path: root/winsup/bz2lib/bzip2.1

bzip2(1)                             General Commands Manual                             bzip2(1)

NAME
       bzip2, bunzip2 - a block-sorting file compressor, v1.0
       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 re-
       placed 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 hap-
       pen, 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 decompression from  standard  input  to
       standard output.

       bunzip2  will  correctly  decompress a file which is the concatenation of two or more com-
       pressed 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 containing multiple compressed file representations.  Such a stream can  be  decom-
       pressed  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  (in-
       cluding  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  decom-
       pressed 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 bil-
       lion 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 consis-
       tency 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 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 out-
              put files.  Also forces bzip2 to break hard links  to  files,  which  it  otherwise
              wouldn't do.

       -k --keep
              Keep (don't delete) input files during compression or decompression.

       -s --small
              Reduce  memory usage, for compression, decompression and testing.  Files are decom-
              pressed 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 verbosity 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 below.

       --     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 compression is read from the
       header of the compressed file, and bunzip2 then allocates itself just enough memory to de-
       compress  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  decom-
       pression 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,  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 constraints allow, since that max-
       imises the compression achieved.  Compression and decompression speed are virtually  unaf-
       fected 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 pro-
       portional  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  allo-
       cate  around  7600k  of memory, but only touch 400k + 20000 * 8 = 560 kbytes of it.  Simi-
       larly, 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 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  indepen-
       dently.   If  a  media or transmission 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 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 integ-
       rity 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  "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 cor-
       rect 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 ..."  (re-
       peated 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 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 ob-
       served 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 mis-
       leading.

       This  manual  page pertains to version 1.0 of bzip2.  Compressed data created by this ver-
       sion is entirely forwards and backwards compatible with the previous public releases, ver-
       sions  0.1pl2, 0.9.0 and 0.9.5, 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  uses  32-bit  integers to represent bit positions 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.

       http://sourceware.cygnus.com/bzip2 http://www.muraroa.demon.co.uk

       The ideas embodied in bzip2 are due to (at least) the following  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  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 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.

                                                                                         bzip2(1)