|BZIP2(1)||General Commands Manual||BZIP2(1)|
block-sorting file compressor
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.
bzcat decompresses files to stdout, and
bzip2recover recovers data from damaged bzip2
The command-line options are deliberately very similar to those of gzip(1), 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.
bunzip2 will by
default not overwrite existing files. If you want this to happen, specify
If no file names are specified,
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
-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:
If the file does not end in one of the recognised endings,
.tbz2, or .tbz,
bzip2 complains that it cannot guess the name of the
original file, and uses the original name with .out
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
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
version 0.9.0 or later. Earlier versions of
will stop after decompressing the first file in the stream.
-dc) decompresses all specified files to the
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,
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
(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.
bzip2 -- -myfilename.
-bestaliases are primarily for GNU gzip(1) compatibility. In particular,
-fastdoesn't make things significantly faster, and
-bestmerely selects the default behaviour.
bzcatare 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
bzip2will not overwrite existing output files. Also forces
bzip2to 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
-sselects 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
-sfor everything. See MEMORY MANAGEMENT below.
-v's increase the verbosity level, spewing out lots of information which is primarily of interest for diagnostic purposes.
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
-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
-9 are irrelevant to and so ignored during
Compression and decompression requirements, in bytes, can be estimated as:
bzip2on 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
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.
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
-t to test
the integrity of the resulting files, and decompress those which are
bzip2recover takes a single argument, the
name of the damaged file, and writes a number of files
“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
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
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
perform best on machines with very large caches.
bzip2 will read arguments from the
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.
0 for a normal exit, 1 for environmental problems (file not found,
invalid flags, I/O errors, etc.), 2 to indicate a corrupt compressed file, 3
for an internal consistency error (e.g., bug) which caused
bzip2 to panic.
Julian Seward ⟨email@example.com⟩
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
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
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
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
|May 14, 2010||NetBSD-7.0.1|