244 lines
7.1 KiB
Groff
244 lines
7.1 KiB
Groff
.\" Copyright (c) 1986, 1990 The Regents of the University of California.
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.\" 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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.\" from: @(#)compress.1 6.10 (Berkeley) 7/30/91
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.\" $Id: compress.1,v 1.2 1993/08/01 07:33:30 mycroft Exp $
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.\"
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.Dd July 30, 1991
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.Dt COMPRESS 1
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.Os BSD 4.3
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.Sh NAME
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.Nm compress ,
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.Nm uncompress ,
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.Nm zcat
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.Nd compress and expand data
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.Sh SYNOPSIS
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.Nm compress
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.Op Fl f
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.Op Fl v
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.Op Fl c
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.Op Fl b Ar bits
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.Op Ar
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.Nm uncompress
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.Op Fl f
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.Op Fl v
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.Op Fl c
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.Ar
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.Nm zcat
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.Op Ar
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.Sh DESCRIPTION
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.Nm Compress
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reduces the size of the named files using adaptive Lempel-Ziv coding.
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Whenever possible,
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each
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.Ar file
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is replaced by one with the extension
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.Ar \&.Z ,
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while keeping the same ownership modes, access and modification times.
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If no files are specified, the standard input is compressed to the
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standard output.
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Compressed files can be restored to their original form using
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.Nm uncompress
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or
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.Nm zcat
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.Bl -tag -width Ds
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.It Fl f
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Force compression of
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.Ar file ,
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even if it does not actually shrink
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or the corresponding
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.Ar file.Z
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file already exists.
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Except when run in the background under
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.Pa /bin/sh ,
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if
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.Fl f
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is not given the user is prompted as to whether an existing
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.Ar file.Z
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file should be overwritten.
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.It Fl c
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(``cat'').
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.Nm Compress/uncompress
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writes to the standard output; no files are changed.
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The nondestructive behavior of
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.Nm zcat
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is identical to that of
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.Nm uncompress
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.Fl c .
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.It Fl b
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Specify
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.Ar bits
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code limit (see below).
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.It Fl v
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Print the percentage reduction of each file.
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.El
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.Pp
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.Nm Compress
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uses the modified Lempel-Ziv algorithm popularized in
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"A Technique for High Performance Data Compression",
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Terry A. Welch,
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.Em IEEE Computer ,
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vol. 17,
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.no 6
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(June 1984), pp. 8-19.
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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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flag is reached (default 16).
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.Ar Bits
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must be between 9 and 16. The default can be changed in the source to allow
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.Nm compress
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to be run on a smaller machine.
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.Pp
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After the
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.Ar bits
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limit is attained,
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.Nm compress
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periodically checks the compression ratio. If it is increasing,
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.Nm compress
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continues to use the existing code dictionary. However,
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if the compression ratio decreases,
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.Nm compress
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discards the table of substrings and rebuilds it from scratch. 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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Note that the
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.Fl b
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flag is omitted for
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.Ar 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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.ne 8
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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
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is reduced by 50\-60%.
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Compression is generally much better than that achieved by
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Huffman coding (as used in the historical command
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pack),
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or adaptive Huffman coding (as
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used in the historical command
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compact),
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and takes less time to compute.
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.Pp
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If an error occurs, exit status is 1;
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if the last file was not compressed because it became larger, the status
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is 2; otherwise the status is 0.
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.Sh DIAGNOSTICS
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.Bl -tag -width flag
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.It "Usage: compress [-fvc] [-b maxbits] [file ...]"
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Invalid options were specified on the command line.
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.It "Missing maxbits"
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Maxbits must follow
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.Fl b .
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.It Ar file : No "not in compressed format"
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The file specified to
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.Ar uncompress
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has not been compressed.
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.It Xo
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.Ar file : No "compressed with"
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.Ar \&xx No "bits, can only handle"
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.Ar \&yy No bits
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.Xc
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.Ar File
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was compressed by a program that could deal with
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more
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.Ar bits
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than the compress code on this machine.
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Recompress the file with smaller
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.Ar bits .
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.It Ar file : No "already has .Z suffix -- no change"
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The file is assumed to be already compressed.
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Rename the file and try again.
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.It Ar file : No "filename too long to tack on .Z"
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The file cannot be compressed because its name is longer than
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12 characters.
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Rename and try again.
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This message does not occur on
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.Bx
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systems.
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.It Ar file No "already exists; do you wish to overwrite (y or n)?"
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Respond "y" if you want the output file to be replaced; "n" if not.
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.It "uncompress: corrupt input"
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A
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.Dv SIGSEGV
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violation was detected which usually means that the input file is
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corrupted.
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.It Compression: Em "xx.xx%"
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Percentage of the input saved by compression.
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(Relevant only for
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.Fl v . )
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.It "-- not a regular file: unchanged"
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When the input file is not a regular file,
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(e.g. a directory), it is
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left unaltered.
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.It "-- has" Ar xx No "other links: unchanged"
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The input file has links; it is left unchanged. See
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.Xr ln 1
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for more information.
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.It "-- file unchanged"
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No savings is achieved by
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compression. The input remains virgin.
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.El
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.Sh FILES
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.Bl -tag -width file.Z
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.It Pa file.Z
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compressed file is file.Z
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.El
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.Sh BUGS
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Although compressed files are compatible between machines with large memory,
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.Fl b Ns Ar 12
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should be used for file transfer to architectures with
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a small process data space (64KB or less, as exhibited by the
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.Tn DEC PDP
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series, the Intel 80286, etc.)
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.Pp
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.Nm Compress
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should be more flexible about the existence of the `.Z' suffix.
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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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