Annotation of src/usr.bin/compress/compress.1, Revision 1.1
1.1 ! deraadt 1: .\" $NetBSD: compress.1,v 1.5 1995/03/26 09:44:34 glass Exp $
! 2: .\"
! 3: .\" Copyright (c) 1986, 1990, 1993
! 4: .\" The Regents of the University of California. All rights reserved.
! 5: .\"
! 6: .\" This code is derived from software contributed to Berkeley by
! 7: .\" James A. Woods, derived from original work by Spencer Thomas
! 8: .\" and Joseph Orost.
! 9: .\"
! 10: .\" Redistribution and use in source and binary forms, with or without
! 11: .\" modification, are permitted provided that the following conditions
! 12: .\" are met:
! 13: .\" 1. Redistributions of source code must retain the above copyright
! 14: .\" notice, this list of conditions and the following disclaimer.
! 15: .\" 2. Redistributions in binary form must reproduce the above copyright
! 16: .\" notice, this list of conditions and the following disclaimer in the
! 17: .\" documentation and/or other materials provided with the distribution.
! 18: .\" 3. All advertising materials mentioning features or use of this software
! 19: .\" must display the following acknowledgement:
! 20: .\" This product includes software developed by the University of
! 21: .\" California, Berkeley and its contributors.
! 22: .\" 4. Neither the name of the University nor the names of its contributors
! 23: .\" may be used to endorse or promote products derived from this software
! 24: .\" without specific prior written permission.
! 25: .\"
! 26: .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
! 27: .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
! 28: .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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! 31: .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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! 33: .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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! 37: .\"
! 38: .\" @(#)compress.1 8.2 (Berkeley) 4/18/94
! 39: .\"
! 40: .Dd April 18, 1994
! 41: .Dt COMPRESS 1
! 42: .Os BSD 4.3
! 43: .Sh NAME
! 44: .Nm compress ,
! 45: .\".Nm uncompress ,
! 46: .Nm uncompress
! 47: .\".Nm zcat
! 48: .Nd compress and expand data
! 49: .Sh SYNOPSIS
! 50: .Nm compress
! 51: .Op Fl cfv
! 52: .Op Fl b Ar bits
! 53: .Op Ar
! 54: .Nm uncompress
! 55: .Op Fl cfv
! 56: .Op Ar
! 57: .\".Nm zcat
! 58: .\".Op Ar
! 59: .Sh DESCRIPTION
! 60: .Nm Compress
! 61: reduces the size of the named files using adaptive Lempel-Ziv coding.
! 62: Each
! 63: .Ar file
! 64: is renamed to the same name plus the extension
! 65: .Dq .Z .
! 66: As many of the modification time, access time, file flags, file mode,
! 67: user ID, and group ID as allowed by permissions are retained in the
! 68: new file.
! 69: If compression would not reduce the size of a
! 70: .Ar file ,
! 71: the file is ignored.
! 72: .Pp
! 73: .Nm Uncompress
! 74: restores the compressed files to their original form, renaming the
! 75: files by deleting the
! 76: .Dq .Z
! 77: extension.
! 78: .\".Pp
! 79: .\".Nm Zcat
! 80: .\"is an alias for
! 81: .\".Dq "uncompress -c" .
! 82: .Pp
! 83: If renaming the files would cause files to be overwritten and the standard
! 84: input device is a terminal, the user is prompted (on the standard error
! 85: output) for confirmation.
! 86: If prompting is not possible or confirmation is not received, the files
! 87: are not overwritten.
! 88: .Pp
! 89: If no files are specified, the standard input is compressed or uncompressed
! 90: to the standard output.
! 91: If either the input and output files are not regular files, the checks for
! 92: reduction in size and file overwriting are not performed, the input file is
! 93: not removed, and the attributes of the input file are not retained.
! 94: .Pp
! 95: The options are as follows:
! 96: .Bl -tag -width Ds
! 97: .It Fl b
! 98: Specify the
! 99: .Ar bits
! 100: code limit (see below).
! 101: .It Fl c
! 102: Compressed or uncompressed output is written to the standard output.
! 103: No files are modified.
! 104: .It Fl f
! 105: Force compression of
! 106: .Ar file ,
! 107: even if it is not actually reduced in size.
! 108: Additionally, files are overwritten without prompting for confirmation.
! 109: .It Fl v
! 110: Print the percentage reduction of each file.
! 111: .El
! 112: .Pp
! 113: .Nm Compress
! 114: uses a modified Lempel-Ziv algorithm.
! 115: Common substrings in the file are first replaced by 9-bit codes 257 and up.
! 116: When code 512 is reached, the algorithm switches to 10-bit codes and
! 117: continues to use more bits until the
! 118: limit specified by the
! 119: .Fl b
! 120: flag is reached (the default is 16).
! 121: .Ar Bits
! 122: must be between 9 and 16.
! 123: .Pp
! 124: After the
! 125: .Ar bits
! 126: limit is reached,
! 127: .Nm compress
! 128: periodically checks the compression ratio.
! 129: If it is increasing,
! 130: .Nm compress
! 131: continues to use the existing code dictionary.
! 132: However, if the compression ratio decreases,
! 133: .Nm compress
! 134: discards the table of substrings and rebuilds it from scratch. This allows
! 135: the algorithm to adapt to the next "block" of the file.
! 136: .Pp
! 137: The
! 138: .Fl b
! 139: flag is omitted for
! 140: .Ar uncompress
! 141: since the
! 142: .Ar bits
! 143: parameter specified during compression
! 144: is encoded within the output, along with
! 145: a magic number to ensure that neither decompression of random data nor
! 146: recompression of compressed data is attempted.
! 147: .Pp
! 148: .ne 8
! 149: The amount of compression obtained depends on the size of the
! 150: input, the number of
! 151: .Ar bits
! 152: per code, and the distribution of common substrings.
! 153: Typically, text such as source code or English is reduced by 50\-60%.
! 154: Compression is generally much better than that achieved by Huffman
! 155: coding (as used in the historical command pack), or adaptive Huffman
! 156: coding (as used in the historical command compact), and takes less
! 157: time to compute.
! 158: .Pp
! 159: The
! 160: .Nm compress
! 161: utility exits 0 on success, and >0 if an error occurs.
! 162: .Sh SEE ALSO
! 163: .Xr zcat 1
! 164: .Rs
! 165: .%A Welch, Terry A.
! 166: .%D June, 1984
! 167: .%T "A Technique for High Performance Data Compression"
! 168: .%J "IEEE Computer"
! 169: .%V 17:6
! 170: .%P pp. 8-19
! 171: .Re
! 172: .Sh HISTORY
! 173: The
! 174: .Nm
! 175: command appeared in
! 176: .Bx 4.3 .
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