BZIP2(1) | General Commands Manual | BZIP2(1) |
bzip2
, bunzip2
,
bzcat
, bzip2recover
—
bzip2 |
[-123456789cdfkLqstVvz ]
[filename file ...] |
bunzip2 |
[-fkLVvs ] [filename
file ...] |
bzcat |
[-s ] [filename
file ...] |
bzip2recover |
filename |
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.
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 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 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 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 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.
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 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.
-
-
bzip2 --
-myfilename.
-1
,
-
-fast
-9
,
-
-best
-
-fast
and
-
-best
aliases are
primarily for GNU gzip(1)
compatibility. In particular,
-
-fast
doesn't make things
significantly faster, and
-
-best
merely selects the
default behaviour.-c
,
-
-stdout
-d
,
-
-decompress
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
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) behaves.
-k
,
-
-keep
-L
,
-
-license
-q
,
-
-quiet
-
-repetitive-fast
-
-repetitive-best
-s
,
-
-small
-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
-V
,
-
-version
-v
,
-
-verbose
-v
's increase the verbosity level, spewing out
lots of information which is primarily of interest for diagnostic
purposes.-z
,
-
-compress
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 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:
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 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.
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 |
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., 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
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.
-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 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.
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.
bzip2
to panic.
http://www.bzip.org
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.
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.
July 13, 2019 | NetBSD 9.4 |