bzip2 (1) - Linux Manuals
bzip2: a block-sorting file compressor, v1.0.8
NAME
bzip2, bunzip2 - a block-sorting file compressor, v1.0.8
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files
SYNOPSIS
bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]bzip2 [ -h|--help ]
bunzip2 [ -fkvsVL ] [ filenames ... ]
bunzip2 [ -h|--help ]
bzcat [ -s ] [ filenames ... ]
bzcat [ -h|--help ]
bzip2recover filename
DESCRIPTION
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 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 standard 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:
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
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
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
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,
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 that
something is wrong. It can't help you
recover the original 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.
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.
During compression, -s selects a block size of 200 k, 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.
Compression and decompression requirements,
in bytes, can be estimated as:
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 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 900 k block size,
bunzip2
will require about 3700 kbytes to decompress. To support decompression
of any file on a 4 megabyte machine,
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 7600 k of memory, but only touch 400 k + 20000 * 8 = 560
kbytes of it. Similarly, the decompressor will allocate 3700 k but only
touch 100 k + 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.
OPTIONS
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 follows
that the flags -1 to -9 are irrelevant to and so ignored
during decompression.