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5 .\" James A. Woods, derived from original work by Spencer Thomas
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36 .\" @(#)compress.1 8.2 (Berkeley) 4/18/94
45 .Nd compress and expand data
64 utility reduces the size of files using adaptive Lempel-Ziv coding.
67 is renamed to the same name plus the extension
73 extension will be ignored except it will cause an
74 error exit after other arguments are processed.
75 If compression would not reduce the size of a
81 utility restores compressed files to their original form, renaming the
85 A file specification need not include the file's
88 If a file's name in its file system does not have a
90 extension, it will not be uncompressed and it will cause
91 an error exit after other arguments are processed.
93 If renaming the files would cause files to be overwritten and the standard
94 input device is a terminal, the user is prompted (on the standard error
95 output) for confirmation.
96 If prompting is not possible or confirmation is not received, the files
99 As many of the modification time, access time, file flags, file mode,
100 user ID, and group ID as allowed by permissions are retained in the
103 If no files are specified or a
105 argument is a single dash
107 the standard input is compressed or uncompressed to the standard output.
108 If either the input and output files are not regular files, the checks for
109 reduction in size and file overwriting are not performed, the input file is
110 not removed, and the attributes of the input file are not retained
113 The options are as follows:
114 .Bl -tag -width ".Fl b Ar bits"
116 The code size (see below) is limited to
118 which must be in the range 9..16.
121 Compressed or uncompressed output is written to the standard output.
122 No files are modified.
126 Compression is attempted even if the results will be larger than the
129 Files are overwritten without prompting for confirmation.
132 files are compressed even if they are not actually reduced in size.
134 Print the percentage reduction of each file.
144 utility uses a modified Lempel-Ziv algorithm.
145 Common substrings in the file are first replaced by 9-bit codes 257 and up.
146 When code 512 is reached, the algorithm switches to 10-bit codes and
147 continues to use more bits until the
148 limit specified by the
150 option or its default is reached.
152 After the limit is reached,
154 periodically checks the compression ratio.
157 continues to use the existing code dictionary.
158 However, if the compression ratio decreases,
160 discards the table of substrings and rebuilds it from scratch.
162 the algorithm to adapt to the next "block" of the file.
166 option is unavailable for
170 parameter specified during compression
171 is encoded within the output, along with
172 a magic number to ensure that neither decompression of random data nor
173 recompression of compressed data is attempted.
175 The amount of compression obtained depends on the size of the
178 per code, and the distribution of common substrings.
179 Typically, text such as source code or English is reduced by 50\-60%.
180 Compression is generally much better than that achieved by Huffman
181 coding (as used in the historical command pack), or adaptive Huffman
182 coding (as used in the historical command compact), and takes less
185 .Ex -std compress uncompress
189 utility exits 2 if attempting to compress a file would not reduce its size
192 option was not specified and if no other error occurs.
203 .%T "A Technique for High Performance Data Compression"
221 Some of these might be considered otherwise-undocumented features.
224 If the utility does not compress a file because doing so would not
225 reduce its size, and a file of the same name except with an
227 extension exists, the named file is not really ignored as stated above;
228 it causes a prompt to confirm the overwriting of the file with the extension.
229 If the operation is confirmed, that file is deleted.
232 If an empty file is compressed (using
237 That seems right, but if
239 is then used on that file, an error will occur.
243 argument is used and the utility prompts the user, the standard input
244 is taken as the user's reply to the prompt.
247 If the specified file does not exist, but a similarly-named one with (for
253 extension does exist, the utility will waste the user's time by not
254 immediately emitting an error message about the missing file and
256 Instead, it first asks for confirmation to overwrite
257 the existing file and then does not overwrite it.