Windows PowerShell command on Get-command bzip2

Manual Pages for UNIX Operating System command usage for man bzip2

User Commands bzip2(1)

NAME

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

bzcat - decompresses files to stdout

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 sort-

ing 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 statist-

ical 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 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.

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 stan-

dard 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 SunOS 5.10 Last change: 1

User Commands bzip2(1)

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. As with compression, supplying no filenames causes decompression from standard input to standard output.

bunzip2 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

compressed and decompressed like this. The resulting out-

puts are fed sequentially to stdout. Compression of multi-

ple files in this manner generates a stream containing mul-

tiple compressed file representations. Such a stream can be

decompressed correctly only by bzip2 version 0.9.0 or later.

Earlier versions 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 con-

venient 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, 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 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 SunOS 5.10 Last change: 2

User Commands bzip2(1)

that something is wrong. It can't help you recover the ori-

ginal 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 compressed 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 them. This really performs a trial decompression and throws away the result.

-f --force

Force overwrite of output files. Normally, bzip2 will

not overwrite existing output files. Also forces 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.

-k --keep

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

-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. SunOS 5.10 Last change: 3

User Commands bzip2(1)

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.

-q --quiet

Suppress non-essential warning messages. Messages per-

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

-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 pri-

marily of interest for diagnostic purposes.

-L --license -V --version

Display the software version, license terms and condi-

tions.

-1 (or --fast) to -9 (or

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 particu-

lar, --fast doesn't make things significantly 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 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 bunzip2 then allocates itself just enough memory to decompress the file. Since block sizes are stored in compressed files, it SunOS 5.10 Last change: 4

User Commands bzip2(1)

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 bzip2 on small machines. It is also impor-

tant 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, bun-

zip2 will require about 3700 kbytes to decompress. To sup-

port decompression of any file on a 4 megabyte machine, bun-

zip2 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 con-

straints 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 fig-

ures tend to understate the advantage of larger block sizes for larger files, since the Corpus is dominated by smaller files. Compress Decompress Decompress Corpus SunOS 5.10 Last change: 5

User Commands bzip2(1)

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 data from the undam-

aged 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 car-

ries its own 32-bit CRC, so damaged blocks can be dis-

tinguished from undamaged ones.

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 bzip2 -t to test the

integrity of the resulting files, and decompress those which are undamaged.

bzip2recover 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 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 damaged single-block files, since a

damaged block cannot be recovered. If you wish to minim-

ise 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 SunOS 5.10 Last change: 6

User Commands bzip2(1)

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 pro-

gress 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 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 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 1.0.6 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 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.

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.

AUTHOR Julian Seward, jsewardbzip.org. SunOS 5.10 Last change: 7

User Commands bzip2(1)

http://www.bzip.org

The ideas embodied in bzip2 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 coding

model in the original bzip, and many refinements), and Alis-

tair 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 dis-

tribution for pointers to sources of documentation. Chris-

tian 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 perfor-

mance. 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.

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes:

_______________________________________

| ATTRIBUTE TYPE | ATTRIBUTE VALUE|

|____________________|__________________|_

| Availability | compress/bzip2 |

|____________________|__________________|_

| Interface Stability| Committed |

|____________________|_________________|

NOTES

Source for bzip2 is available on http://opensolaris.org.

SunOS 5.10 Last change: 8