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bzip2(1)                                                              bzip2(1)



NAME
       bzip2, bunzip2 - a block-sorting file compressor, v1.0.4
       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  compres-
       sors, 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 cor-
       rectly restored at decompression time.  File name handling is naive in the sense that there is no mechanism for
       preserving  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 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.  Compression of
       multiple files in this manner generates a stream containing multiple 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 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 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 corruption of the compressed data, and against unde-
       tected bugs in bzip2 (hopefully very unlikely).  The chances of data  corruption  going  undetected  is  micro-
       scopic, 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  uncom-
       pressed 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, 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 overwrite 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 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 verbosity
              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 (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 compatibility.  In
              particular, --fast doesn't make things significantly 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 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 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  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 approx-
       imately 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.  Simi-
       larly, 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 col-
       umn 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 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  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 damaged single-block  files,  since  a damaged  block  cannot  be recovered.  If
       you wish to minimise any potential data loss through media  or  transmission errors, you  might  consider  com-
       pressing with a smaller block size.


PERFORMANCE NOTES
       The sorting phase of compression gathers together similar strings in the file.  Because of this, files contain-
       ing 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  ver-
       sions,  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 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 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 for-
       wards 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 concate-
       nated 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  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, 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 transformation), David Wheeler (again, for the Huffman coder), Peter Fenwick (for the  struc-
       tured  coding model in the original bzip, and many refinements), and Alistair Moffat, Radford Neal and Ian Wit-
       ten (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
       encouraged me to look for faster sorting algorithms, so as to speed up compression.  Bela Lubkin encouraged  me
       to  improve the worst-case compression 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)