path: root/winsup/bz2lib/bzip2.txt

NAME
       bzip2, bunzip2 - a block-sorting file compressor, v1.0
       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
       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,  per-
       missions, and, when possible, ownership as the correspond-
       ing 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 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  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 decom-
       pression 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 com-
       pressed and decompressed like this.  The resulting outputs
       are  fed  sequentially to stdout.  Compression of multiple
       files in this manner generates a stream containing  multi-
       ple 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 decom-
       pressing 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 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 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 decompress.

       -z --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.

       -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.

       -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.

              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
              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 ver-
              bosity level, spewing out lots of information which
              is primarily of interest for diagnostic purposes.

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

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

       --     Treats all subsequent arguments as file names, even
              if they start with a dash.  This is so you can han-
              dle 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 ver-
              sions, 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)  respec-
       tively.   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.  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 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 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      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 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
       wildcards in subsequent processing -- for example,  "bzip2
       -dc   rec*file.bz2 > recovered_data" -- lists 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 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
       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 mon-
       itor 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  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.


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 of bzip2.  Com-
       pressed  data created by this version is entirely forwards
       and  backwards  compatible  with   the   previous   public
       releases,  versions  0.1pl2, 0.9.0 and 0.9.5, but with the
       following exception: 0.9.0 and above can correctly  decom-
       press multiple concatenated compressed files.  0.1pl2 can-
       not do this; it will stop  after  decompressing  just  the
       first file in the stream.

       bzip2recover  uses  32-bit integers to represent bit posi-
       tions in compressed files, so it cannot handle  compressed
       files  more than 512 megabytes long.  This could easily be
       fixed.


AUTHOR
       Julian Seward, jseward@acm.org.

       http://sourceware.cygnus.com/bzip2
       http://www.muraroa.demon.co.uk

       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 cod-
       ing 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 man-
       ual 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  compres-
       sion.  Bela Lubkin encouraged me to improve the worst-case
       compression performance.  Many people sent patches, helped
       with  portability problems, lent machines, gave advice and
       were generally helpful.