bzip2.1

ncbi源码

.PU
.TH bzip2 1
.SH NAME
bzip2, bunzip2 \- a block-sorting file compressor, v1.0.2
.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.