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bzip2, bunzip2 - a block-sorting file compressor, v1.0
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files
bzip2
[ -cdfkqstvzVL123456789 ] [ filenames ... ]
bunzip2 [ -fkvsVL ] [ filenames ... ]
bzcat [ -s ] [ filenames ... ]
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 possible, 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, 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 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 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 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 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 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 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.
- -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 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.
Also forces bzip2 to break hard links to files, which it otherwise
wouldn't do.
- -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 verbosity 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 below.
- --
- 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 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) 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 + ( 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. 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 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 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 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.
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 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 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 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 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.
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 of bzip2. Compressed data created by this version is entirely
forwards and backwards compatible with the previous public releases, versions
0.1pl2, 0.9.0 and 0.9.5, 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 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.
Julian Seward, jseward@acm.org.
http://sourceware.cygnus.com/bzip2
http://www.muraroa.demon.co.uk
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. Many people sent patches, helped with
portability problems, lent machines, gave advice and were generally helpful.
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