bunzip2



bzip2(1)                                                              bzip2(1)




NAME

       bzip2, bunzip2 - a block-sorting file compressor, v1.0.2
       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 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
       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.  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 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.

       -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:   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



RECOVERING DATA FROM DAMAGED FILES

       bzip2 compresses files in blocks, usually 900kbytes 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  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 1.0.2 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
       and 1.0.1, but with the following exception: 0.9.0 and above  can  cor-
       rectly  decompress multiple concatenated compressed files.  0.1pl2 can-
       not do this; it will stop after decompressing just the  first  file  in
       the stream.

       bzip2recover versions prior to this one, 1.0.2, used 32-bit integers to
       represent bit positions in compressed files, so  it  could  not  handle
       compressed files more than 512 megabytes long.  Version 1.0.2 and above
       uses 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.





AUTHOR

       Julian Seward, jseward@acm.org.

       http://sources.redhat.com/bzip2

       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.  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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