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bzip2.txt
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NAME
bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
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 sta-
tistical 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 com-
mand-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, per-
missions, and, when possible, ownership as the correspond-
ing 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 preserv-
ing 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 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 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 decom-
pression 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 com-
pressed and decompressed like this. The resulting outputs
are fed sequentially to stdout. Compression of multiple
files in this manner generates a stream containing multi-
ple 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 decom-
pressing 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 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 identical to the original. This guards against corrup-
tion 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 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.
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 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), however, 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 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 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 (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 MANAGEMENT below. The --fast and --best
aliases are primarily for GNU gzip compatibility.
In particular, --fast doesn't make things signifi-
cantly 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 han-
dle 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 ver-
sions, 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) respec-
tively. 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 + ( 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 decompression 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,
bunzip2 has an option to decompress using approximately
half this amount of memory, about 2300 kbytes. Decompres-
sion 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 virtu-
ally 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 7600k of memory, but only
touch 400k + 20000 * 8 = 560 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 Compres-
sion 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 domi-
nated 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 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
"rec00001file.bz2", "rec00002file.bz2", etc, containing
the extracted blocks. The output filenames are
designed so that the use of wildcards in subsequent pro-
cessing -- for example, "bzip2 -dc rec*file.bz2 > recov-
ered_data" -- processes the files in the correct 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 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 mon-
itor 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 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 1.0.6 of bzip2. Com-
pressed data created by this version is entirely forwards
and backwards 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 support 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.
AUTHOR
Julian Seward, jsewardbzip.org.
http://www.bzip.org
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. Donna Robinson XMLised the docu-
mentation. 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.