bzip2(1)														  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 ...	]
       bzip2 [ -h|--help ]
       bunzip2 [ -fkvsVL ] [ filenames ...  ]
       bunzip2 [ -h|--help ]
       bzcat [ -s ] [ filenames ...  ]
       bzcat [ -h|--help ]
       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.

       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 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 filename 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 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 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 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 convenient 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 expan‐
       sion 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 identi‐
       cal  to	the  original.	This guards against corruption of the compressed data, and against undetected bugs in bzip2 (hope‐
       fully 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 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 decom‐
	      press.

       -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),  how‐
	      ever, 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 2300 k  of  mem‐
	      ory, albeit at about half the normal speed.

	      During  compression,  -s	selects  a  block size of 200 k, 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 pertaining 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 primarily of interest for diagnostic purposes.

       -h --help
	      Print a help message and exit.

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

       -1 (or --fast) to -9 (or --best)
	      Set  the block size to 100 k, 200 k ...  900 k when compressing.	Has no effect when decompressing.  See MEMORY MAN‐
	      AGEMENT below.  The --fast and --best aliases are primarily for  GNU  gzip  compatibility.   In  particular,  --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 mem‐
       ory  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 decompres‐
       sion.

       Compression and decompression requirements, in bytes, can be estimated as:

	      Compression:   400 k + ( 8 x block size )

	      Decompression: 100 k + ( 4 x block size ), or
			     100 k + ( 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 appreci‐
       ate that the decompression memory requirement is set at compression time by the choice of block size.

       For  files  compressed with the default 900 k 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 necessary.  The
       relevant flag is -s.

       In general, try and use the largest block size memory constraints 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 7600 k of memory, but only touch 400 k + 20000 * 8 = 560 kbytes of it.  Similarly, the  decompressor  will  allocate
       3700 k but only touch 100 k + 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.

		  Compress   Decompress   Decompress   Corpus
	   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 900 kbytes long.  Each block is handled independently.  If a media  or  transmis‐
       sion  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 undam‐
       aged.

       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 wild‐
       cards in subsequent processing -- for example, "bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in the cor‐
       rect 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 aver‐
       age-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 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 can‐
       not 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 limita‐
       tion, run it without arguments.	In any event you can build yourself an unlimited version if you can recompile it with May‐
       beUInt64 set to be an unsigned 64-bit integer.




AUTHOR
       Julian Seward, jsewardbzip.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 generally helpful.



															  bzip2(1)