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
              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
       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 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
                             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
       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
       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.



                                                                      bzip2(1)
