.PU

.TH bzip2 1

.SH NAME

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

.br

bzcat \- decompresses files to stdout

.br

bzip2recover \- recovers data from damaged bzip2 files

.SH SYNOPSIS

.ll +8

.B bzip2

.RB [ " \-cdfkqstvzVL123456789 " ]

[

.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, 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.

.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 all

specified files.  Files which were not created by 

.I bzip2

will be detected and ignored, and a warning issued.  

.I 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, 

.I .bz2, 

.I .bz, 

.I .tbz2

or

.I .tbz, 

.I bzip2 

complains that it cannot

guess the name of the original file, and uses the original name

with

.I .out

appended.

As with compression, 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.

.I bzip2

will read arguments from the environment variables

.I BZIP2

and

.I 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, 

.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

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 OPTIONS

.TP

.B \-c --stdout

Compress or decompress to standard output.

.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

invocation 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.  Also forces 

.I 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.

.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 below.

.TP

.B \-q --quiet

Suppress non-essential warning messages.  Messages pertaining to

I/O errors and other critical events will not be suppressed.

.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 (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.

.TP

.B \--

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.

.TP

.B \--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.

.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 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 + ( 8 x block size )

       Decompression: 100k + ( 4 x block size ), or

                      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 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

.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 "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.

.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 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.

.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 1.0.6 of

.I 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.

.I 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.

.SH AUTHOR

Julian Seward, jsewardbzip.org.

http://www.bzip.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.  

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.