258 lines
7.5 KiB
Groff
258 lines
7.5 KiB
Groff
.\" Copyright (c) 1986, 1990, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" James A. Woods, derived from original work by Spencer Thomas
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.\" and Joseph Orost.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)compress.1 8.2 (Berkeley) 4/18/94
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.\" $FreeBSD$
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.\"
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.Dd May 17, 2002
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.Dt COMPRESS 1
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.Os
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.Sh NAME
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.Nm compress ,
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.Nm uncompress
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.Nd compress and expand data
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.Sh SYNOPSIS
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.Nm
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.Op Fl fv
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.Op Fl b Ar bits
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.Op Ar
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.Nm
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.Fl c
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.Op Fl b Ar bits
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.Op Ar file
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.Nm uncompress
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.Op Fl f
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.Op Ar
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.Nm uncompress
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.Fl c
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.Op Ar file
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.Sh DESCRIPTION
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The
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.Nm
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utility reduces the size of files using adaptive Lempel-Ziv coding.
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Each
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.Ar file
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is renamed to the same name plus the extension
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.Pa .Z .
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A
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.Ar file
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argument with a
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.Pa .Z
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extension will be ignored except it will cause an
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error exit after other arguments are processed.
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If compression would not reduce the size of a
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.Ar file ,
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the file is ignored.
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.Pp
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The
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.Nm uncompress
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utility restores compressed files to their original form, renaming the
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files by deleting the
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.Pa .Z
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extensions.
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A file specification need not include the file's
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.Pa .Z
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extension.
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If a file's name in its file system does not have a
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.Pa .Z
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extension, it will not be uncompressed and it will cause
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an error exit after other arguments are processed.
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.Pp
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If renaming the files would cause files to be overwritten and the standard
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input device is a terminal, the user is prompted (on the standard error
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output) for confirmation.
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If prompting is not possible or confirmation is not received, the files
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are not overwritten.
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.Pp
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As many of the modification time, access time, file flags, file mode,
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user ID, and group ID as allowed by permissions are retained in the
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new file.
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.Pp
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If no files are specified or a
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.Ar file
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argument is a single dash
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.Pq Sq Fl ,
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the standard input is compressed or uncompressed to the standard output.
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If either the input and output files are not regular files, the checks for
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reduction in size and file overwriting are not performed, the input file is
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not removed, and the attributes of the input file are not retained
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in the output file.
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.Pp
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The options are as follows:
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.Bl -tag -width ".Fl b Ar bits"
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.It Fl b Ar bits
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The code size (see below) is limited to
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.Ar bits ,
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which must be in the range 9..16.
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The default is 16.
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.It Fl c
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Compressed or uncompressed output is written to the standard output.
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No files are modified.
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The
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.Fl v
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option is ignored.
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Compression is attempted even if the results will be larger than the
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original.
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.It Fl f
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Files are overwritten without prompting for confirmation.
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Also, for
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.Nm compress ,
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files are compressed even if they are not actually reduced in size.
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.It Fl v
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Print the percentage reduction of each file.
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Ignored by
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.Nm uncompress
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or if the
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.Fl c
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option is also used.
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.El
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.Pp
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The
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.Nm
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utility uses a modified Lempel-Ziv algorithm.
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Common substrings in the file are first replaced by 9-bit codes 257 and up.
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When code 512 is reached, the algorithm switches to 10-bit codes and
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continues to use more bits until the
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limit specified by the
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.Fl b
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option or its default is reached.
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.Pp
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After the limit is reached,
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.Nm
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periodically checks the compression ratio.
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If it is increasing,
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.Nm
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continues to use the existing code dictionary.
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However, if the compression ratio decreases,
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.Nm
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discards the table of substrings and rebuilds it from scratch.
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This allows
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the algorithm to adapt to the next "block" of the file.
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.Pp
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The
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.Fl b
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option is unavailable for
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.Nm uncompress
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since the
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.Ar bits
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parameter specified during compression
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is encoded within the output, along with
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a magic number to ensure that neither decompression of random data nor
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recompression of compressed data is attempted.
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.Pp
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The amount of compression obtained depends on the size of the
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input, the number of
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.Ar bits
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per code, and the distribution of common substrings.
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Typically, text such as source code or English is reduced by 50\-60%.
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Compression is generally much better than that achieved by Huffman
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coding (as used in the historical command pack), or adaptive Huffman
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coding (as used in the historical command compact), and takes less
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time to compute.
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.Sh EXIT STATUS
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.Ex -std compress uncompress
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.Pp
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The
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.Nm compress
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utility exits 2 if attempting to compress a file would not reduce its size
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and the
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.Fl f
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option was not specified and if no other error occurs.
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.Sh SEE ALSO
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.Xr gunzip 1 ,
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.Xr gzexe 1 ,
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.Xr gzip 1 ,
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.Xr zcat 1 ,
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.Xr zmore 1 ,
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.Xr znew 1
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.Rs
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.%A Welch, Terry A.
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.%D June, 1984
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.%T "A Technique for High Performance Data Compression"
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.%J "IEEE Computer"
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.%V 17:6
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.%P pp. 8-19
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.Re
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.Sh STANDARDS
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The
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.Nm compress
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and
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.Nm uncompress
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utilities conform to
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.St -p1003.1-2001 .
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.Sh HISTORY
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The
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.Nm
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command appeared in
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.Bx 4.3 .
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.Sh BUGS
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Some of these might be considered otherwise-undocumented features.
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.Pp
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.Nm compress :
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If the utility does not compress a file because doing so would not
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reduce its size, and a file of the same name except with an
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.Pa .Z
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extension exists, the named file is not really ignored as stated above;
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it causes a prompt to confirm the overwriting of the file with the extension.
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If the operation is confirmed, that file is deleted.
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.Pp
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.Nm uncompress :
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If an empty file is compressed (using
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.Fl f ) ,
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the resulting
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.Pa .Z
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file is also empty.
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That seems right, but if
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.Nm uncompress
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is then used on that file, an error will occur.
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.Pp
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Both utilities: If a
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.Sq Fl
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argument is used and the utility prompts the user, the standard input
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is taken as the user's reply to the prompt.
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.Pp
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Both utilities:
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If the specified file does not exist, but a similarly-named one with (for
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.Nm compress )
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or without (for
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.Nm uncompress )
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a
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.Pa .Z
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extension does exist, the utility will waste the user's time by not
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immediately emitting an error message about the missing file and
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continuing.
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Instead, it first asks for confirmation to overwrite
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the existing file and then does not overwrite it.
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