to stdout
     bzip2recover - recovers data from damaged bzip2 files

SYNOPSIS
     bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
     bunzip2 [ -fkvsVL ] [ filenames ... ]
     bzcat [ -s ] [ filenames ... ]
     bzip2recover filename

DESCRIPTION
     bzip2 compresses files using the Burrows-Wheeler block sort-
     ing 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 statist-
     ical 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
     command-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, permissions,
     and, when possible, ownership 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, permissions, ownerships or dates in filesystems
     which lack these concepts, 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 stan-
     dard 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 all specified files.
     Files which were not created by bzip2 will be detected and
     ignored, and a warning issued. bzip2 attempts to guess the
     filename for the decompressed file from that of the
     compressed file as follows:

	    filename.bz2    becomes   filename
	    filename.bz	    becomes   filename

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

	    filename.tbz2   becomes   filename.tar
	    filename.tbz    becomes   filename.tar
	    anyothername    becomes   anyothername.out

     If the file does not end in one of the recognised endings,
     .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it cannot
     guess the name of the original file, and uses the original
     name with .out appended.

     As with compression, 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
     compressed and decompressed like this.  The resulting out-
     puts are fed sequentially to stdout.  Compression of multi-
     ple files in this manner generates a stream containing mul-
     tiple 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 decompressing the
     first file in the stream.

     bzcat (or bzip2 -dc) decompresses all specified files to the
     standard output.

     bzip2 will read arguments from the environment variables
     BZIP2 and BZIP, in that order, and will process them before
     any arguments read from the command line.	This gives a con-
     venient way to supply default arguments.

     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 corruption
     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

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

     that something is wrong.  It can't help you recover the ori-
     ginal 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.

OPTIONS
     -c --stdout
	  Compress or decompress to standard output.

     -d --decompress
	  Force decompression. bzip2, bunzip2 and 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
	  bzip2 to decompress.

     -z --compress
	  The complement to -d: forces compression, regardless of
	  the invocation name.

     -t --test
	  Check integrity of the specified file(s), but don't
	  decompress them. This really performs a trial
	  decompression and throws away the result.

     -f --force
	  Force overwrite of output files.  Normally, bzip2 will
	  not overwrite existing output files.	Also forces bzip2
	  to break hard links to files, which it otherwise
	  wouldn't do.

	  bzip2 normally declines to decompress files which don't
	  have the correct magic header bytes.	If forced (-f),
	  however, it will pass such files through unmodified.
	  This is how GNU gzip behaves.

     -k --keep
	  Keep (don't delete) input files during compression or
	  decompression.

     -s --small
	  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.

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

	  During compression, -s selects a block size of 200k,
	  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.

     -q --quiet
	  Suppress non-essential warning messages.  Messages per-
	  taining to I/O errors and other critical events will
	  not be suppressed.

     -v --verbose
	  Verbose mode -- show the compression ratio for each
	  file processed. Further -v's increase the verbosity
	  level, spewing out lots of information which is pri-
	  marily of interest for diagnostic purposes.

     -L --license -V --version
	  Display the software version, license terms and condi-
	  tions.

     -1 (or --fast) to -9 (or
	  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 compatibility.  In particu-
	  lar, --fast doesn't make things significantly faster.
	  And --best merely selects the default behaviour.

     --	  Treats all subsequent arguments as file names, even if
	  they start with a dash.  This is so you can handle
	  files with names beginning with a dash, for example:
	  bzip2 -- -myfilename.

     --repetitive-fast --repetitive-best
	  These flags are redundant in versions 0.9.5 and above.
	  They provided some coarse control over the behaviour of
	  the sorting algorithm in earlier versions, which was
	  sometimes useful.  0.9.5 and above have an improved
	  algorithm which renders these flags irrelevant.

MEMORY MANAGEMENT
     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 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

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     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 + ( 8 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 impor-
     tant 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, bun-
     zip2 will require about 3700 kbytes to decompress.	 To sup-
     port decompression of any file on a 4 megabyte machine, bun-
     zip2 has an option to decompress using approximately half
     this amount of memory, about 2300 kbytes.	Decompression
     speed is also halved, so you should use this option only
     where necessary.  The relevant flag is -s.

     In general, try and use the largest block size memory con-
     straints allow, since that maximises the compression
     achieved.	Compression and decompression speed are virtually
     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 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.

