manual_2.html

高效率的一种通用压缩/解压程序

            -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, [e-mail].

       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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