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


   bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ...  ]
   bzip2 [ -h|--help ]
   bunzip2 [ -fkvsVL ] [ filenames ...  ]
   bunzip2 [ -h|--help ]
   bzcat [ -s ] [ filenames ...  ]
   bzcat [ -h|--help ]
   bzip2recover filename


   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
     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

   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

   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 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.


   -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

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

   -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 memory,  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 MANAGEMENT  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

   --     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.


   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

          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  appreciate  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


   bzip2 compresses files in blocks, usually 900 kbytes 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 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.


   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  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.


   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.


   Julian Seward,

   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.



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