bzip2.1

高效率的一种通用压缩/解压程序

.PU
.TH bzip2 1
.SH NAME
bzip2, bunzip2 \- a block-sorting file compressor, v0.9.0
.br
bzcat \- decompresses files to stdout
.br
bzip2recover \- recovers data from damaged bzip2 files

.SH SYNOPSIS
.ll +8
.B bzip2
.RB [ " \-cdfkstvzVL123456789 " ]
[
.I "filenames \&..."
]
.ll -8
.br
.B bunzip2
.RB [ " \-fkvsVL " ]
[
.I "filenames \&..."
]
.br 
.B bzcat
.RB [ " \-s " ]
[
.I "filenames \&..."
]
.br
.B bzip2recover
.I "filename"

.SH DESCRIPTION
.I 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 
.I GNU Gzip,
but they are not identical.

.I 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 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, permissions and dates in filesystems 
which lack these concepts, or have serious file name length
restrictions, such as MS-DOS.

.I bzip2
and
.I bunzip2
will by default not overwrite existing files; 
if you want this to happen, specify the \-f flag.

If no file names are specified,
.I bzip2
compresses from standard input to standard output.
In this case,
.I bzip2
will decline to write compressed output to a terminal, as
this would be entirely incomprehensible and therefore pointless.

.I bunzip2
(or
.I bzip2 \-d
) decompresses and restores all specified files whose names
end in ".bz2".
Files without this suffix are ignored.  
Again, supplying no filenames
causes decompression from standard input to standard output.

.I bunzip2
will correctly decompress a file which is the concatenation
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 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
.I bzip2
version 0.9.0 or later.  Earlier versions of
.I bzip2
will stop after decompressing the first file in the stream.

.I bzcat
(or
.I 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,
.I 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
.I 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
.I 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
.I bzip2 
to panic.

.SH MEMORY MANAGEMENT
.I 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 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
.I 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
.br
                     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 
.I 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, 
.I bunzip2
will require about 3700 kbytes to decompress.
To support decompression of any file on a 4 megabyte machine,
.I 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
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 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      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

.SH OPTIONS
.TP
.B \-c --stdout
Compress or decompress to standard output.  \-c will decompress
multiple files to stdout, but will only compress a single file to
stdout.
.TP
.B \-d --decompress
Force decompression.
.I bzip2,
.I bunzip2
and
.I 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
.I bzip2
to decompress.
.TP 
.B \-z --compress
The complement to \-d: forces compression, regardless of the invokation
name.
.TP
.B \-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.
.TP
.B \-f --force
Force overwrite of output files.  Normally,
.I bzip2
will not overwrite existing output files.
.TP
.B \-k --keep
Keep (don't delete) input files during compression or decompression.
.TP
.B \-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.
.TP
.B \-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.
.TP
.B \-L --license -V --version
Display the software version, license terms and conditions.
.TP
.B \-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.
.TP
.B \--repetitive-fast
.I 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, 
.I 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.

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

.SH RECOVERING DATA FROM DAMAGED FILES
.I bzip2
compresses files in blocks, usually 900kbytes 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 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.

.I 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
.I bzip2 -t
to test the integrity of the resulting files, 
and decompress those which are undamaged.

.I 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 "right" order.

.I 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.

.SH 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.
Decompression 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 
.I 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.

.I 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 observed to give
disproportionately large performance improvements.
I imagine 
.I bzip2
will perform best on machines with very large caches.

.SH CAVEATS
I/O error messages are not as helpful as they could be.
.I 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 
.I bzip2.  
Compressed 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 decompressing just the first file in the stream.

Wildcard expansion for Windows 95 and NT 
is flaky.

.I 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.

.SH AUTHOR
Julian Seward, [e-mail].

http://www.muraroa.demon.co.uk

The ideas embodied in 
.I 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
.I bzip, 
and many refinements),
and
Alistair Moffat, Radford Neal and Ian Witten (for the arithmetic
coder in the original
.I 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.
Many people sent patches, helped with portability problems,
lent machines, gave advice and were generally helpful.