Annotation of src/usr.bin/compress/compress.1, Revision 1.5
1.5 ! denny 1: .\" $OpenBSD: compress.1,v 1.4 1997/07/06 20:22:53 mickey Exp $
1.1 deraadt 2: .\" $NetBSD: compress.1,v 1.5 1995/03/26 09:44:34 glass Exp $
3: .\"
4: .\" Copyright (c) 1986, 1990, 1993
5: .\" The Regents of the University of California. All rights reserved.
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7: .\" This code is derived from software contributed to Berkeley by
8: .\" James A. Woods, derived from original work by Spencer Thomas
9: .\" and Joseph Orost.
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12: .\" modification, are permitted provided that the following conditions
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39: .\" @(#)compress.1 8.2 (Berkeley) 4/18/94
40: .\"
41: .Dd April 18, 1994
42: .Dt COMPRESS 1
43: .Os BSD 4.3
44: .Sh NAME
45: .Nm compress ,
46: .\".Nm uncompress ,
47: .Nm uncompress
48: .\".Nm zcat
49: .Nd compress and expand data
50: .Sh SYNOPSIS
51: .Nm compress
1.4 mickey 52: .Op Fl cftgOqv
1.1 deraadt 53: .Op Fl b Ar bits
1.4 mickey 54: .Op Fl o Ar filename
1.1 deraadt 55: .Op Ar
56: .Nm uncompress
1.4 mickey 57: .Op Fl cftoqv
58: .Op Fl o Ar filename
1.1 deraadt 59: .Op Ar
60: .\".Nm zcat
61: .\".Op Ar
62: .Sh DESCRIPTION
63: .Nm Compress
64: reduces the size of the named files using adaptive Lempel-Ziv coding.
65: Each
66: .Ar file
67: is renamed to the same name plus the extension
68: .Dq .Z .
69: As many of the modification time, access time, file flags, file mode,
70: user ID, and group ID as allowed by permissions are retained in the
71: new file.
72: If compression would not reduce the size of a
73: .Ar file ,
74: the file is ignored.
75: .Pp
76: .Nm Uncompress
77: restores the compressed files to their original form, renaming the
78: files by deleting the
79: .Dq .Z
80: extension.
81: .\".Pp
82: .\".Nm Zcat
83: .\"is an alias for
84: .\".Dq "uncompress -c" .
85: .Pp
86: If renaming the files would cause files to be overwritten and the standard
87: input device is a terminal, the user is prompted (on the standard error
88: output) for confirmation.
89: If prompting is not possible or confirmation is not received, the files
90: are not overwritten.
91: .Pp
92: If no files are specified, the standard input is compressed or uncompressed
93: to the standard output.
94: If either the input and output files are not regular files, the checks for
95: reduction in size and file overwriting are not performed, the input file is
96: not removed, and the attributes of the input file are not retained.
97: .Pp
98: The options are as follows:
99: .Bl -tag -width Ds
100: .It Fl b
101: Specify the
102: .Ar bits
103: code limit (see below).
104: .It Fl c
105: Compressed or uncompressed output is written to the standard output.
106: No files are modified.
1.4 mickey 107: .It Fl d
108: Decompress the source files instead of compressing them.
1.1 deraadt 109: .It Fl f
110: Force compression of
111: .Ar file ,
112: even if it is not actually reduced in size.
113: Additionally, files are overwritten without prompting for confirmation.
1.4 mickey 114: .It Fl g
115: Use deflate scheme wich reported to provide better compression rates.
116: .It Fl O
117: Use old compression method (default is based on the program name).
118: .It Fl o
119: Set the output file name.
120: .It Fl t
121: Test the entefrity of each file leaving any files intact.
122: .It Fl q
123: Be quite, suppress any messages.
1.1 deraadt 124: .It Fl v
125: Print the percentage reduction of each file.
126: .El
127: .Pp
128: .Nm Compress
129: uses a modified Lempel-Ziv algorithm.
130: Common substrings in the file are first replaced by 9-bit codes 257 and up.
131: When code 512 is reached, the algorithm switches to 10-bit codes and
132: continues to use more bits until the
133: limit specified by the
134: .Fl b
135: flag is reached (the default is 16).
136: .Ar Bits
137: must be between 9 and 16.
138: .Pp
139: After the
140: .Ar bits
141: limit is reached,
142: .Nm compress
143: periodically checks the compression ratio.
144: If it is increasing,
145: .Nm compress
146: continues to use the existing code dictionary.
147: However, if the compression ratio decreases,
148: .Nm compress
149: discards the table of substrings and rebuilds it from scratch. This allows
150: the algorithm to adapt to the next "block" of the file.
151: .Pp
152: The
153: .Fl b
154: flag is omitted for
1.3 deraadt 155: .Nm uncompress
1.1 deraadt 156: since the
157: .Ar bits
158: parameter specified during compression
159: is encoded within the output, along with
160: a magic number to ensure that neither decompression of random data nor
161: recompression of compressed data is attempted.
162: .Pp
163: .ne 8
164: The amount of compression obtained depends on the size of the
165: input, the number of
166: .Ar bits
167: per code, and the distribution of common substrings.
168: Typically, text such as source code or English is reduced by 50\-60%.
169: Compression is generally much better than that achieved by Huffman
170: coding (as used in the historical command pack), or adaptive Huffman
171: coding (as used in the historical command compact), and takes less
172: time to compute.
173: .Pp
174: The
175: .Nm compress
1.5 ! denny 176: utility exits 0 on success, 1 if an error occurs, and 2 if one or
! 177: more files were not compressed because they would have grown in
! 178: size (and -f was not specifed).
1.1 deraadt 179: .Sh SEE ALSO
180: .Rs
181: .%A Welch, Terry A.
182: .%D June, 1984
183: .%T "A Technique for High Performance Data Compression"
184: .%J "IEEE Computer"
185: .%V 17:6
186: .%P pp. 8-19
187: .Re
1.5 ! denny 188: .Sh STANDARDS
! 189: The
! 190: .Nm compress
! 191: utility is compliant with the
! 192: .St -p1003.2-92
! 193: specification.
1.1 deraadt 194: .Sh HISTORY
195: The
196: .Nm
197: command appeared in
198: .Bx 4.3 .
1.4 mickey 199: The deflate compression support added in
200: .Ox 2.1 .
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