     Here is a table which summarises the maximum memory usage
     for different block sizes.	 Also recorded is the total
     compressed size for 14 files of the Calgary Text Compression
     Corpus totalling 3,141,622 bytes.	This column gives some
     feel for how compression varies with block size. These fig-
     ures tend to understate the advantage of larger block sizes
     for larger files, since the Corpus is dominated by smaller
     files.

		Compress   Decompress	Decompress   Corpus

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

	 Flag	  usage	     usage	 -s usage     Size

	  -1	  1200k	      500k	   350k	     914704
	  -2	  2000k	      900k	   600k	     877703
	  -3	  2800k	     1300k	   850k	     860338
	  -4	  3600k	     1700k	  1100k	     846899
	  -5	  4400k	     2100k	  1350k	     845160
	  -6	  5200k	     2500k	  1600k	     838626
	  -7	  6100k	     2900k	  1850k	     834096
	  -8	  6800k	     3300k	  2100k	     828642
	  -9	  7600k	     3700k	  2350k	     828642

RECOVERING DATA FROM DAMAGED FILES
     bzip2 compresses files in blocks, usually 900kbytes long.
     Each block is handled independently.  If a media or
     transmission error causes a multi-block .bz2 file to become
     damaged, it may be possible to recover data from the undam-
     aged blocks in the file.

     The compressed representation of each block is delimited by
     a 48-bit pattern, which makes it possible to find the block
     boundaries with reasonable certainty.  Each block also car-
     ries its own 32-bit CRC, so damaged blocks can be dis-
     tinguished from undamaged ones.

     bzip2recover is a simple program whose purpose is to search
     for blocks in .bz2 files, and write each block out into its
     own .bz2 file.  You can then use bzip2 -t to test the
     integrity of the resulting files, and decompress those which
     are undamaged.

     bzip2recover takes a single argument, the name of the dam-
     aged 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 the correct order.

     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 minim-
     ise any potential data loss through media	or  transmission
     errors, you might consider compressing with a smaller block
     size.

PERFORMANCE NOTES
     The sorting phase of compression gathers together similar
     strings in the file.  Because of this, files containing very

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

     long runs of repeated 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. For previous versions, this figure was more
     like 100:1.  You can use the -vvvv option to monitor pro-
     gress in great detail, if you want.

     Decompression speed is unaffected by these phenomena.

     bzip2 usually allocates several megabytes of memory to
     operate in, and then charges all over it in a fairly random
     fashion.  This means 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 bzip2 will perform best on machines
     with very large caches.

CAVEATS
     I/O error messages are not as helpful as they could be.
     bzip2 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 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 exception: 0.9.0 and above can
     correctly decompress multiple concatenated compressed files.
     0.1pl2 cannot do this; it will stop after decompressing just
     the first file in the stream.

     bzip2recover versions prior to 1.0.2 used 32-bit integers to
     represent bit positions in compressed files, so they could
     not handle compressed files more than 512 megabytes long.
     Versions 1.0.2 and above use 64-bit ints on some platforms
     which support them (GNU supported targets, and Windows).  To
     establish whether or not bzip2recover was built with such a
     limitation, run it without arguments.  In any event you can
     build yourself an unlimited version if you can recompile it
     with MaybeUInt64 set to be an unsigned 64-bit integer.

AUTHOR
     Julian Seward, jsewardbzip.org.

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bzip2(1)	    UNIX Programmer's Manual		 bzip2(1)

     http://www.bzip.org

     The ideas embodied in bzip2 are due to (at least) the fol-
     lowing people: Michael Burrows and David Wheeler (for the
     block sorting transformation), David Wheeler (again, for the
     Huffman coder), Peter Fenwick (for the structured coding
     model in the original bzip, and many refinements), and Alis-
     tair Moffat, Radford Neal and Ian Witten (for the arithmetic
     coder in the original bzip). I am much indebted for their
     help, support and advice.	See the manual in the source dis-
     tribution for pointers to sources of documentation.  Chris-
     tian 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 perfor-
     mance. Donna Robinson XMLised the documentation. The bz*
     scripts are derived from those of GNU gzip. Many people sent
     patches, helped with portability problems, lent machines,
     gave advice and were generally helpful.

MirOS BSD #10-current  Printed 15.11.2010			8

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