path: root/bzip2.1

.PU
.TH bzip2 1
.SH NAME
bzip2, bunzip2 \- a block-sorting file compressor, v0.1
.br
bzip2recover \- recovers data from damaged bzip2 files

.SH SYNOPSIS
.ll +8
.B bzip2
.RB [ " \-cdfkstvVL123456789 " ]
[
.I "filenames \&..."
]
.ll -8
.br
.B bunzip2
.RB [ " \-kvsVL " ]
[
.I "filenames \&..."
]
.br
.B bzip2recover
.I "filename"

.SH DESCRIPTION
.I 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 
.I GNU Gzip,
but they are not identical.

.I 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 and permissions
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 and dates in filesystems 
which lack these concepts, or have serious file name length
restrictions, such as MS-DOS.

.I Bzip2
and
.I bunzip2
will not overwrite existing files; if you want this to happen,
you should delete them first.

If no file names are specified,
.I bzip2
compresses from standard input to standard output.
In this case,
.I bzip2
will decline to write compressed output to a terminal, as
this would be entirely incomprehensible and therefore pointless.

.I Bunzip2
(or
.I bzip2 \-d
) decompresses and restores all specified files whose names
end in ".bz2".
Files without this suffix are ignored.  
Again, supplying no filenames
causes decompression from standard input to standard output.

You can also compress or decompress files to
the standard output by giving the \-c flag.
You can decompress multiple files like this, but you may
only compress a single file this way, since it would otherwise
be difficult to separate out the compressed representations of
the original files.

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,
.I 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
.I 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
that something is wrong.  It can't help you recover the
original uncompressed data.
You can use
.I 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
.I bzip2 
to panic.

.SH MEMORY MANAGEMENT
.I Bzip2
compresses large files in blocks.  The block size affects both the 
compression ratio achieved, and the amount of memory needed both 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
.I 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:   400k + ( 7 x block size )

      Decompression: 100k + ( 5 x block size ), or
.br
                     100k + ( 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 
.I 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, 
.I bunzip2
will require about 4600 kbytes to decompress.
To support decompression of any file on a 4 megabyte machine,
.I 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
6700k of memory, but only touch 400k + 20000 * 7 = 540
kbytes of it.  Similarly, the decompressor will allocate 4600k but
only touch 100k + 20000 * 5 = 200 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      1100k       600k         350k      914704
     -2      1800k      1100k         600k      877703
     -3      2500k      1600k         850k      860338
     -4      3200k      2100k        1100k      846899
     -5      3900k      2600k        1350k      845160
     -6      4600k      3100k        1600k      838626
     -7      5400k      3600k        1850k      834096
     -8      6000k      4100k        2100k      828642
     -9      6700k      4600k        2350k      828642

.SH OPTIONS
.TP
.B \-c  --stdout
Compress or decompress to standard output.  \-c will decompress
multiple files to stdout, but will only compress a single file to
stdout.
.TP
.B \-d --decompress
Force decompression.
.I Bzip2
and
.I bunzip2
are really the same program, and the decision about whether to
compress or decompress is done on the basis of which name is
used.  This flag overrides that mechanism, and forces
.I bzip2
to decompress.
.TP 
.B \-f --compress
The complement to \-d: forces compression, regardless of the invokation
name.
.TP
.B \-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,
using the low-memory decompression algorithm (see \-s).
.TP
.B \-k --keep
Keep (don't delete) input files during compression or decompression.
.TP
.B \-s --small
Reduce memory usage, both for compression and decompression.
Files are decompressed 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 somewhat more slowly than
usual.

During compression, -s selects a block size of 200k, 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 above.

.TP
.B \-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.
.TP
.B \-L --license
Display the software version, license terms and conditions.
.TP
.B \-V --version
Same as \-L.
.TP
.B \-1 to \-9 
Set the block size to 100 k, 200 k .. 900 k when
compressing.  Has no effect when decompressing.
See MEMORY MANAGEMENT above.
.TP
.B \--repetitive-fast
.I bzip2
injects some small pseudo-random variations
into very repetitive blocks to limit
worst-case performance during compression.
If sorting runs into difficulties, the block
is randomised, and sorting is restarted.  
Very roughly, 
.I bzip2
persists for three times as long as a well-behaved input
would take before resorting to randomisation.
This flag makes it give up much sooner.

