BZIP2 linux command manual

bzip2(1)                                                           bzip2(1)



NAME
       bzip2, bunzip2 - a block-sorting file compressor, v1.0.2
       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 modi-
       fication  date, permissions, and, when possible, ownership as the cor-
       responding  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 con-
       cepts, 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 com-
       pressed output to a terminal, as this would be entirely incomprehensi-
       ble 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 con-
       taining multiple compressed file representations.  Such a  stream  can
       be  decompressed correctly only by bzip2 version 0.9.0 or later.  Ear-
       lier 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  hun-
       dred  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 orig-
       inal.  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 uncom-
       pressed 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 decom-
              pression.

       -s --small
              Reduce memory usage, for compression, decompression  and  test-
              ing.   Files are decompressed and tested using a modified algo-
              rithm 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 lim-
              its  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 MEM-
              ORY 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  pro-
              cessed.  Further -v's increase the verbosity level, spewing out
              lots of information which is primarily of interest for diagnos-
              tic 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 com-
              patibility.  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 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 compres-
       sion  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, compress-
       ing a file 20,000 bytes long with the flag -9 will cause the  compres-
       sor  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 differ-
       ent 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 deter-
       mined  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 1.0.2 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 and 1.0.1, 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 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  sup-
       ported   targets,   and   Windows).    To  establish  whether  or  not
       bzip2recover was built with such a limitation, run  it  without  argu-
       ments.   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 following peo-
       ple: Michael Burrows and David Wheeler (for the block  sorting  trans-
       formation),  David  Wheeler (again, for the Huffman coder), Peter Fen-
       wick (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 distri-
       bution 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 com-
       pression  performance.   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)