bzip2(1)                     General Commands Manual                    bzip2(1)

       bzip2, bunzip2 - a block-sorting file compressor, v1.0.8
       bzcat - decompresses files to stdout
       bzip2recover - recovers data from damaged bzip2 files

       bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ...  ]
       bunzip2 [ -fkvsVL ] [ filenames ...  ]
       bzcat [ -s ] [ filenames ...  ]
       bzip2recover filename

       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

       bzcat (or bzip2 -dc) decompresses all specified files to the standard

       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.

       -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

       -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

       -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

       -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

       --     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

       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

              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

       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.

       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.

       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 1.0.8 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.

       Julian Seward, jseward@acm.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.