bzip2(1)                                              General Commands Manual                                              bzip2(1)

       bzip2, bunzip2 - a block-sorting file compressor, v1.0.8
       bzcat - decompresses files to stdout
       bzip2recover - recovers data from damaged bzip2 files

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

       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 modification date, permissions, and, when pos‐
       sible,  ownership  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, permissions,  owner‐
       ships 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

              filename.bz2    becomes   filename
         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 con‐
       catenation of the corresponding uncompressed files.  Integrity testing (-t) of concatenated compressed files  is  also  sup‐

       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 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 ar‐
       guments 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  identi‐
       cal 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 that something is
       wrong.  It can't help you recover the original uncompressed data.  You can use bzip2recover to try to recover data from dam‐
       aged files.

       Return values: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors, &c), 2 to indi‐
       cate 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 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), how‐
              ever, 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 2300 k of memory,
              albeit at about half the normal speed.

              During compression, -s selects a block size of 200 k, which limits memory use to around the same figure, at  the  ex‐
              pense of your compression ratio.  In short, if your machine is low on memory (8 megabytes or less), use -s for every‐
              thing.  See MEMORY MANAGEMENT below.

       -q --quiet
              Suppress non-essential warning messages.  Messages pertaining to I/O errors and other critical  events  will  not  be

       -v --verbose
              Verbose mode -- show the compression ratio for each file processed.  Further -v's increase the verbosity level, spew‐
              ing out lots of information which is primarily of interest for diagnostic purposes.

       -h --help
              Print a help message and exit.

       -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  MAN‐
              AGEMENT  below.   The  --fast  and  --best  aliases  are primarily for GNU gzip compatibility.  In particular, --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 mem‐
       ory 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 follows that the flags -1 to -9 are irrelevant to and so ignored during decompres‐

       Compression and decompression requirements, in bytes, can be estimated as:

              Compression:   400 k + ( 8 x block size )

              Decompression: 100 k + ( 4 x block size ), or
                             100 k + ( 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 appreci‐
       ate that the decompression memory requirement is set at compression time by the choice of block size.

       For files compressed with the default 900 k 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, compressing a file 20,000 bytes long with the flag -9 will cause the compressor  to  allocate  around
       7600 k of memory, but only touch 400 k + 20000 * 8 = 560 kbytes of it.  Similarly, the decompressor will allocate 3700 k but
       only touch 100 k + 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

       bzip2 compresses files in blocks, usually 900 kbytes 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

       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  undam‐

       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 or‐

       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.

       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 determined by the speed at which your ma‐
       chine 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

       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 de‐
       tails of what the problem is sometimes seem rather misleading.

       This manual page pertains to version 1.0.8 of bzip2.  Compressed data created by this version is entirely forwards and back‐
       wards 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 sup‐
       port 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,

       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 compression.  Bela Lubkin encouraged me to improve the worst-case compression performance.  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.