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1 .\" $NetBSD: compress.1,v 1.13 2003/01/23 08:53:46 wiz Exp $
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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.
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34 .\" @(#)compress.1 8.2 (Berkeley) 4/18/94
35 .\"
36 .Dd January 23, 2003
37 .Dt COMPRESS 1
38 .Os
39 .Sh NAME
40 .Nm compress ,
41 .\".Nm uncompress ,
42 .Nm uncompress
43 .\".Nm zcat
44 .Nd compress and expand data
45 .Sh SYNOPSIS
46 .Nm
47 .Op Fl cdfv
48 .Op Fl b Ar bits
49 .Op Ar
50 .Nm uncompress
51 .Op Fl cdfv
52 .Op Ar
53 .\".Nm zcat
54 .\".Op Ar
55 .Sh DESCRIPTION
56 .Nm
57 reduces the size of the named files using adaptive Lempel-Ziv coding.
58 Each
59 .Ar file
60 is renamed to the same name plus the extension
61 .Dq .Z .
62 As many of the modification time, access time, file flags, file mode,
63 user ID, and group ID as allowed by permissions are retained in the
64 new file.
65 If compression would not reduce the size of a
66 .Ar file ,
67 the file is ignored.
68 .Pp
69 .Nm uncompress
70 restores the compressed files to their original form, renaming the
71 files by deleting the
72 .Dq .Z
73 extension.
74 .\".Pp
75 .\".Nm Zcat
76 .\"is an alias for
77 .\".Dq "uncompress -c" .
78 .Pp
79 If renaming the files would cause files to be overwritten and the standard
80 input device is a terminal, the user is prompted (on the standard error
81 output) for confirmation.
82 If prompting is not possible or confirmation is not received, the files
83 are not overwritten.
84 .Pp
85 If no files are specified, the standard input is compressed or uncompressed
86 to the standard output.
87 If either the input and output files are not regular files, the checks for
88 reduction in size and file overwriting are not performed, the input file is
89 not removed, and the attributes of the input file are not retained.
90 .Pp
91 The options are as follows:
92 .Bl -tag -width Ds
93 .It Fl b
94 Specify the
95 .Ar bits
96 code limit (see below).
97 .It Fl c
98 Compressed or uncompressed output is written to the standard output.
99 No files are modified.
100 .It Fl d
101 Force decompression.
102 .It Fl f
103 Force compression of
104 .Ar file ,
105 even if it is not actually reduced in size.
106 Additionally, files are overwritten without prompting for confirmation.
107 .It Fl v
108 Print the percentage reduction of each file.
109 .El
110 .Pp
111 .Nm
112 uses a modified Lempel-Ziv algorithm.
113 Common substrings in the file are first replaced by 9-bit codes 257 and up.
114 When code 512 is reached, the algorithm switches to 10-bit codes and
115 continues to use more bits until the
116 limit specified by the
117 .Fl b
118 flag is reached (the default is 16).
119 .Ar Bits
120 must be between 9 and 16.
121 .Pp
122 After the
123 .Ar bits
124 limit is reached,
125 .Nm
126 periodically checks the compression ratio.
127 If it is increasing,
128 .Nm
129 continues to use the existing code dictionary.
130 However, if the compression ratio decreases,
131 .Nm
132 discards the table of substrings and rebuilds it from scratch. This allows
133 the algorithm to adapt to the next "block" of the file.
134 .Pp
135 The
136 .Fl b
137 flag is omitted for
138 .Ar uncompress
139 since the
140 .Ar bits
141 parameter specified during compression
142 is encoded within the output, along with
143 a magic number to ensure that neither decompression of random data nor
144 recompression of compressed data is attempted.
145 .Pp
146 The amount of compression obtained depends on the size of the
147 input, the number of
148 .Ar bits
149 per code, and the distribution of common substrings.
150 Typically, text such as source code or English is reduced by 50\-60%.
151 Compression is generally much better than that achieved by Huffman
152 coding (as used in the historical command pack), or adaptive Huffman
153 coding (as used in the historical command compact), and takes less
154 time to compute.
155 .Pp
156 The
157 .Nm
158 utility exits 0 on success, and \*[Gt]0 if an error occurs.
159 .Sh SEE ALSO
160 .Xr zcat 1
161 .Rs
162 .%A Welch, Terry A.
163 .%D June, 1984
164 .%T "A Technique for High Performance Data Compression"
165 .%J "IEEE Computer"
166 .%V 17:6
167 .%P pp. 8-19
168 .Re
169 .Sh HISTORY
170 The
171 .Nm
172 command appeared in
173 .Bx 4.3 .