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 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

              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 decom-

       pression from standard input to standard output.

       bunzip2 will correctly decompress a file which is the con-

       catenation 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 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

              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 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

              default behaviour.

       --     Treats all subsequent arguments as file names, even

              if they start with a dash.  This is so you can han-

              dle 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 ver-

              sions, 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)  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

       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 Compres-

       sion 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 domi-

       nated 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 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

       ..."   (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 mon-

       itor 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  ran-

       dom  fashion.   This means that performance, both for com-

       pressing 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 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 1.0.6 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, 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 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-

       sion.  Bela Lubkin encouraged me to improve the worst-case

       compression performance.  Donna Robinson XMLised the docu-

       mentation.   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.