bzip2, bunzip2 - a block-sorting file compressor, v1.0.8
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files
bzip2 [ -cdfkqstvzVL123456789 ] [
filenames ... ]
bzip2 [ -h|--help ]
bunzip2 [ -fkvsVL ] [ filenames
... ]
bunzip2 [ -h|--help ]
bzcat [ -s ] [ filenames ...
]
bzcat [ -h|--help ]
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.
Compress or decompress to standard output.
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.
The complement to -d: forces compression, regardless of the invocation name.
Check integrity of the specified file(s), but don't decompress them. This really performs a trial decompression and throws away the result.
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.
Keep (don't delete) input files during compression or decompression.
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 2300 k of memory, albeit at about half the normal speed.
During compression, -s selects a block size of 200 k, 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.
Suppress non-essential warning messages. Messages pertaining to I/O errors and other critical events will not be suppressed.
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.
Print a help message and exit.
Display the software version, license terms and conditions.
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 significantly 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 handle files with names beginning with a dash, for example: bzip2 -- -myfilename.
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: 400 k + ( 8 x block size )
Decompression: 100 k + ( 4 x block size ), or 100 k + ( 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 900 k 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 7600 k of memory, but only touch 400 k + 20000 * 8 = 560 kbytes of it. Similarly, the decompressor will allocate 3700 k but only touch 100 k + 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 900 kbytes 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 "rec00001file.bz2",
"rec00002file.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"
-- 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 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 1.0.8 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, 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.
Julian Seward, jseward@acm.org.
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. Donna Robinson XMLised
the documentation. 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.