bzip2(1) General Commands Manual bzip2(1)
NAME
bzip2, bunzip2 - a block-sorting file compressor, v1.0.8
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files
SYNOPSIS
bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
bzip2 [ -h|--help ]
bunzip2 [ -fkvsVL ] [ filenames ... ]
bunzip2 [ -h|--help ]
bzcat [ -s ] [ filenames ... ]
bzcat [ -h|--help ]
bzip2recover filename
DESCRIPTION
bzip2 compresses files using the Burrows-Wheeler block sorting text com-
pression algorithm, and Huffman coding. Compression is generally con-
siderably better than that achieved by more conventional LZ77/LZ78-based
compressors, and approaches the performance of the PPM family of statis-
tical 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
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 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 concate-
nated 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. Compres-
sion of multiple files in this manner generates a stream containing mul-
tiple compressed file representations. Such a stream can be decom-
pressed correctly only by bzip2 version 0.9.0 or later. Earlier ver-
sions 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 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
corruption going undetected is microscopic, about one chance in four
billion for each file processed. Be aware, though, that the check oc-
curs 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 invo-
cation 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 over-
write 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 be-
haves.
-k --keep
Keep (don't delete) input files during compression or decompres-
sion.
-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 2300 k of memory, albeit at about
half the normal speed.
During compression, -s selects a block size of 200 k, which lim-
its 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 information which is primarily of interest for diag-
nostic purposes.
-h --help
Print a help message and exit.
-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 compati-
bility. 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 begin-
ning with a dash, for example: bzip2 -- -myfilename.
--repetitive-fast --repetitive-best
These flags are redundant in versions 0.9.5 and above. They pro-
vided some coarse control over the behaviour of the sorting algo-
rithm in earlier versions, which was sometimes useful. 0.9.5 and
above have an improved algorithm which renders these flags irrel-
evant.
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: 400 k + ( 8 x block size )
Decompression: 100 k + ( 4 x block size ), or
100 k + ( 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 re-
quirement is set at compression time by the choice of block size.
For files compressed with the default 900 k block size, bunzip2 will re-
quire 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 7600 k of memory, but only touch 400 k + 20000 * 8 = 560 kbytes
of it. Similarly, the decompressor will allocate 3700 k but only touch
100 k + 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 900 kbytes 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 rea-
sonable certainty. Each block also carries its own 32-bit CRC, so dam-
aged 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 de-
compress 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.
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 com-
press more slowly than normal. Versions 0.9.5 and above fare much bet-
ter 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. Be-
cause 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
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.
AUTHOR
Julian Seward, jseward@acm.org.
https://sourceware.org/bzip2/
The ideas embodied in bzip2 are due to (at least) the following people:
Michael Burrows and David Wheeler (for the block sorting transforma-
tion), 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 portabil-
ity problems, lent machines, gave advice and were generally helpful.
bzip2(1)
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