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.TH BUNZIP2 1 "9 June 2003"
bunzip2 \- a block-sorting file decompressor, v1.0.2gs1
bzcat \- decompresses files to stdout
bzip2recover \- recovers data from damaged bzip2 files
.B bunzip2
.RB [ " \-fkvsVL " ]
.I "filenames \&..."
.B bzcat
.RB [ " \-s " ]
.I "filenames \&..."
.B bzip2recover
.I "filename"
.I bunzip2
decompresses files created by
.I bzip2
using the Burrows-Wheeler block sorting
text compression algorithm, and Huffman coding.
.I bzip2
generally achieves
considerably better compression 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 gunzip,
but they are not identical.
.I bunzip2
will by default not overwrite existing
files. If you want this to happen, specify the \-f flag.
.I bunzip2
decompresses all specified files. Files which were not created by
.I bzip2
will be detected and ignored, and a warning issued.
.I bunzip2
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
.I .tbz,
.I bunzip2
complains that it cannot
guess the name of the original file, and uses the original name
.I .out
Supplying no filenames causes decompression from
standard input to standard output.
File name handling is
naive in the sense that there is no mechanism for preserving original
file names, permissions, ownerships or dates in operating systems or
filesystems which lack these concepts, or have serious file name length
restrictions, such as MS-DOS or GS/OS.
.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 decompress files to the standard output by
giving the \-c flag. Multiple files may be
decompressed like this. The resulting outputs are fed sequentially to stdout.
.I bzcat
.I bunzip2
.I \-c)
decompresses all specified files to
the standard output.
.I bunzip2
will read arguments from the environment variables
in that order, and will process them
before any arguments read from the command line. This gives a
convenient way to supply default arguments.
As a self-check for your protection,
.I bzip2
.I bunzip2
use 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
.I bunzip2
(hopefully very unlikely). The
chances of data corruption going undetected are 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.
This manual page pertains to version 1.0.2gs1 of
.I bunzip2.
It is fully campatible with compressed data created with all of the previous
public releases of bzip2, versions
0.1pl2, 0.9.0, 0.9.5, 1.0.0 and 1.0.1, as well as version 1.0.2.
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
.I bunzip2
to panic.
.IP "\fB\-c\fP \fB\--stdout\fP"
Decompress to standard output.
.IP "\fB\-d\fP \fB\--decompress\fP"
Force decompression. This flag is unnecessary on bunzip2 for GNO,
since it always decompresses.
.IP "\fB\-t\fP \fB\--test\fP"
Check integrity of the specified file(s), but don't decompress them.
This really performs a trial decompression and throws away the result.
.IP "\fB\-f\fP \fB\--force\fP"
Force overwrite of output files. Normally,
.I bunzip2
will not overwrite
existing output files.
.I bunzip2
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.
.IP "\fB\-k\fP \fB\--keep\fP"
Keep (don't delete) input files during decompression.
.IP "\fB\-s\fP \fB\--small\fP"
Reduce memory usage, for 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.
In short, if your machine is low on memory (5 megabytes or
less), you will probably need to use \-s. See MEMORY MANAGEMENT below.
.IP "\fB\-q\fP \fB\--quiet\fP"
Suppress non-essential warning messages. Messages pertaining to
I/O errors and other critical events will not be suppressed.
.IP "\fB\-v\fP \fB\--verbose\fP"
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.
.IP "\fB\-L\fP \fB\--license\fP \fB\-V\fP \fB\--version\fP"
Display the software version, license terms and conditions.
.IP "\fB\--\fP"
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: bunzip2 \-- \-myfilename.
.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 block size can be specified
to be 100,000 bytes through 900,000 bytes (the
default). 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.
Decompression requirements, in bytes, can be estimated as:
100k + ( 4 x block size ), or
100k + ( 2.5 x block size ) if using \-s
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.
Decompression speeds 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 a 900k block size will cause the decompressor to
allocate 3700k but only touch 100k + 20000 * 4 = 180 kbytes
when decompressing it.
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.
Block Decompress Decompress Corpus
Size usage -s usage Size
100k 500k 350k 914704
200k 900k 600k 877703
300k 1300k 850k 860338
400k 1700k 1100k 846899
500k 2100k 1350k 845160
600k 2500k 1600k 838626
700k 2900k 1850k 834096
800k 3300k 2100k 828642
900k 3700k 2350k 828642
.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 bunzip2
to test the
integrity of the resulting files, and decompress those which are
.I bzip2recover
takes a single argument, the name of the damaged file,
and writes a number of files named "rec0001file.bz2",
"rec0002file.bz2", etc, containing the extracted blocks.
The output filenames are designed so that the use of
wildcards in subsequent processing -- for example,
"bunzip2 -c 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.
.I bunzip2
usually allocates several megabytes of memory to operate
in, and then charges all over it in a fairly random fashion. This means
that performance is largely determined by the speed at which your machine can
access main memory or (if you have a caching accelerator) serve 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 that
.I bunzip2
will perform best on machines with very large caches.
I/O error messages are not as helpful as they could be.
.I bunzip2
tries hard to detect I/O errors and exit cleanly, but the details of
what the problem is sometimes seem rather misleading.
.I bzip2recover
for GNO uses 32-bit integers to represent bit positions in compressed files,
so it cannot handle compressed files more than 512 megabytes long.
Julian Seward, jseward@acm.org.
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. Many people sent patches, helped
with portability problems, lent machines, gave advice and were generally
This version of
.I bunzip2
for GNO has been ported by Stephen Heumann <sheumann@myrealbox.com> from
Julian Seward's
.I bzip2
version 1.0.2 for other platforms.
This program contains material from the ORCA/C Run-Time Libraries,
copyright 1987-1996 by Byte Works, Inc. Used with permission.
It also incorporates a public domain stristr routine by Fred Cole,
Bob Stout, and Greg Thayer, which was obtained from http://www.snippets.org .