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bzip2(1) bzip2(1)
NAME
bzip2, bunzip2 - a block-sorting file compressor, v1.0.2
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 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 possi-
ble, ownership as the corresponding original, so that these properties can be cor-
rectly restored at decompression time. File name handling is naive in the sense
that there is no mechanism for preserving original file names, permissions, owner-
ships 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 cre-
ated 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 uncom-
pressed 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 man-
ner 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 exist-
ing 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 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 (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 ver-
sions, which was sometimes useful. 0.9.5 and above have an improved algo-
rithm which renders these flags irrelevant.
MEMORY MANAGEMENT
bzip2 compresses large files in blocks. The block size affects both the compres-
sion ratio achieved, and the amount of memory needed for compression and decompres-
sion. 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 compres-
sion comes from the first two or three hundred k of block size, a fact worth bear-
ing 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 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 mem-
ory 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
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 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 undam-
aged.
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 com-
pressing 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 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 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 perfor-
mance 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 1.0.2 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 and 1.0.1, but with the following
exception: 0.9.0 and above can correctly decompress multiple concatenated com-
pressed files. 0.1pl2 cannot do this; it will stop after decompressing just the
first file in the stream.
bzip2recover versions prior to this one, 1.0.2, used 32-bit integers to represent
bit positions in compressed files, so it could not handle compressed files more
than 512 megabytes long. Version 1.0.2 and above uses 64-bit ints on some plat-
forms which support them (GNU supported targets, and Windows). To establish
whether or not bzip2recover was built with such a limitation, run it without argu-
ments. In any event you can build yourself an unlimited version if you can recom-
pile it with MaybeUInt64 set to be an unsigned 64-bit integer.
AUTHOR
Julian Seward, jseward AT acm.org.
http://sources.redhat.com/bzip2
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 encour-
aged me to improve the worst-case compression performance. 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.
bzip2(1)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
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