bzip2 and libbzip2 - How to use bzip2

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# How to use `bzip2`

This chapter contains a copy of the `bzip2` man page, and nothing else.

    NAME
           bzip2, bunzip2 - a block-sorting file compressor, v0.9.0
           bzcat - decompresses files to stdout
           bzip2recover - recovers data from damaged bzip2 files

    SYNOPSIS
           bzip2 [ -cdfkstvzVL123456789 ] [ 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  and
           permissions  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,  per-
           missions  and  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 compressed output to a terminal,  as
           this  would  be  entirely  incomprehensible  and therefore
           pointless.

           bunzip2 (or bzip2 -d ) decompresses and restores all spec-
           ified files whose names end in ".bz2".  Files without this
           suffix are ignored.  Again, supplying no filenames  causes
           decompression 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.

           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 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.

    MEMORY MANAGEMENT
           Bzip2 compresses large files in blocks.   The  block  size
           affects  both  the  compression  ratio  achieved,  and the
           amount of memory needed both for  compression  and  decom-
           pression.   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 + ( 7 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 6700k of memory, but only
           touch 400k + 20000 * 7 = 540 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      1100k       500k         350k      914704
                -2      1800k       900k         600k      877703
                -3      2500k      1300k         850k      860338
                -4      3200k      1700k        1100k      846899
                -5      3900k      2100k        1350k      845160
                -6      4600k      2500k        1600k      838626
                -7      5400k      2900k        1850k      834096
                -8      6000k      3300k        2100k      828642
                -9      6700k      3700k        2350k      828642

    OPTIONS
           -c --stdout
                  Compress or decompress to standard output.  -c will
                  decompress multiple files to stdout, but will  only
                  compress a single file to stdout.

           -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, regard-
                  less of the invokation 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.

           -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 above.

           -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 to -9
                  Set the block size to 100 k, 200 k ..  900  k  when
                  compressing.   Has  no  effect  when decompressing.
                  See MEMORY MANAGEMENT above.

           --repetitive-fast
                  bzip2 injects some small  pseudo-random  variations
                  into  very  repetitive  blocks  to limit worst-case
                  performance during compression.   If  sorting  runs
                  into  difficulties,  the  block  is randomised, and
                  sorting is restarted.  Very roughly, bzip2 persists
                  for  three  times  as  long as a well-behaved input
                  would take before resorting to randomisation.  This
                  flag makes it give up much sooner.

           --repetitive-best
                  Opposite  of  --repetitive-fast;  try  a lot harder
                  before resorting to randomisation.

    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 "rec0001file.bz2",
           "rec0002file.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" -- lists the files in
           the "right" 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
           extraordinarily slowly.  You can use the -vvvvv option  to
           monitor progress in great detail, if you want.  Decompres-
           sion speed is unaffected.

           Such pathological cases seem rare in  practice,  appearing
           mostly in artificially-constructed test files, and in low-
           level disk images.  It may be inadvisable to use bzip2  to
           compress  the  latter.   If you do get a file which causes
           severe slowness in compression, try making the block  size
           as small as possible, with flag -1.

           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 0.9.0 of bzip2.  Com-
           pressed  data created by this version is entirely forwards
           and backwards compatible with the previous public release,
           version  0.1pl2,  but  with the following exception: 0.9.0
           can correctly decompress multiple concatenated  compressed
           files.   0.1pl2  cannot do this; it will stop after decom-
           pressing just the first file in the stream.

           Wildcard expansion for Windows 95 and NT is flaky.

           bzip2recover uses 32-bit integers to represent  bit  posi-
           tions  in compressed files, so it cannot handle compressed
           files more than 512 megabytes long.  This could easily  be
           fixed.

    AUTHOR
           Julian Seward, jseward@acm.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.  Many people sent patches, helped
           with portability problems, lent machines, gave advice  and
           were generally helpful.

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