bzip2(1)
bzip2(1) USER COMMANDS 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 ... ]
bunzip2 [ -fkvsVL ] [ filenames ... ]
bzcat [ -s ] [ filenames ... ]
bzip2recover filename
DESCRIPTION
bzip2 compresses files using the Burrows-Wheeler block sort-
ing 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 statist-
ical 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 stan-
dard 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
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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 con-
catenation 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 out-
puts are fed sequentially to stdout. Compression of multi-
ple files in this manner generates a stream containing mul-
tiple 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 con-
venient 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
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that something is wrong. It can't help you recover the ori-
ginal 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
2300k of memory, albeit at about half the normal speed.
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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 per-
taining 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 pri-
marily of interest for diagnostic purposes.
-L --license -V --version
Display the software version, license terms and condi-
tions.
-1 (or --fast) to -9 (or
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 particu-
lar, --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
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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 impor-
tant 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, bun-
zip2 will require about 3700 kbytes to decompress. To sup-
port decompression of any file on a 4 megabyte machine, bun-
zip2 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 con-
straints 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 fig-
ures tend to understate the advantage of larger block sizes
for larger files, since the Corpus is dominated by smaller
files.
Compress Decompress Decompress Corpus
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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 undam-
aged 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 car-
ries its own 32-bit CRC, so damaged blocks can be dis-
tinguished 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 dam-
aged 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 minim-
ise 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
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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 pro-
gress 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 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, jseward@acm.org.
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https://sourceware.org/bzip2/
The ideas embodied in bzip2 are due to (at least) the fol-
lowing 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 Alis-
tair 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 dis-
tribution for pointers to sources of documentation. Chris-
tian 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 perfor-
mance. 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.
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