.TP
.B \--repetitive-best
Opposite of \--repetitive-fast; try a lot harder before 
resorting to randomisation.

.SH RECOVERING DATA FROM DAMAGED FILES
.I 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 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.

.I 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
.I bzip2 -t
to test the integrity of the resulting files, 
and decompress those which are undamaged.

.I bzip2recover
takes a single argument, the name of the damaged file,
and writes a number of files "rec0001file.bz2", "rec0002file.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" --
lists the files in the "right" order.

.I 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.

.SH 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 extraordinarily slowly.
You can use the
\-vvvvv 
option to monitor progress in great detail, if you want.
Decompression speed is unaffected.

Such pathological cases
seem rare in practice, appearing mostly in artificially-constructed
test files, and in low-level disk images.  It may be inadvisable to
use 
.I bzip2
to compress the latter.  
If you do get a file which causes severe slowness in compression,
try making the block size as small as possible, with flag \-1.

Incompressible or virtually-incompressible data may decompress
rather more slowly than one would hope.  This is due to 
a naive implementation of the move-to-front coder.

.I 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 
.I bzip2
will perform best on machines with very large caches.

Test mode (\-t) uses the low-memory decompression algorithm
(\-s).  This means test mode does not run as fast as it could;
it could run as fast as the normal decompression machinery.
This could easily be fixed at the cost of some code bloat.

.SH CAVEATS
I/O error messages are not as helpful as they could be.
.I 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 0.1 of 
.I bzip2.  
It may well happen that some future version will
use a different compressed file format.  If you try to 
decompress, using 0.1, a .bz2 file created with some
future version which uses a different compressed file format,
0.1 will complain that your file "is not a bzip2 file".
If that happens, you should obtain a more recent version
of 
.I bzip2
and use that to decompress the file.

Wildcard expansion for Windows 95 and NT 
is flaky.

.I bzip2recover
uses 32-bit integers to represent bit positions in
compressed files, so it cannot handle compressed files
more than 512 megabytes long.  This could easily be fixed.

.I bzip2recover
sometimes reports a very small, incomplete final block.
This is spurious and can be safely ignored.

.SH RELATIONSHIP TO bzip-0.21
This program is a descendant of the 
.I bzip
program, version 0.21, which I released in August 1996.  
The primary difference of
.I bzip2
is its avoidance of the possibly patented algorithms
which were used in 0.21.  
.I bzip2
also brings various useful refinements (\-s, \-t),
uses less memory, decompresses significantly faster, and
has support for recovering data from damaged files.

Because
.I bzip2
uses Huffman coding to construct the compressed bitstream,
rather than the arithmetic coding used in 0.21,
the compressed representations generated by the two programs
are incompatible, and they will not interoperate.  The change
in suffix from .bz to .bz2 reflects this.  It would have been
helpful to at least allow
.I bzip2
to decompress files created by 0.21, but this would
defeat the primary aim of having a patent-free compressor.

Huffman coding necessarily involves some coding inefficiency
compared to arithmetic coding.  This means that
.I bzip2
compresses about 1% worse than 0.21, an unfortunate but
unavoidable fact-of-life.  On the other hand, decompression
is approximately 50% faster for the same reason, and the
change in file format gave an opportunity to add data-recovery
features.  So it is not all bad.

.SH AUTHOR
Julian Seward, jseward@acm.org.

The ideas embodied in 
.I bzip
and
.I 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 0.21, 
and many refinements),
and
Alistair Moffat, Radford Neal and Ian Witten (for the arithmetic
coder in 0.21).  I am much indebted for their help, support and advice.
See the file ALGORITHMS 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.
Many people sent patches, helped with portability problems,
lent machines, gave advice and were generally helpful.