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NetBSD 6.1.5 - man page for bzcat (netbsd section 1)

BZIP2(1)			   BSD General Commands Manual				 BZIP2(1)

NAME
     bzip2, bunzip2, bzcat, bzip2recover -- block-sorting file compressor

SYNOPSIS
     bzip2 [-123456789cdfkLqstVvz] [filename file ...]

     bunzip2 [-fkLVvs] [filename file ...]

     bzcat [-s] [filename file ...]

     bzip2recover filename

DESCRIPTION
     bzip2 compresses files using the Burrows-Wheeler block sorting text compression algorithm,
     and Huffman coding.  Compression is generally considerably better than that achieved by more
     conventional LZ77/LZ78-based compressors, and approaches the performance of the PPM family
     of statistical compressors.

     bzcat decompresses files to stdout, and bzip2recover recovers data from damaged bzip2 files.

     The command-line options are deliberately very similar to those of gzip(1), 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 com-
     pressed file has the same modification date, permissions, and, when possible, ownership as
     the corresponding original, so that these properties can be correctly restored at decompres-
     sion time.  File name handling is naive in the sense that there is no mechanism for preserv-
     ing 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 standard input to standard output.  In
     this case, bzip2 will decline to write compressed output to a terminal, as this would be
     entirely incomprehensible and therefore pointless.

     bunzip2 (or bzip2 -d) decompresses all specified files.  Files which were not created by
     bzip2 will be detected and ignored, and a warning issued.	bzip2 attempts to guess the file-
     name for the decompressed file from that of the compressed file as follows:

	   filename.bz2     becomes    filename
	   filename.bz	    becomes    filename
	   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 concatenation of two or more com-
     pressed 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 outputs are fed
     sequentially to stdout.  Compression of multiple files in this manner generates a stream
     containing multiple compressed file representations.  Such a stream can be decompressed cor-
     rectly 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.

     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 compres-
     sion 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 expan-
     sion of around 0.5%.

     As a self-check for your protection, bzip2 uses 32-bit CRCs to make sure that the decom-
     pressed 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 that something is wrong.  It can't help you recover the original uncom-
     pressed data.  You can use bzip2recover to try to recover data from damaged files.

OPTIONS
     -- 		  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.

     -1, --fast 	  to

     -9, --best 	  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(1) compatibility.  In
			  particular, --fast doesn't make things significantly faster, and --best
			  merely selects the default behaviour.

     -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.

     -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 cor-
			  rect magic header bytes.  If forced (-f), however, it will pass such
			  files through unmodified.  This is how GNU gzip(1) behaves.

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

     -L, --license	  Display the license terms and conditions.

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

     --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 ear-
			  lier versions, which was sometimes useful.  0.9.5 and above have an
			  improved algorithm which renders these flags irrelevant.

     -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 decom-
			  pressed in 2300k of memory, albeit at about half the normal speed.
			  During compression, -s selects a block size of 200k, which limits mem-
			  ory 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.

     -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.

     -V, --version	  Display the software version.

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

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

   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 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 important to appreciate that the decompression memory
     requirement is set at compression time by the choice of block size.

     For files compressed with the default 900k block size, bunzip2 will require about 3700
     kbytes to decompress.  To support decompression of any file on a 4 megabyte machine, bunzip2
     has an option to decompress using approximately half this amount of memory, about 2300
     kbytes.  Decompression speed is also halved, so you should use this option only where neces-
     sary.  The relevant flag is -s.

     In general, try and use the largest block size memory constraints allow, since that max-
     imises the compression achieved.  Compression and decompression speed are virtually unaf-
     fected 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 pro-
     portional 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 figures tend to understate the advantage of larger block sizes for larger
     files, since the Corpus is dominated by smaller files.

     Flag   Compression   Decompression   Decompression -s   Corpus 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 indepen-
     dently.  If a media or transmission error causes a multi-block .bz2 file to become damaged,
     it may be possible to recover data from the undamaged 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 carries
     its own 32-bit CRC, so damaged blocks can be distinguished 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 damaged file, and writes a number of
     files ``rec00001file.bz2'', ``rec00002file.bz2'', etc., containing the extracted blocks.
     The output filenames are designed so that the use of wildcards in subsequent processing --
     for example,
	   bzip2 -dc rec*file.bz2 > recovered_data
     -- processes the files in 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 minimise 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 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 progress 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 decom-
     pressing, 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.

ENVIRONMENT
     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 convenient
     way to supply default arguments.

EXIT STATUS
     0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O
     errors, etc.), 2 to indicate a corrupt compressed file, 3 for an internal consistency error
     (e.g., bug) which caused bzip2 to panic.

AUTHORS
     Julian Seward <jseward@bzip.org>

     http://www.bzip.org

     The ideas embodied in bzip2 are due to (at least) the following 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 Alistair 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 distribution for pointers to sources of documentation.  Christian von Roques
     encouraged me to look for faster sorting algorithms, so as to speed up compression.  Bela
     Lubkin encouraged me to improve the worst-case compression performance.  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 gener-
     ally helpful.

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 mis-
     leading.

     This manual page pertains to version 1.0.5 of bzip2.  Compressed data created by this ver-
     sion is entirely forwards and backwards compatible with the previous public releases, ver-
     sions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1, 1.0.2 and 1.0.3, 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 com-
     pressed files, so they could not handle compressed files more than 512 megabytes long.  Ver-
     sions 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 limi-
     tation, 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.

BSD					   May 14, 2010 				      BSD


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