


bzip2(1)                                                 bzip2(1)


NAME
       bzip2, bunzip2 - a block-sorting file compressor, v0.1
       bzip2recover - recovers data from damaged bzip2 files


SYNOPSIS
       bzip2 [ -cdfkstvVL123456789 ] [ filenames ...  ]
       bunzip2 [ -kvsVL ] [ filenames ...  ]
       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 sta-
       tistical 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 com-
       mand-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  and
       permissions  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,  per-
       missions  and  dates  in filesystems which lack these con-
       cepts, or have serious file name length restrictions, such
       as MS-DOS.

       Bzip2  and  bunzip2  will not overwrite existing files; if
       you want this to happen, you should delete them first.

       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 and restores all spec-
       ified files whose names end in ".bz2".  Files without this
       suffix are ignored.  Again, supplying no filenames  causes
       decompression from standard input to standard output.

       You  can also compress or decompress files to the standard
       output by giving the -c flag.  You can decompress multiple
       files  like  this, but you may only compress a single file
       this way, since it would otherwise be difficult  to  sepa-
       rate  out  the  compressed representations of the original
       files.



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bzip2(1)                                                 bzip2(1)


       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 corrup-
       tion  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 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.


MEMORY MANAGEMENT
       Bzip2 compresses large files in blocks.   The  block  size
       affects  both  the  compression  ratio  achieved,  and the
       amount of memory needed both for  compression  and  decom-
       pression.   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.   Com-
       pression  and decompression requirements, in bytes, can be
       estimated as:

             Compression:   400k + ( 7 x block size )

             Decompression: 100k + ( 5 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.



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bzip2(1)                                                 bzip2(1)


       For files compressed with the  default  900k  block  size,
       bunzip2  will require about 4600 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.  Decompres-
       sion  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  virtu-
       ally 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 6700k of memory, but only
       touch 400k + 20000 * 7 = 540 kbytes of it.  Similarly, the
       decompressor  will  allocate  4600k  but only touch 100k +
       20000 * 5 = 200 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  Compres-
       sion  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  domi-
       nated by smaller files.

                  Compress   Decompress   Decompress   Corpus
           Flag     usage      usage       -s usage     Size

            -1      1100k       600k         350k      914704
            -2      1800k      1100k         600k      877703
            -3      2500k      1600k         850k      860338
            -4      3200k      2100k        1100k      846899
            -5      3900k      2600k        1350k      845160
            -6      4600k      3100k        1600k      838626
            -7      5400k      3600k        1850k      834096
            -8      6000k      4100k        2100k      828642
            -9      6700k      4600k        2350k      828642


OPTIONS
       -c --stdout
              Compress or decompress to standard output.  -c will
              decompress multiple files to stdout, but will  only
              compress a single file to stdout.





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bzip2(1)                                                 bzip2(1)


       -d --decompress
              Force  decompression.  Bzip2 and bunzip2 are really
              the same program, and the decision about whether to
              compress  or  decompress  is  done  on the basis of
              which name is used.  This flag overrides that mech-
              anism, and forces bzip2 to decompress.

       -f --compress
              The  complement  to -d: forces compression, regard-
              less of the invokation 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, using the
              low-memory decompression algorithm (see -s).

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

       -s --small
              Reduce  memory  usage,  both  for  compression  and
              decompression.  Files are decompressed using a mod-
              ified algorithm which only requires 2.5  bytes  per
              block  byte.   This  means  any  file can be decom-
              pressed in 2300k of memory,  albeit  somewhat  more
              slowly than usual.

              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 above.


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

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

       -V --version
              Same as -L.

       -1 to -9
              Set the block size to 100 k, 200 k ..  900  k  when
              compressing.   Has  no  effect  when decompressing.
              See MEMORY MANAGEMENT above.



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bzip2(1)                                                 bzip2(1)


       --repetitive-fast
              bzip2 injects some small  pseudo-random  variations
              into  very  repetitive  blocks  to limit worst-case
              performance during compression.   If  sorting  runs
              into  difficulties,  the  block  is randomised, and
              sorting is restarted.  Very roughly, bzip2 persists
              for  three  times  as  long as a well-behaved input
              would take before resorting to randomisation.  This
              flag makes it give up much sooner.


       --repetitive-best
              Opposite  of  --repetitive-fast;  try  a lot harder
              before resorting to randomisation.


