bzip2

       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,
       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.

              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 2300k of  memory,  albeit  at  about
              half the normal speed.

              During  compression, -s selects a block size of 200k, 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.

       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.
       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 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 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

       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 recov-
       ered.  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 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 1.0.4 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 1.0.3, 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.

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)
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