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BZIP2(1) NetBSD General Commands Manual BZIP2(1)
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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 com-
pression algorithm, and Huffman coding. Compression is generally consid-
erably better than that achieved by more conventional LZ77/LZ78-based
compressors, and approaches the performance of the PPM family of statis-
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 corresponding
original, so that these properties can be correctly restored at decom-
pression 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
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
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 giv-
ing 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 cor-
rectly 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
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 over-
head 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 unde-
tected 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.
-- Treats all subsequent arguments as file names, even
if they start with a dash. This is so you can han-
dle files with names beginning with a dash, for
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 signifi-
cantly faster, and --best merely selects the default
-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)
-k, --keep Keep (don't delete) input files during compression
-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 suppressed.
--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 ver-
sions, which was sometimes useful. 0.9.5 and above
have an improved algorithm which renders these flags
-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 pri-
marily of interest for diagnostic purposes.
-z, --compress The complement to Fl d : forces compression, regard-
less 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 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
Compression: 400k + ( 8 x block size )
Decompression: 100k + ( 4 x block size ), or 100k + ( 2.5 x block
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 require-
ment 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. Decompres-
sion speed is also halved, so you should use this option only where nec-
essary. 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 col-
umn 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 pat-
tern, which makes it possible to find the block boundaries with reason-
able 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 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 dam-
aged 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.
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 com-
press 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 deter-
mined 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.
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
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 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. 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 prob-
lem 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
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 your-
self an unlimited version if you can recompile it with MaybeUInt64 set to
be an unsigned 64-bit integer.
NetBSD 9.1 July 13, 2019 NetBSD 9.1