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 consider-
ably 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 possible, ownership
as the corresponding origi-
nal, so that these properties
can be correctly restored at decompression time. File name handling is
naive in the sense that there is no mecha-
nism 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 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 cor-
responding 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 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 cor-
ruption 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 com-
pressed 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.
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 infor-
mation 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 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 compres-
sion 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 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. 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 decom-
pression 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.
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
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 undamaged.
bzip2recover takes a single argument, the
name of the damaged file, and writes a number of files "rec00001file.bz2",
"rec00002file.bz2", etc, con-
taining 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 transmis-
sion 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 sym-
bols, 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 per-
formance, 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 prob-
lem 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 compressed
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 com-
pressed files more than 512 megabytes long.
Version 1.0.2 and above uses 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, [email protected].
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 encouraged 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 gen-
erally helpful.