bzip2(1)                    General Commands Manual                   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  decom-
       pression  time.  File name handling is naive in the sense that there is
       no mechanism for preserving original file  names,  permissions,  owner-
       ships  or dates in filesystems which lack these concepts, or have seri-
       ous 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  concate-
       nated 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.  Com-
       pression of multiple files in this manner generates a stream containing
       multiple  compressed file representations.  Such a stream can be decom-
       pressed correctly only by bzip2 version 0.9.0 or later.   Earlier  ver-
       sions  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 hun-
       dred 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 origi-
       nal.   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 some-
       thing 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 com-
       pressed 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,  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 over-
              write 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
              behaves.

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

       -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 lim-
              its 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  MAN-
              AGEMENT 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  pro-
              cessed.   Further -v's increase the verbosity level, spewing out
              lots of information which is primarily of interest for  diagnos-
              tic 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 compat-
              ibility.  In particular, --fast  doesn't  make  things  signifi-
              cantly 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  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 compres-
       sion 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:   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


RECOVERING DATA FROM DAMAGED FILES
       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 rea-
       sonable certainty.  Each block also carries its own 32-bit CRC, so dam-
       aged 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 dam-
       aged single-block files, since a damaged block cannot be recovered.  If
       you wish to minimise 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 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  deter-
       mined  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 improve-
       ments.  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 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  tar-
       gets, 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 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 transforma-
       tion), David Wheeler (again, for the Huffman coder), Peter Fenwick (for
       the  structured  coding  model  in  the original bzip, and many refine-
       ments), 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 encour-
       aged me to look for faster sorting algorithms, so as to speed  up  com-
       pression.  Bela Lubkin encouraged me to improve the worst-case compres-
       sion 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)
