Linux / Unix Command: bzip2

Command Library

NAME

bzip2, bunzip2 - a block-sorting file compressor, v1.0.2
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files

EXAMPLES

SYNOPSIS

bzip2[ -cdfkqstvzVL123456789 ][ filenames ...]
bunzip2[ -fkvsVL ][ filenames ...]
bzcat[ -s ][ filenames ...]
bzip2recover filename

 

DESCRIPTION

bzip2compresses files using the Burrows-Wheeler block sortingtext compression algorithm, and Huffman coding. Compression isgenerally considerably better than that achieved by more conventionalLZ77/LZ78-based compressors, and approaches the performance of the PPMfamily of statistical compressors.

The command-line options are deliberately very similar to those of GNU gzip, but they are not identical.

bzip2expects a list of file names to accompany thecommand-line flags. Each file is replaced by a compressed version ofitself, with the name "original_name.bz2". Each compressed filehas the same modification date, permissions, and, when possible,ownership as the corresponding original, so that these properties canbe correctly restored at decompression time. File name handling isnaive in the sense that there is no mechanism for preserving originalfile names, permissions, ownerships or dates in filesystems which lackthese concepts, or have serious file name length restrictions, such asMS-DOS.

bzip2andbunzip2will by default not overwrite existingfiles. If you want this to happen, specify the -f flag.

If no file names are specified,bzip2compresses from standardinput to standard output. In this case,bzip2will decline towrite compressed output to a terminal, as this would be entirelyincomprehensible and therefore pointless.

bunzip2(orbzip2 -d) decompresses allspecified files. Files which were not created by bzip2will be detected and ignored, and a warning issued. bzip2attempts 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, .bz2, .bz, .tbz2or.tbz, bzip2 complains that it cannotguess the name of the original file, and uses the original namewith.outappended.

As with compression, supplying nofilenames causes decompression from standard input to standard output.

bunzip2 will correctly decompress a file which is theconcatenation of two or more compressed files. The result is theconcatenation of the corresponding uncompressed files. Integritytesting (-t) of concatenated compressed files is also supported.

You can also compress or decompress files to the standard output bygiving the -c flag. Multiple files may be compressed anddecompressed like this. The resulting outputs are fed sequentially tostdout. Compression of multiple files in this manner generates a streamcontaining multiple compressed file representations. Such a streamcan be decompressed correctly only bybzip2 version 0.9.0 orlater. Earlier versions ofbzip2will stop after decompressingthe first file in the stream.

bzcat(orbzip2 -dc) decompresses all specified files tothe standard output.

bzip2will read arguments from the environment variablesBZIP2andBZIP,in that order, and will process thembefore 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 slightlylarger than the original. Files of less than about one hundred bytestend to get larger, since the compression mechanism has a constantoverhead in the region of 50 bytes. Random data (including the outputof most file compressors) is coded at about 8.05 bits per byte, givingan expansion of around 0.5%.

As a self-check for your protection, bzip2uses 32-bit CRCs tomake sure that the decompressed version of a file is identical to theoriginal. This guards against corruption of the compressed data, andagainst undetected bugs inbzip2(hopefully very unlikely). Thechances of data corruption going undetected is microscopic, about onechance in four billion for each file processed. Be aware, though, thatthe check occurs upon decompression, so it can only tell you thatsomething is wrong. It can't help you recover the original uncompresseddata. You can use bzip2recoverto try to recover data fromdamaged files.

Return values: 0 for a normal exit, 1 for environmental problems (filenot found, invalid flags, I/O errors, &c), 2 to indicate a corruptcompressed file, 3 for an internal consistency error (eg, bug) whichcausedbzip2to panic.

 

OPTIONS

-c --stdout

Compress or decompress to standard output.

-d --decompress

Force decompression. bzip2, bunzip2 and bzcat arereally the same program, and the decision about what actions to take isdone on the basis of which name is used. This flag overrides thatmechanism, and forces bzip2to decompress.

-z --compress

The complement to -d: forces compression, regardless of theinvocation 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 overwriteexisting output files. Also forces bzip2 to break hard linksto files, which it otherwise wouldn't do.

bzip2 normally declines to decompress files which don't have thecorrect magic header bytes. If forced (-f), however, it will passsuch files through unmodified. This is how GNU gzip behaves.

-k --keep

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

-s --small

Reduce memory usage, for compression, decompression and testing. Filesare decompressed and tested using a modified algorithm which onlyrequires 2.5 bytes per block byte. This means any file can bedecompressed in 2300k of memory, albeit at about half the normal speed.

During compression, -s selects a block size of 200k, which limitsmemory use to around the same figure, at the expense of your compressionratio. In short, if your machine is low on memory (8 megabytes orless), use -s for everything. See MEMORY MANAGEMENT below.

-q --quiet

Suppress non-essential warning messages. Messages pertaining toI/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 ofinformation which is primarily of interest for diagnostic purposes.

-L --license -V --version

Display the software version, license terms and conditions.

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

Set the block size to 100 k, 200 k .. 900 k when compressing. Has noeffect when decompressing. See MEMORY MANAGEMENT below.The --fast and --best aliases are primarily for GNU gzip compatibility. In particular, --fast doesn't make thingssignificantly faster. And --best merely selects the default behaviour.

--

Treats all subsequent arguments as file names, even if they startwith a dash. This is so you can handle files with names beginningwith a dash, for example: bzip2 -- -myfilename.

--repetitive-fast --repetitive-best

These flags are redundant in versions 0.9.5 and above. They providedsome coarse control over the behaviour of the sorting algorithm inearlier versions, which was sometimes useful. 0.9.5 and above have animproved algorithm which renders these flags irrelevant.

 

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