bzip2(1) bzip2(1)

NNAAMMEE

bzip2, bunzip2 − a block‐sorting file compressor, v1.0.8

bzcat − decompresses files to stdout

bzip2recover − recovers data from damaged bzip2 files

SSYYNNOOPPSSIISS

bbzziipp22 [ −−ccddffkkqqssttvvzzVVLL112233445566778899 ] [ _f_i_l_e_n_a_m_e_s _._._. ]

bbuunnzziipp22 [ −−ffkkvvssVVLL ] [ _f_i_l_e_n_a_m_e_s _._._. ]

bbzzccaatt [ −−ss ] [ _f_i_l_e_n_a_m_e_s _._._. ]

bbzziipp22rreeccoovveerr _f_i_l_e_n_a_m_e

DDEESSCCRRIIPPTTIIOONN

_b_z_i_p_2 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 sta­

tistical compressors.

The command‐line options are deliberately very similar to

those of _G_N_U _g_z_i_p_, but they are not identical.

_b_z_i_p_2 expects a list of file names to accompany the com­

mand‐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, per­

missions, and, when possible, ownership as the correspond­

ing 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 preserv­

ing original file names, permissions, ownerships or dates

in filesystems which lack these concepts, or have serious

file name length restrictions, such as MS‐DOS.

_b_z_i_p_2 and _b_u_n_z_i_p_2 will by default not overwrite existing

files. If you want this to happen, specify the −f flag.

If no file names are specified, _b_z_i_p_2 compresses from

standard input to standard output. In this case, _b_z_i_p_2

will decline to write compressed output to a terminal, as

this would be entirely incomprehensible and therefore

pointless.

_b_u_n_z_i_p_2 (or _b_z_i_p_2 _−_d_) decompresses all specified files.

Files which were not created by _b_z_i_p_2 will be detected and

ignored, and a warning issued. _b_z_i_p_2 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,

_._b_z_2_, _._b_z_, _._t_b_z_2 or _._t_b_z_, _b_z_i_p_2 complains that it cannot

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

name with _._o_u_t appended.

As with compression, supplying no filenames causes decom­

pression from standard input to standard output.

_b_u_n_z_i_p_2 will correctly decompress a file which is the con­

catenation 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 com­

pressed and decompressed like this. The resulting outputs

are fed sequentially to stdout. Compression of multiple

files in this manner generates a stream containing multi­

ple compressed file representations. Such a stream can be

decompressed correctly only by _b_z_i_p_2 version 0.9.0 or

later. Earlier versions of _b_z_i_p_2 will stop after decom­

pressing the first file in the stream.

_b_z_c_a_t (or _b_z_i_p_2 _‐_d_c_) decompresses all specified files to

the standard output.

_b_z_i_p_2 will read arguments from the environment variables

_B_Z_I_P_2 and _B_Z_I_P_, 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, _b_z_i_p_2 uses 32‐bit

CRCs to make sure that the decompressed version of a file

is identical to the original. This guards against corrup­

tion of the compressed data, and against undetected bugs

in _b_z_i_p_2 (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

_b_z_i_p_2_r_e_c_o_v_e_r 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 _b_z_i_p_2 to panic.

OOPPTTIIOONNSS

−−cc ‐‐‐‐ssttddoouutt

Compress or decompress to standard output.

−−dd ‐‐‐‐ddeeccoommpprreessss

Force decompression. _b_z_i_p_2_, _b_u_n_z_i_p_2 and _b_z_c_a_t 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 _b_z_i_p_2 to decompress.

−−zz ‐‐‐‐ccoommpprreessss

The complement to −d: forces compression,

regardless of the invocation name.

−−tt ‐‐‐‐tteesstt

Check integrity of the specified file(s), but don’t

decompress them. This really performs a trial

decompression and throws away the result.

−−ff ‐‐‐‐ffoorrccee

Force overwrite of output files. Normally, _b_z_i_p_2

will not overwrite existing output files. Also

forces _b_z_i_p_2 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.

−−kk ‐‐‐‐kkeeeepp

Keep (don’t delete) input files during compression

or decompression.

