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 bzip2(1)                                                           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 ... ]
      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 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



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 bzip2(1)                                                           bzip2(1)




             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 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



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 bzip2(1)                                                           bzip2(1)




      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.





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 bzip2(1)                                                           bzip2(1)




      -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
           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 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
      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



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 bzip2(1)                                                           bzip2(1)




                            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






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 bzip2(1)                                                           bzip2(1)




 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 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
      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



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 bzip2(1)                                                           bzip2(1)




      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 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 structured coding 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 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.







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