bzip2recover - Online in the Cloud

PROGRAM:

NAME

bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
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. 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.

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
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.

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 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.

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 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 invocation 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 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 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 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 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 1.0.6 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.

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