bzip2recover
recovers data from damaged bzip2 files
see also :
bunzip2
Synopsis
bzip2 [
-cdfkqstvzVL123456789 ] [ filenames ...
]
bzip2 [ -h|--help ]
bunzip2 [ -fkvsVL ] [ filenames ...
]
bunzip2 [ -h|--help ]
bzcat [ -s ] [ filenames ... ]
bzcat [ -h|--help ]
bzip2recover filename
add an example, a script, a trick and tips
examples
source
decompress.o \
bzlib.o
all: libbz2.a bzip2 bzip2recover test
bzip2: libbz2.a bzip2.o
$(CC) $(CFLAGS) $(LDFLAGS) -o bzip2 bzip2.o -L.
-lbz2
$(CC) $(CFLAGS) $(LDFLAGS) -o bzip2 bzip2.o -L.
-lbz2
bzip2recover: bzip2recover.o
$(CC) $(CFLAGS) $(LDFLAGS) -o bzip2recover
bzip2recover.o
source
randtable.o \
compress.o \
decompress.o \
bzlib.o
all: libbz2.a bzip2 bzip2recover test
bzip2: libbz2.a bzip2.o
bzip2: libbz2.a bzip2.o
$(CC) $(CFLAGS) $(LDFLAGS) -o bzip2 bzip2.o -L.
-lbz2
bzip2recover: bzip2recover.o
$(CC) $(CFLAGS) $(LDFLAGS) -o bzip2recover
bzip2recover.o
source
randtable.o \
compress.o \
decompress.o \
bzlib.o
all: libbz2.a bzip2 bzip2recover test
bzip2: libbz2.a bzip2.o
bzip2: libbz2.a bzip2.o
$(CC) $(CFLAGS) $(LDFLAGS) -o bzip2 bzip2.o -L.
-lbz2
bzip2recover: bzip2recover.o
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.
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.
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
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.
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.
author
Julian Seward,
jsewardbzip.org.
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.