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Updated: July 2014

bunzip2 (1)


bunzip2 - sorting file compressor, v1.0.6 bzcat - decompresses files to stdout bzip2recover - recovers data from damaged bzip2 files


bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ...  ]
bunzip2 [ -fkvsVL ] [ filenames ...  ]
bzcat [ -s ] [ filenames ...  ]
bzip2recover filename


User Commands                                            bzip2(1)

     bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
     bzcat - decompresses files to stdout
     bzip2recover - recovers data from damaged bzip2 files

     bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ...  ]
     bunzip2 [ -fkvsVL ] [ filenames ...  ]
     bzcat [ -s ] [ filenames ...  ]
     bzip2recover filename

     bzip2 compresses files using the Burrows-Wheeler block sort-
     ing text compression algorithm, and  Huffman  coding.   Com-
     pression is generally considerably better than that achieved
     by  more  conventional  LZ77/LZ78-based   compressors,   and
     approaches  the performance of the PPM family of statistical

     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 com-
     pressed file has the same  modification  date,  permissions,
     and, when possible, ownership as the corresponding original,
     so that these properties can be correctly restored at decom-
     pression  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  stan-
     dard  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 com-
     pressed file as follows:

            filename.bz2    becomes   filename
       becomes   filename

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User Commands                                            bzip2(1)

            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  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 con-
     venient 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  com-
     pression  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

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User Commands                                            bzip2(1)

     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.

     -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 decom-
          pression 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

     -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.

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User Commands                                            bzip2(1)

          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 per-
          taining  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 primar-
          ily of interest for diagnostic purposes.

     -L --license -V --version
          Display  the software version, license terms and condi-

     -1 (or --fast) to -9 (or
          Set the block size to 100 k, 200 k ..  900 k when  com-
          pressing.   Has no effect when decompressing.  See MEM-
          ORY MANAGEMENT below.  The --fast  and  --best  aliases
          are  primarily for GNU gzip compatibility.  In particu-
          lar, --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.

     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

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User Commands                                            bzip2(1)

     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  impor-
     tant 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,  bun-
     zip2  will require about 3700 kbytes to decompress.  To sup-
     port decompression of any file on a 4 megabyte machine, bun-
     zip2  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  con-
     straints   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

     Here is a table which summarises the  maximum  memory  usage
     for  different block sizes.  Also recorded is the total com-
     pressed 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 fig-
     ures  tend to understate the advantage of larger block sizes
     for larger files, since the Corpus is dominated  by  smaller

                Compress   Decompress   Decompress   Corpus

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User Commands                                            bzip2(1)

         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

     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 dam-
     aged, 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 car-
     ries its own 32-bit CRC, so damaged blocks  can  be  distin-
     guished 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  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  min-
     imise  any  potential data loss through media  or  transmis-
     sion errors, you might consider compressing with  a  smaller
     block size.

     The  sorting  phase  of compression gathers together similar
     strings in the file.  Because of this, files containing very

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User Commands                                            bzip2(1)

     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 oper-
     ate  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.

     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

     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.

     Julian Seward,

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User Commands                                            bzip2(1)

     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 coding
     model in the original bzip, and many refinements), and Alis-
     tair 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 dis-
     tribution for pointers to sources of documentation.   Chris-
     tian  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 perfor-
     mance.  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.

     See   attributes(5)   for   descriptions  of  the  following

     |Availability   | compress/bzip2   |
     |Stability      | Committed        |
     This  software  was   built   from   source   available   at    The  original
     community       source       was       downloaded       from

     Further  information about this software can be found on the
     open source community website at

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