Manual: bzip2recover
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 compression algorithm, and Huffman coding. Comā
pression 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 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 verā
sion 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 comā
pressed 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 comā
pressed 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.
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 overhead in the reā
gion 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 bzip2reā
cover 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 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 acā
tions 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 invocation name.
-t --test
Check integrity of the specified file(s), but don't decompress them. This really performs a trial decompresā
sion and throws away the result.
-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 behaves.
-k --keep
Keep (don't delete) input files during compression or decompression.
-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 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.
-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 diagnostic 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 compatibility. 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 beginning with a dash, for example: bzip2 -- -myfilename.
--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.
MEMORY MANAGEMENT
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:
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 requirement is set at compression time by the choice of block size.
For files compressed with the default 900 k 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 7600 k of memory, but only touch 400 k + 20000 * 8 = 560 kbytes of it. Similarly, the decompresā
sor 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 comā
pressed 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 unā
damaged 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" -- 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 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.
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.
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 maā
chines 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 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 arithā
metic 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(1)