Manual: bunzip2
bzip2(1) General Commands Manual bzip2(1)
NAME
bzip2, bunzip2 - a block-sorting file compressor, v1.0.8
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 Huffā
man 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 compresā
sors.
The command-line options are deliberately very similar to those of GNU gzip, but they are not identiā
cal.
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 incomā
prehensible 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 decomā
pressed 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 outā
put.
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 comā
pressed 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 compresā
sors) 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 verā
sion 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 beā
haves.
-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 decomā
pressing. 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 decompresā
sion 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 durā
ing 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 deā
compress. To support decompression of any file on a 4 megabyte machine, bunzip2 has an option to deā
compress 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 fileā
names 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 recovā
ered. If you wish to minimise any potential data loss through media or transmission 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 hunā
dred 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 1.0.8 of bzip2. Compressed data created by this version is enā
tirely 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 decomā
pressing 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, jseward@acm.org.
https://sourceware.org/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 inā
debted 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 perā
formance. 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.
bzip2(1)