bzip2recover - recovers data from damaged bzip2 files
SYNOPSIS
bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
bunzip2 [ -fkvsVL ] [ filenames ... ]
bzcat [ -s ] [ filenames ... ]
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 decom-
pression time. File name handling is naive in the sense that there is
no mechanism for preserving original file names, permissions, owner-
ships or dates in filesystems which lack these concepts, or have seri-
ous 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.
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 hun-
dred 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 origi-
nal. 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 some-
thing 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
invocation name.
-t –test
Check integrity of the specified file(s), but don’t decompress
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 2300k of memory, albeit at about
half the normal speed.
During compression, -s selects a block size of 200k, 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 MAN-
AGEMENT 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 pro-
cessed. Further -v’s increase the verbosity level, spewing out
lots of information which is primarily of interest for diagnos-
tic purposes.
-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 compat-
ibility. In particular, –fast doesn’t make things signifi-
cantly 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
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 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.
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 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
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 recov-
ered. 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 deter-
mined 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 improve-
ments. 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 1.0.4 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 1.0.3, 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 tar-
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 refine-
ments), 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 encour-
aged me to look for faster sorting algorithms, so as to speed up com-
pression. Bela Lubkin encouraged me to improve the worst-case compres-
sion 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)