compress/compress.1

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   5 .\" James A. Woods, derived from original work by Spencer Thomas
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  32 .\"     @(#)compress.1  8.2 (Berkeley) 4/18/94
  33 .\" $FreeBSD: src/usr.bin/compress/compress.1,v 1.23 2010/12/11 08:32:16 joel Exp $
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  35 .Dd May 17, 2002
  36 .Dt COMPRESS 1
  37 .Os
  38 .Sh NAME
  39 .Nm compress ,
  40 .Nm uncompress
  41 .Nd compress and expand data
  42 .Sh SYNOPSIS
  43 .Nm
  44 .Op Fl fv
  45 .Op Fl b Ar bits
  46 .Op Ar
  47 .Nm
  48 .Fl c
  49 .Op Fl b Ar bits
  50 .Op Ar file
  51 .Nm uncompress
  52 .Op Fl fv
  53 .Op Ar
  54 .Nm uncompress
  55 .Fl c
  56 .Op Ar file
  57 .Sh DESCRIPTION
  58 The
  59 .Nm
  60 utility reduces the size of files using adaptive Lempel-Ziv coding.
  61 Each
  62 .Ar file
  63 is renamed to the same name plus the extension
  64 .Pa .Z .
  65 A
  66 .Ar file
  67 argument with a
  68 .Pa .Z
  69 extension will be ignored except it will cause an
  70 error exit after other arguments are processed.
  71 If compression would not reduce the size of a
  72 .Ar file ,
  73 the file is ignored.
  74 .Pp
  75 The
  76 .Nm uncompress
  77 utility restores compressed files to their original form, renaming the
  78 files by deleting the
  79 .Pa .Z
  80 extensions.
  81 A file specification need not include the file's
  82 .Pa .Z
  83 extension.
  84 If a file's name in its file system does not have a
  85 .Pa .Z
  86 extension, it will not be uncompressed and it will cause
  87 an error exit after other arguments are processed.
  88 .Pp
  89 If renaming the files would cause files to be overwritten and the standard
  90 input device is a terminal, the user is prompted (on the standard error
  91 output) for confirmation.
  92 If prompting is not possible or confirmation is not received, the files
  93 are not overwritten.
  94 .Pp
  95 As many of the modification time, access time, file flags, file mode,
  96 user ID, and group ID as allowed by permissions are retained in the
  97 new file.
  98 .Pp
  99 If no files are specified or a
 100 .Ar file
 101 argument is a single dash
 102 .Pq Sq Fl ,
 103 the standard input is compressed or uncompressed to the standard output.
 104 If either the input and output files are not regular files, the checks for
 105 reduction in size and file overwriting are not performed, the input file is
 106 not removed, and the attributes of the input file are not retained
 107 in the output file.
 108 .Pp
 109 The options are as follows:
 110 .Bl -tag -width ".Fl b Ar bits"
 111 .It Fl b Ar bits
 112 The code size (see below) is limited to
 113 .Ar bits ,
 114 which must be in the range 9..16.
 115 The default is 16.
 116 .It Fl c
 117 Compressed or uncompressed output is written to the standard output.
 118 No files are modified.
 119 The
 120 .Fl v
 121 option is ignored.
 122 Compression is attempted even if the results will be larger than the
 123 original.
 124 .It Fl f
 125 Files are overwritten without prompting for confirmation.
 126 Also, for
 127 .Nm compress ,
 128 files are compressed even if they are not actually reduced in size.
 129 .It Fl v
 130 Print the percentage reduction of each file.
 131 Ignored by
 132 .Nm uncompress
 133 or if the
 134 .Fl c
 135 option is also used.
 136 .El
 137 .Pp
 138 The
 139 .Nm
 140 utility uses a modified Lempel-Ziv algorithm.
 141 Common substrings in the file are first replaced by 9-bit codes 257 and up.
 142 When code 512 is reached, the algorithm switches to 10-bit codes and
 143 continues to use more bits until the
 144 limit specified by the
 145 .Fl b
 146 option or its default is reached.
 147 .Pp
 148 After the limit is reached,
 149 .Nm
 150 periodically checks the compression ratio.
 151 If it is increasing,
 152 .Nm
 153 continues to use the existing code dictionary.
 154 However, if the compression ratio decreases,
 155 .Nm
 156 discards the table of substrings and rebuilds it from scratch.
 157 This allows
 158 the algorithm to adapt to the next "block" of the file.
 159 .Pp
 160 The
 161 .Fl b
 162 option is unavailable for
 163 .Nm uncompress
 164 since the
 165 .Ar bits
 166 parameter specified during compression
 167 is encoded within the output, along with
 168 a magic number to ensure that neither decompression of random data nor
 169 recompression of compressed data is attempted.
 170 .Pp
 171 The amount of compression obtained depends on the size of the
 172 input, the number of
 173 .Ar bits
 174 per code, and the distribution of common substrings.
 175 Typically, text such as source code or English is reduced by 50\-60%.
 176 Compression is generally much better than that achieved by Huffman
 177 coding (as used in the historical command pack), or adaptive Huffman
 178 coding (as used in the historical command compact), and takes less
 179 time to compute.
 180 .Sh EXIT STATUS
 181 .Ex -std compress uncompress
 182 .Pp
 183 The
 184 .Nm compress
 185 utility exits 2 if attempting to compress a file would not reduce its size
 186 and the
 187 .Fl f
 188 option was not specified and if no other error occurs.
 189 .Sh SEE ALSO
 190 .Xr gunzip 1 ,
 191 .Xr gzexe 1 ,
 192 .Xr gzip 1 ,
 193 .Xr zcat 1 ,
 194 .Xr zmore 1 ,
 195 .Xr znew 1
 196 .Rs
 197 .%A Welch, Terry A.
 198 .%D June, 1984
 199 .%T "A Technique for High Performance Data Compression"
 200 .%J "IEEE Computer"
 201 .%V 17:6
 202 .%P pp. 8-19
 203 .Re
 204 .Sh STANDARDS
 205 The
 206 .Nm compress
 207 and
 208 .Nm uncompress
 209 utilities conform to
 210 .St -p1003.1-2001 .
 211 .Sh HISTORY
 212 The
 213 .Nm
 214 command appeared in
 215 .Bx 4.3 .
 216 .Sh BUGS
 217 Some of these might be considered otherwise-undocumented features.
 218 .Pp
 219 .Nm compress :
 220 If the utility does not compress a file because doing so would not
 221 reduce its size, and a file of the same name except with an
 222 .Pa .Z
 223 extension exists, the named file is not really ignored as stated above;
 224 it causes a prompt to confirm the overwriting of the file with the extension.
 225 If the operation is confirmed, that file is deleted.
 226 .Pp
 227 .Nm uncompress :
 228 If an empty file is compressed (using
 229 .Fl f ) ,
 230 the resulting
 231 .Pa .Z
 232 file is also empty.
 233 That seems right, but if
 234 .Nm uncompress
 235 is then used on that file, an error will occur.
 236 .Pp
 237 Both utilities: If a
 238 .Sq Fl
 239 argument is used and the utility prompts the user, the standard input
 240 is taken as the user's reply to the prompt.
 241 .Pp
 242 Both utilities:
 243 If the specified file does not exist, but a similarly-named one with (for
 244 .Nm compress )
 245 or without (for
 246 .Nm uncompress )
 247 a
 248 .Pa .Z
 249 extension does exist, the utility will waste the user's time by not
 250 immediately emitting an error message about the missing file and
 251 continuing.
 252 Instead, it first asks for confirmation to overwrite
 253 the existing file and then does not overwrite it.