RECOVERING DATA FROM DAMAGED FILES
       bzip2 compresses files in blocks, usually 900kbytes  long.
       Each block is handled independently.  If a media or trans-
       mission 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 dam-
       aged file, and writes a number of files "rec0001file.bz2",
       "rec0002file.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" -- lists the files in the
       "right" 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 min-
       imise any potential data loss through media  or  transmis-
       sion errors, you might consider compressing with a smaller
       block size.


PERFORMANCE NOTES
       The sorting phase of compression gathers together  similar



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bzip2(1)                                                 bzip2(1)


       strings  in  the  file.  Because of this, files containing
       very long runs of  repeated  symbols,  like  "aabaabaabaab
       ..."   (repeated   several  hundred  times)  may  compress
       extraordinarily slowly.  You can use the -vvvvv option  to
       monitor progress in great detail, if you want.  Decompres-
       sion speed is unaffected.

       Such pathological cases seem rare in  practice,  appearing
       mostly in artificially-constructed test files, and in low-
       level disk images.  It may be inadvisable to use bzip2  to
       compress  the  latter.   If you do get a file which causes
       severe slowness in compression, try making the block  size
       as small as possible, with flag -1.

       Incompressible or virtually-incompressible data may decom-
       press rather more slowly than one would hope.  This is due
       to a naive implementation of the move-to-front coder.

       bzip2  usually  allocates  several  megabytes of memory to
       operate in, and then charges all over it in a fairly  ran-
       dom  fashion.   This means that performance, both for com-
       pressing 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 per-
       form best on machines with very large caches.

       Test mode (-t) uses the low-memory decompression algorithm
       (-s).  This means test mode does not run  as  fast  as  it
       could;  it  could  run as fast as the normal decompression
       machinery.  This could easily be fixed at the cost of some
       code bloat.


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 0.1 of bzip2.  It may
       well  happen that some future version will use a different
       compressed file format.  If you try to  decompress,  using
       0.1,  a  .bz2  file created with some future version which
       uses a different compressed file format, 0.1 will complain
       that  your  file  "is not a bzip2 file".  If that happens,
       you should obtain a more recent version of bzip2  and  use
       that to decompress the file.

       Wildcard expansion for Windows 95 and NT is flaky.

       bzip2recover  uses  32-bit integers to represent bit posi-
       tions in compressed files, so it cannot handle  compressed



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bzip2(1)                                                 bzip2(1)


       files  more than 512 megabytes long.  This could easily be
       fixed.

       bzip2recover sometimes reports a  very  small,  incomplete
       final  block.  This is spurious and can be safely ignored.


RELATIONSHIP TO bzip-0.21
       This program is a descendant of the bzip program,  version
       0.21,  which  I released in August 1996.  The primary dif-
       ference of bzip2 is its avoidance of the possibly patented
       algorithms  which  were  used  in 0.21.  bzip2 also brings
       various useful refinements (-s,  -t),  uses  less  memory,
       decompresses  significantly  faster,  and  has support for
       recovering data from damaged files.

       Because bzip2 uses Huffman coding to  construct  the  com-
       pressed  bitstream, rather than the arithmetic coding used
       in 0.21, the compressed representations generated  by  the
       two  programs are incompatible, and they will not interop-
       erate.  The change in suffix from  .bz  to  .bz2  reflects
       this.   It would have been helpful to at least allow bzip2
       to decompress files created by 0.21, but this would defeat
       the primary aim of having a patent-free compressor.

       For a more precise statement about patent issues in bzip2,
       please see the README file in the distribution.

       Huffman  coding  necessarily  involves some coding ineffi-
       ciency compared to arithmetic  coding.   This  means  that
       bzip2  compresses about 1% worse than 0.21, an unfortunate
       but unavoidable fact-of-life.  On the other  hand,  decom-
       pression  is approximately 50% faster for the same reason,
       and the change in file format gave an opportunity  to  add
       data-recovery features.  So it is not all bad.


AUTHOR
       Julian Seward, jseward@acm.org.

       The ideas embodied in bzip and 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 0.21, and many refinements),
       and Alistair Moffat, Radford Neal and Ian Witten (for  the
       arithmetic  coder  in 0.21).  I am much indebted for their
       help, support and advice.  See the file ALGORITHMS in  the
       source  distribution for pointers to sources of documenta-
       tion.  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  com-
       pression  performance.   Many  people sent patches, helped
       with portability problems, lent machines, gave advice  and
       were generally helpful.





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