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 ... ]
bzip2 [
-h|--help ]
bunzip2 [
-fkvsVL ] [
filenames ... ]
bunzip2 [
-h|--help ]
bzcat [
-s ] [
filenames ... ]
bzcat [
-h|--help ]
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 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.
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 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.
- -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.
- -h --help
- Print a help message and exit.
- -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 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.
- --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.
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) 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
RECOVERING DATA FROM DAMAGED FILES¶
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.
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. 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.
AUTHOR¶
Julian Seward, jsewardbzip.org.
http://www.bzip.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.
