.PU

.TH bzip2 1

.SH NAME

bzip2, bunzip2 \- a block-sorting file compressor, v0.9.0

.br

bzcat \- decompresses files to stdout

.br

bzip2recover \- recovers data from damaged bzip2 files

.SH SYNOPSIS

.ll +8

.B bzip2

.RB [ " \-cdfkstvzVL123456789 " ]

[

.I "filenames \&..."

]

.ll -8

.br

.B bunzip2

.RB [ " \-fkvsVL " ]

[

.I "filenames \&..."

]

.br

.B bzcat

.RB [ " \-s " ]

[

.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 by default not overwrite existing files;

if you want this to happen, specify the \-f flag.

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.

.I 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

.I bzip2

version 0.9.0 or later. Earlier versions of

.I bzip2

will stop after decompressing the first file in the stream.

.I bzcat

(or

.I bzip2 \-dc

) decompresses all specified files to the standard output.

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 + ( 4 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 3700 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 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 1100k 500k 350k 914704

-2 1800k 900k 600k 877703

-3 2500k 1300k 850k 860338

-4 3200k 1700k 1100k 846899

-5 3900k 2100k 1350k 845160

-6 4600k 2500k 1600k 838626

-7 5400k 2900k 1850k 834096

-8 6000k 3300k 2100k 828642

-9 6700k 3700k 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,

.I bunzip2

and

.I 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

.I bzip2

to decompress.

.TP

.B \-z --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.

.TP

.B \-f --force

Force overwrite of output files. Normally,

.I bzip2

will not overwrite existing output files.

.TP

.B \-k --keep

Keep (don't delete) input files during compression or decompression.

.TP

.B \-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 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 -V --version

Display the software version, license terms and conditions.

.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.

.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.

.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 0.9.0 of

.I bzip2.

Compressed data created by this version is entirely forwards and

backwards compatible with the previous public release, version 0.1pl2,

but with the following exception: 0.9.0 can correctly decompress

multiple concatenated compressed files. 0.1pl2 cannot do this; it

will stop after decompressing just the first file in the stream.

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.

.SH AUTHOR

Julian Seward, jseward@acm.org.

The ideas embodied in

.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 the original

.I bzip,

and many refinements),

and

Alistair Moffat, Radford Neal and Ian Witten (for the arithmetic

coder in the original

.I 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.

Many people sent patches, helped with portability problems,

lent machines, gave advice and were generally helpful.