     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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