454 lines
16 KiB
Groff
454 lines
16 KiB
Groff
.PU
|
|
.TH bzip2 1
|
|
.SH NAME
|
|
bzip2, bunzip2 \- a block-sorting file compressor, v1.0.2
|
|
.br
|
|
bzcat \- decompresses files to stdout
|
|
.br
|
|
bzip2recover \- recovers data from damaged bzip2 files
|
|
|
|
.SH SYNOPSIS
|
|
.ll +8
|
|
.B bzip2
|
|
.RB [ " \-cdfkqstvzVL123456789 " ]
|
|
[
|
|
.I "filenames \&..."
|
|
]
|
|
.ll -8
|
|
.br
|
|
.B bunzip2
|
|
.RB [ " \-fkvsVL " ]
|
|
[
|
|
.I "filenames \&..."
|
|
]
|
|
.br
|
|
.B bzcat
|
|
.RB [ " \-s " ]
|
|
[
|
|
.I "filenames \&..."
|
|
]
|
|
.br
|
|
.B bzip2recover
|
|
.I "filename"
|
|
|
|
.SH DESCRIPTION
|
|
.I 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.
|
|
|
|
The command-line options are deliberately very similar to
|
|
those of
|
|
.I GNU gzip,
|
|
but they are not identical.
|
|
|
|
.I 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.
|
|
|
|
.I bzip2
|
|
and
|
|
.I bunzip2
|
|
will by default not overwrite existing
|
|
files. If you want this to happen, specify the \-f flag.
|
|
|
|
If no file names are specified,
|
|
.I bzip2
|
|
compresses from standard
|
|
input to standard output. In this case,
|
|
.I bzip2
|
|
will decline to
|
|
write compressed output to a terminal, as this would be entirely
|
|
incomprehensible and therefore pointless.
|
|
|
|
.I bunzip2
|
|
(or
|
|
.I bzip2 \-d)
|
|
decompresses all
|
|
specified files. Files which were not created by
|
|
.I bzip2
|
|
will be detected and ignored, and a warning issued.
|
|
.I 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,
|
|
.I .bz2,
|
|
.I .bz,
|
|
.I .tbz2
|
|
or
|
|
.I .tbz,
|
|
.I bzip2
|
|
complains that it cannot
|
|
guess the name of the original file, and uses the original name
|
|
with
|
|
.I .out
|
|
appended.
|
|
|
|
As with compression, supplying no
|
|
filenames causes decompression from
|
|
standard input to standard output.
|
|
|
|
.I 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
|
|
.I bzip2
|
|
version 0.9.0 or
|
|
later. Earlier versions of
|
|
.I bzip2
|
|
will stop after decompressing
|
|
the first file in the stream.
|
|
|
|
.I bzcat
|
|
(or
|
|
.I bzip2 -dc)
|
|
decompresses all specified files to
|
|
the standard output.
|
|
|
|
.I bzip2
|
|
will read arguments from the environment variables
|
|
.I BZIP2
|
|
and
|
|
.I 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,
|
|
.I
|
|
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
|
|
.I 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
|
|
.I 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
|
|
.I bzip2
|
|
to panic.
|
|
|
|
.SH OPTIONS
|
|
.TP
|
|
.B \-c --stdout
|
|
Compress or decompress to standard output.
|
|
.TP
|
|
.B \-d --decompress
|
|
Force decompression.
|
|
.I bzip2,
|
|
.I bunzip2
|
|
and
|
|
.I 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
|
|
.I bzip2
|
|
to decompress.
|
|
.TP
|
|
.B \-z --compress
|
|
The complement to \-d: forces compression, regardless of the
|
|
invocation name.
|
|
.TP
|
|
.B \-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.
|
|
.TP
|
|
.B \-f --force
|
|
Force overwrite of output files. Normally,
|
|
.I bzip2
|
|
will not overwrite
|
|
existing output files. Also forces
|
|
.I 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.
|
|
.TP
|
|
.B \-k --keep
|
|
Keep (don't delete) input files during compression
|
|
or decompression.
|
|
.TP
|
|
.B \-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.
|
|
.TP
|
|
.B \-q --quiet
|
|
Suppress non-essential warning messages. Messages pertaining to
|
|
I/O errors and other critical events will not be suppressed.
|
|
.TP
|
|
.B \-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.
|
|
.TP
|
|
.B \-L --license -V --version
|
|
Display the software version, license terms and conditions.
|
|
.TP
|
|
.B \-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.
|
|
.TP
|
|
.B \--
|
|
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.
|
|
.TP
|
|
.B \--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.
|
|
|
|
.SH MEMORY MANAGEMENT
|
|
.I 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
|
|
.I 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
|
|
.I 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,
|
|
.I bunzip2
|
|
will require about 3700 kbytes to decompress. To support decompression
|
|
of any file on a 4 megabyte machine,
|
|
.I 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
|
|
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.
|
|
|
|
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
|
|
|
|
.SH RECOVERING DATA FROM DAMAGED FILES
|
|
.I 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.
|
|
|
|
.I 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
|
|
.I bzip2
|
|
\-t
|
|
to test the
|
|
integrity of the resulting files, and decompress those which are
|
|
undamaged.
|
|
|
|
.I 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 example,
|
|
"bzip2 -dc rec*file.bz2 > recovered_data" -- processes the files in
|
|
the correct order.
|
|
|
|
.I 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.
|
|
|
|
.SH 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 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.
|
|
|
|
.I 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
|
|
.I bzip2
|
|
will perform best on machines with very large caches.
|
|
|
|
.SH CAVEATS
|
|
I/O error messages are not as helpful as they could be.
|
|
.I 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 1.0.2 of
|
|
.I 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.
|
|
|
|
.I 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 compressed
|
|
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.
|
|
|
|
|
|
|
|
.SH AUTHOR
|
|
Julian Seward, jseward@acm.org.
|
|
|
|
http://sources.redhat.com/bzip2
|
|
|
|
The ideas embodied in
|
|
.I 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
|
|
.I bzip,
|
|
and many refinements), and Alistair Moffat, Radford Neal and Ian Witten
|
|
(for the arithmetic coder in the original
|
|
.I 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 generally
|
|
helpful.
|