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NAME
bzip2, bunzip2 - a block-sorting file compressor, v0.9.0
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files
SYNOPSIS
bzip2 [ -cdfkstvzVL123456789 ] [ 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 statist-
ical 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 and permis-
sions as the corresponding original, so that these proper-
ties can be correctly restored at decompression time. File
name handling is naive in the sense that there is no mechan-
ism 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.
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 and restores all speci-
fied files whose names end in ".bz2". Files without this
suffix are ignored. Again, supplying no filenames causes
decompression 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
compressed and decompressed like this. The resulting out-
puts are fed sequentially to stdout. Compression of multi-
ple files in this manner generates a stream containing mul-
tiple 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.
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
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
that 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.
MEMORY MANAGEMENT
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 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
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 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 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 fig-
ures 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
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, 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 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 verbosity
level, spewing out lots of information which is pri-
marily of interest for diagnostic purposes.
-L --license -V --version
Display the software version, license terms and condi-
tions.
-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 ran-
domisation. 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
transmission error causes a multi-block .bz2 file to become
damaged, it may be possible to recover data from the undam-
aged 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 dis-
tinguished 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 wild-
cards 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 dam-
aged 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 ..."
(repeated several hundred times) may compress extraordi-
narily 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 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 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.
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.
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.
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, [email protected].
http://www.muraroa.demon.co.uk
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. Many people sent patches, helped with portability
problems, lent machines, gave advice and were generally
helpful.
NOTES
Source for bzip2 is available in the SUNWbzipS package.
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