|
0
|
1 .PU
|
|
|
2 .TH COMPRESS 1 local
|
|
|
3 .SH NAME
|
|
|
4 compress, uncompress, zcat \- compress and expand data
|
|
|
5 .SH SYNOPSIS
|
|
|
6 .ll +8
|
|
|
7 .B compress
|
|
|
8 [
|
|
|
9 .B \-f
|
|
|
10 ] [
|
|
|
11 .B \-v
|
|
|
12 ] [
|
|
|
13 .B \-c
|
|
|
14 ] [
|
|
|
15 .B \-V
|
|
|
16 ] [
|
|
|
17 .B \-b
|
|
|
18 .I bits
|
|
|
19 ] [
|
|
|
20 .I "name \&..."
|
|
|
21 ]
|
|
|
22 .ll -8
|
|
|
23 .br
|
|
|
24 .B uncompress
|
|
|
25 [
|
|
|
26 .B \-f
|
|
|
27 ] [
|
|
|
28 .B \-v
|
|
|
29 ] [
|
|
|
30 .B \-c
|
|
|
31 ] [
|
|
|
32 .B \-V
|
|
|
33 ] [
|
|
|
34 .I "name \&..."
|
|
|
35 ]
|
|
|
36 .br
|
|
|
37 .B zcat
|
|
|
38 [
|
|
|
39 .B \-V
|
|
|
40 ] [
|
|
|
41 .I "name \&..."
|
|
|
42 ]
|
|
|
43 .SH DESCRIPTION
|
|
|
44 .I Compress
|
|
|
45 reduces the size of the named files using adaptive Lempel-Ziv coding.
|
|
|
46 Whenever possible,
|
|
|
47 each file is replaced by one with the extension
|
|
|
48 .B "\&.Z,"
|
|
|
49 while keeping the same ownership modes, access and modification times.
|
|
|
50 If no files are specified, the standard input is compressed to the
|
|
|
51 standard output.
|
|
|
52 Compressed files can be restored to their original form using
|
|
|
53 .I uncompress
|
|
|
54 or
|
|
|
55 .I zcat.
|
|
|
56 .PP
|
|
|
57 The
|
|
|
58 .B \-f
|
|
|
59 option will force compression of
|
|
|
60 .I name.
|
|
|
61 This is useful for compressing an entire directory,
|
|
|
62 even if some of the files do not actually shrink.
|
|
|
63 If
|
|
|
64 .B \-f
|
|
|
65 is not given and
|
|
|
66 .I compress
|
|
|
67 is run in the foreground,
|
|
|
68 the user is prompted as to whether an existing file should be overwritten.
|
|
|
69 .PP
|
|
|
70 The
|
|
|
71 .B \-c
|
|
|
72 option makes
|
|
|
73 .I compress/uncompress
|
|
|
74 write to the standard output; no files are changed.
|
|
|
75 The nondestructive behavior of
|
|
|
76 .I zcat
|
|
|
77 is identical to that of
|
|
|
78 .I uncompress
|
|
|
79 .B \-c.
|
|
|
80 .PP
|
|
|
81 .I Compress
|
|
|
82 uses the modified Lempel-Ziv algorithm popularized in
|
|
|
83 "A Technique for High Performance Data Compression",
|
|
|
84 Terry A. Welch,
|
|
|
85 .I "IEEE Computer,"
|
|
|
86 vol. 17, no. 6 (June 1984), pp. 8-19.
|
|
|
87 Common substrings in the file are first replaced by 9-bit codes 257 and up.
|
|
|
88 When code 512 is reached, the algorithm switches to 10-bit codes and
|
|
|
89 continues to use more bits until the
|
|
|
90 limit specified by the
|
|
|
91 .B \-b
|
|
|
92 flag is reached (default 16).
|
|
|
93 .I Bits
|
|
|
94 must be between 9 and 16. The default can be changed in the source to allow
|
|
|
95 .I compress
|
|
|
96 to be run on a smaller machine.
|
|
|
97 .PP
|
|
|
98 After the
|
|
|
99 .I bits
|
|
|
100 limit is attained,
|
|
|
101 .I compress
|
|
|
102 periodically checks the compression ratio. If it is increasing,
|
|
|
103 .I compress
|
|
|
104 continues to use the existing code dictionary. However,
|
|
|
105 if the compression ratio decreases,
|
|
|
106 .I compress
|
|
|
107 discards the table of substrings and rebuilds it from scratch. This allows
|
|
|
108 the algorithm to adapt to the next "block" of the file.
|
|
|
109 .PP
|
|
|
110 Note that the
|
|
|
111 .B \-b
|
|
|
112 flag is omitted for
|
|
|
113 .I uncompress,
|
|
|
114 since the
|
|
|
115 .I bits
|
|
|
116 parameter specified during compression
|
|
|
117 is encoded within the output, along with
|
|
|
118 a magic number to ensure that neither decompression of random data nor
|
|
|
119 recompression of compressed data is attempted.
