BZIP2(1) BSD General Commands Manual BZIP2(1)
bzip2, bunzip2, bzcat, bzip2recover -- block-sorting file compressor
bzip2 [-123456789cdfkLqstVvz] [filename file ...]
bunzip2 [-fkLVvs] [filename file ...]
bzcat [-s] [filename file ...]
bzip2 compresses files using the Burrows-Wheeler block sorting text compression algorithm, and Huffman coding. Compression is generally con-
siderably better than that achieved by more conventional LZ77/LZ78-based compressors, and approaches the performance of the PPM family of
bzcat decompresses files to stdout, and bzip2recover recovers data from damaged bzip2 files.
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 corre-
sponding 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 com-
pressed 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 ver-
sions of bzip2 will stop after decompressing the first file in the stream.
bzcat (or bzip2 -dc) decompresses all specified files to the standard output.
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 out-
put 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 origi-
nal. 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.
-- 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.
-1, --fast to
-9, --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(1) compatibility. In particular, --fast doesn't make
things significantly faster, and --best merely selects the default behaviour.
-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.
-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(1) behaves.
-k, --keep Keep (don't delete) input files during compression or decompression.
-L, --license Display the license terms and conditions.
-q, --quiet Suppress non-essential warning messages. Messages pertaining to I/O errors and other critical events will not be sup-
--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.
-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.
-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.
-V, --version Display the software version.
-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.
-z, --compress The complement to Fl d : forces compression, regardless of the invocation name.
bzip2 compresses large files in blocks. The block size affects both the compression ratio achieved, and the amount of memory needed for com-
pression and decompression. The flags -1 through -9 specify the block size to be 100,000 bytes through 900,000 bytes (the default) respec-
tively. At decompression time, the block size used for compression is read from the header of the compressed file, and bunzip2 then allo-
cates 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 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 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. Decom-
pression 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 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.
Flag Compression Decompression Decompression -s Corpus 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 rea-
sonable 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 exam-
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 dam-
aged single-block files, since a damaged block cannot be recovered. If you wish to minimise any potential data loss through media or trans-
mission 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 improve-
ments. I imagine bzip2 will perform best on machines with very large caches.
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.
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 transforma-
tion), David Wheeler (again, for the Huffman coder), Peter Fenwick (for the structured coding model in the original bzip, and many refine-
ments), 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 encour-
aged me to look for faster sorting algorithms, so as to speed up compression. Bela Lubkin encouraged me to improve the worst-case compres-
sion 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.
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 1.0.5 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 1.0.3, 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.
May 14, 2010 BSD