           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 sta
           tistical 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 com
           mand-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,  per
           missions, and, when possible, ownership as the correspond
           ing 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 preserv
           ing  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

           also supported.

           You can also compress or decompress files to the  standard
           output  by giving the -c flag.  Multiple files may be com
           pressed and decompressed like this.  The resulting outputs
           are  fed  sequentially to stdout.  Compression of multiple
           files in this manner generates a stream containing  multi
           ple 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 decom
           pressing 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 corrup
           tion  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

           -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 ver
                  bosity 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  signifi
                  cantly  faster.   And  --best  merely  selects  the

## 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)  respec
           tively.   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.  Decompres
           sion 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 con
           straints  allow,  since  that  maximises  the  compression
           achieved.   Compression and decompression speed are virtu
           ally 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

                -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 trans
           mission 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 dam
           aged    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 pro
           cessing  -- for example, "bzip2 -dc  rec*file.bz2 > recov
           ered_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 min
           imise any potential data loss through media  or  transmis
           sion 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
           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 per
           form 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 1.0.2 of bzip2.  Com
           pressed  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 and 1.0.1,
           but with the following exception: 0.9.0 and above can cor
           rectly  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  this  one,  1.0.2, used
           32-bit integers to represent bit positions  in  compressed
           files,  so  it could not handle compressed files more than
           512 megabytes long.  Version 1.0.2 and above  uses  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, jseward@acm.org.

           http://sources.redhat.com/bzip2

           The ideas embodied in bzip2 are due to (at least) the fol
           lowing  people: Michael Burrows and David Wheeler (for the
           block sorting transformation), David Wheeler  (again,  for
           the Huffman coder), Peter Fenwick (for the structured cod
           ing 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 man
           ual 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  compres