−−ss ‐‐‐‐ssmmaallll

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.

−−qq ‐‐‐‐qquuiieett

Suppress non‐essential warning messages. Messages

pertaining to I/O errors and other critical events

will not be suppressed.

−−vv ‐‐‐‐vveerrbboossee

Verbose mode ‐‐ show the compression ratio for each

file processed. Further −v’s increase the ver­

bosity level, spewing out lots of information which

is primarily of interest for diagnostic purposes.

−−LL ‐‐‐‐lliicceennssee ‐‐VV ‐‐‐‐vveerrssiioonn

Display the software version, license terms and

conditions.

−−11 ((oorr −−−−ffaasstt)) ttoo −−99 ((oorr −−−−bbeesstt))

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 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 han­

dle files with names beginning with a dash, for

example: bzip2 −‐ −myfilename.

−−‐‐rreeppeettiittiivvee‐‐ffaasstt ‐‐‐‐rreeppeettiittiivvee‐‐bbeesstt

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 ver­

sions, which was sometimes useful. 0.9.5 and above

have an improved algorithm which renders these

flags irrelevant.

MMEEMMOORRYY MMAANNAAGGEEMMEENNTT

_b_z_i_p_2 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) respec­

tively. At decompression time, the block size used for

compression is read from the header of the compressed

file, and _b_u_n_z_i_p_2 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 _b_z_i_p_2 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,

_b_u_n_z_i_p_2 will require about 3700 kbytes to decompress. To

support decompression of any file on a 4 megabyte machine,

_b_u_n_z_i_p_2 has an option to decompress using approximately

half this amount of memory, about 2300 kbytes. Decompres­

sion 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 con­

straints allow, since that maximises the compression

achieved. Compression and decompression speed are virtu­

ally 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 Compres­

sion 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 domi­

nated 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

RREECCOOVVEERRIINNGG DDAATTAA FFRROOMM DDAAMMAAGGEEDD FFIILLEESS

_b_z_i_p_2 compresses files in blocks, usually 900kbytes long.

Each block is handled independently. If a media or trans­

mission 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.

_b_z_i_p_2_r_e_c_o_v_e_r 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 _b_z_i_p_2 −t to test

the integrity of the resulting files, and decompress those

which are undamaged.

_b_z_i_p_2_r_e_c_o_v_e_r takes a single argument, the name of the dam­

aged 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 pro­

cessing ‐‐ for example, "bzip2 ‐dc rec*file.bz2 > recov­

ered_data" ‐‐ processes the files in the correct order.

_b_z_i_p_2_r_e_c_o_v_e_r 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 min­

imise any potential data loss through media or transmis­

sion errors, you might consider compressing with a smaller

block size.

PPEERRFFOORRMMAANNCCEE NNOOTTEESS

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 mon­

itor progress in great detail, if you want.

Decompression speed is unaffected by these phenomena.

_b_z_i_p_2 usually allocates several megabytes of memory to

operate in, and then charges all over it in a fairly ran­

dom fashion. This means that performance, both for com­

pressing 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 _b_z_i_p_2 will per­

form best on machines with very large caches.

CCAAVVEEAATTSS

I/O error messages are not as helpful as they could be.

_b_z_i_p_2 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 1.0.8 of _b_z_i_p_2_. Com­

pressed 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.

_b_z_i_p_2_r_e_c_o_v_e_r 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.

AAUUTTHHOORR

Julian Seward, jseward@acm.org.

https://sourceware.org/bzip2/

The ideas embodied in _b_z_i_p_2 are due to (at least) the fol­

lowing people: Michael Burrows and David Wheeler (for the

block sorting transformation), David Wheeler (again, for

the Huffman coder), Peter Fenwick (for the structured cod­

ing model in the original _b_z_i_p_, and many refinements), and

Alistair Moffat, Radford Neal and Ian Witten (for the

arithmetic coder in the original _b_z_i_p_)_. I am much

indebted for their help, support and advice. See the man­

ual 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 compres­

sion. Bela Lubkin encouraged me to improve the worst‐case

compression performance. Donna Robinson XMLised the docu­

mentation. 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)