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5 .\" James A. Woods, derived from original work by Spencer Thomas
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38 .Nd compress and expand data
57 utility reduces the size of files using adaptive Lempel-Ziv coding.
60 is renamed to the same name plus the extension
66 extension will be ignored except it will cause an
67 error exit after other arguments are processed.
68 If compression would not reduce the size of a
74 utility restores compressed files to their original form, renaming the
78 A file specification need not include the file's
81 If a file's name in its file system does not have a
83 extension, it will not be uncompressed and it will cause
84 an error exit after other arguments are processed.
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
92 As many of the modification time, access time, file flags, file mode,
93 user ID, and group ID as allowed by permissions are retained in the
96 If no files are specified or a
98 argument is a single dash
100 the standard input is compressed or uncompressed to the standard output.
101 If either the input and output files are not regular files, the checks for
102 reduction in size and file overwriting are not performed, the input file is
103 not removed, and the attributes of the input file are not retained
106 The options are as follows:
107 .Bl -tag -width ".Fl b Ar bits"
109 The code size (see below) is limited to
111 which must be in the range 9..16.
114 Compressed or uncompressed output is written to the standard output.
115 No files are modified.
119 Compression is attempted even if the results will be larger than the
122 Files are overwritten without prompting for confirmation.
125 files are compressed even if they are not actually reduced in size.
127 Print the percentage reduction of each file.
137 utility uses a modified Lempel-Ziv algorithm.
138 Common substrings in the file are first replaced by 9-bit codes 257 and up.
139 When code 512 is reached, the algorithm switches to 10-bit codes and
140 continues to use more bits until the
141 limit specified by the
143 option or its default is reached.
145 After the limit is reached,
147 periodically checks the compression ratio.
150 continues to use the existing code dictionary.
151 However, if the compression ratio decreases,
153 discards the table of substrings and rebuilds it from scratch.
155 the algorithm to adapt to the next "block" of the file.
159 option is unavailable for
163 parameter specified during compression
164 is encoded within the output, along with
165 a magic number to ensure that neither decompression of random data nor
166 recompression of compressed data is attempted.
168 The amount of compression obtained depends on the size of the
171 per code, and the distribution of common substrings.
172 Typically, text such as source code or English is reduced by 50\-60%.
173 Compression is generally much better than that achieved by Huffman
174 coding (as used in the historical command pack), or adaptive Huffman
175 coding (as used in the historical command compact), and takes less
180 is a soft or hard link
182 will replace it with a compressed copy of the file pointed to by the link.
183 The link's target file is left uncompressed.
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.
196 with a single line of text:
197 .Bd -literal -offset indent
198 echo "This is a test" > test_file
201 Try to reduce the size of the file using a 10-bit code and show the exit status:
202 .Bd -literal -offset indent
203 $ compress -b 10 test_file
208 Try to compress the file and show compression percentage:
209 .Bd -literal -offset indent
210 $ compress -v test_file
211 test_file: file would grow; left unmodified
214 Same as above but forcing compression:
215 .Bd -literal -offset indent
216 $ compress -f -v test_file
217 test_file.Z: 79% expansion
220 Compress and uncompress the string
223 .Bd -literal -offset indent
224 $ echo "hello" | compress | uncompress
237 .%T "A Technique for High Performance Data Compression"
255 The program does not handle links well and has no link-handling options.
257 Some of these might be considered otherwise-undocumented features.
260 If the utility does not compress a file because doing so would not
261 reduce its size, and a file of the same name except with an
263 extension exists, the named file is not really ignored as stated above;
264 it causes a prompt to confirm the overwriting of the file with the extension.
265 If the operation is confirmed, that file is deleted.
268 If an empty file is compressed (using
273 That seems right, but if
275 is then used on that file, an error will occur.
279 argument is used and the utility prompts the user, the standard input
280 is taken as the user's reply to the prompt.
283 If the specified file does not exist, but a similarly-named one with (for
289 extension does exist, the utility will waste the user's time by not
290 immediately emitting an error message about the missing file and
292 Instead, it first asks for confirmation to overwrite
293 the existing file and then does not overwrite it.