|
|
|
120 .PP
|
|
|
121 .ne 8
|
|
|
122 The amount of compression obtained depends on the size of the
|
|
|
123 input, the number of
|
|
|
124 .I bits
|
|
|
125 per code, and the distribution of common substrings.
|
|
|
126 Typically, text such as source code or English
|
|
|
127 is reduced by 50\-60%.
|
|
|
128 Compression is generally much better than that achieved by
|
|
|
129 Huffman coding (as used in
|
|
|
130 .IR pack ),
|
|
|
131 or adaptive Huffman coding
|
|
|
132 .RI ( compact ),
|
|
|
133 and takes less time to compute.
|
|
|
134 .PP
|
|
|
135 Under the
|
|
|
136 .B \-v
|
|
|
137 option,
|
|
|
138 a message is printed yielding the percentage of
|
|
|
139 reduction for each file compressed.
|
|
|
140 .PP
|
|
|
141 If the
|
|
|
142 .B \-V
|
|
|
143 option is specified, the current version and compile options are printed on
|
|
|
144 stderr.
|
|
|
145 .PP
|
|
|
146 Exit status is normally 0;
|
|
|
147 if the last file is larger after (attempted) compression, the status is 2;
|
|
|
148 if an error occurs, exit status is 1.
|
|
|
149 .SH "SEE ALSO"
|
|
|
150 pack(1), compact(1)
|
|
|
151 .SH "DIAGNOSTICS"
|
|
|
152 Usage: compress [\-dfvcV] [\-b maxbits] [file ...]
|
|
|
153 .in +8
|
|
|
154 Invalid options were specified on the command line.
|
|
|
155 .in -8
|
|
|
156 Missing maxbits
|
|
|
157 .in +8
|
|
|
158 Maxbits must follow
|
|
|
159 .BR \-b \.
|
|
|
160 .in -8
|
|
|
161 .IR file :
|
|
|
162 not in compressed format
|
|
|
163 .in +8
|
|
|
164 The file specified to
|
|
|
165 .I uncompress
|
|
|
166 has not been compressed.
|
|
|
167 .in -8
|
|
|
168 .IR file :
|
|
|
169 compressed with
|
|
|
170 .I xx
|
|
|
171 bits, can only handle
|
|
|
172 .I yy
|
|
|
173 bits
|
|
|
174 .in +8
|
|
|
175 .I File
|
|
|
176 was compressed by a program that could deal with
|
|
|
177 more
|
|
|
178 .I bits
|
|
|
179 than the compress code on this machine.
|
|
|
180 Recompress the file with smaller
|
|
|
181 .IR bits \.
|
|
|
182 .in -8
|
|
|
183 .IR file :
|
|
|
184 already has .Z suffix -- no change
|
|
|
185 .in +8
|
|
|
186 The file is assumed to be already compressed.
|
|
|
187 Rename the file and try again.
|
|
|
188 .in -8
|
|
|
189 .IR file :
|
|
|
190 filename too long to tack on .Z
|
|
|
191 .in +8
|
|
|
192 The file cannot be compressed because its name is longer than
|
|
|
193 12 characters.
|
|
|
194 Rename and try again.
|
|
|
195 This message does not occur on BSD systems.
|
|
|
196 .in -8
|
|
|
197 .I file
|
|
|
198 already exists; do you wish to overwrite (y or n)?
|
|
|
199 .in +8
|
|
|
200 Respond "y" if you want the output file to be replaced; "n" if not.
|
|
|
201 .in -8
|
|
|
202 uncompress: corrupt input
|
|
|
203 .in +8
|
|
|
204 A SIGSEGV violation was detected which usually means that the input file has
|
|
|
205 been corrupted.
|
|
|
206 .in -8
|
|
|
207 Compression:
|
|
|
208 .I "xx.xx%"
|
|
|
209 .in +8
|
|
|
210 Percentage of the input saved by compression.
|
|
|
211 (Relevant only for
|
|
|
212 .BR \-v \.)
|
|
|
213 .in -8
|
|
|
214 -- not a regular file: unchanged
|
|
|
215 .in +8
|
|
|
216 When the input file is not a regular file,
|
|
|
217 (e.g. a directory), it is
|
|
|
218 left unaltered.
|
|
|
219 .in -8
|
|
|
220 -- has
|
|
|
221 .I xx
|
|
|
222 other links: unchanged
|
|
|
223 .in +8
|
|
|
224 The input file has links; it is left unchanged. See
|
|
|
225 .IR ln "(1)"
|
|
|
226 for more information.
|
|
|
227 .in -8
|
|
|
228 -- file unchanged
|
|
|
229 .in +8
|
|
|
230 No savings is achieved by
|
|
|
231 compression. The input remains virgin.
|
|
|
232 .in -8
|
|
|
233 .SH "BUGS"
|
|
|
234 Although compressed files are compatible between machines with large memory,
|
|
|
235 .BR \-b \12
|
|
|
236 should be used for file transfer to architectures with
|
|
|
237 a small process data space (64KB or less, as exhibited by the DEC PDP
|
|
|
238 series, the Intel 80286, etc.)
|
