Provided by: bzip2_1.0.6-1_amd64 bug

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 ...  ]
       bzip2 [ -h|--help ]
       bunzip2 [ -fkvsVL ] [ filenames ...  ]
       bunzip2 [ -h|--help ]
       bzcat [ -s ] [ filenames ...  ]
       bzcat [ -h|--help ]
       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.

       -h --help
              Print a help message and exit.

       -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)