compress/compress.1

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  36 .\"     @(#)compress.1  8.2 (Berkeley) 4/18/94
  37 .\" $FreeBSD: src/usr.bin/compress/compress.1,v 1.19 2004/07/05 17:12:53 ru Exp $
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  39 .Dd May 17, 2002
  40 .Dt COMPRESS 1
  41 .Os
  42 .Sh NAME
  43 .Nm compress ,
  44 .Nm uncompress
  45 .Nd compress and expand data
  46 .Sh SYNOPSIS
  47 .Nm
  48 .Op Fl cfv
  49 .Op Fl b Ar bits
  50 .Op Ar
  51 .Nm uncompress
  52 .Op Fl cfv
  53 .Op Ar
  54 .Sh DESCRIPTION
  55 The
  56 .Nm
  57 utility 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 The
  70 .Nm uncompress
  71 utility restores the compressed files to their original form, renaming the
  72 files by deleting the
  73 .Dq .Z
  74 extension.
  75 .Pp
  76 If renaming the files would cause files to be overwritten and the standard
  77 input device is a terminal, the user is prompted (on the standard error
  78 output) for confirmation.
  79 If prompting is not possible or confirmation is not received, the files
  80 are not overwritten.
  81 .Pp
  82 If no files are specified or a
  83 .Ar file
  84 argument is a single dash
  85 .Pq Sq Fl ,
  86 the standard input is compressed or uncompressed 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 indent
  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 f
 101 Force compression of
 102 .Ar file ,
 103 even if it is not actually reduced in size.
 104 Additionally, files are overwritten without prompting for confirmation.
 105 .It Fl v
 106 Print the percentage reduction of each file.
 107 .El
 108 .Pp
 109 The
 110 .Nm
 111 utility uses a modified Lempel-Ziv algorithm.
 112 Common substrings in the file are first replaced by 9-bit codes 257 and up.
 113 When code 512 is reached, the algorithm switches to 10-bit codes and
 114 continues to use more bits until the
 115 limit specified by the
 116 .Fl b
 117 flag is reached (the default is 16).
 118 .Ar Bits
 119 must be between 9 and 16.
 120 .Pp
 121 After the
 122 .Ar bits
 123 limit is reached,
 124 .Nm
 125 periodically checks the compression ratio.
 126 If it is increasing,
 127 .Nm
 128 continues to use the existing code dictionary.
 129 However, if the compression ratio decreases,
 130 .Nm
 131 discards the table of substrings and rebuilds it from scratch.
 132 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 .Nm 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 .Sh DIAGNOSTICS
 156 .Ex -std compress uncompress
 157 .Pp
 158 The
 159 .Nm compress
 160 utility exits 2 if attempting to compress the file would not reduce its size
 161 and the
 162 .Fl f
 163 option was not specified.
 164 .Sh SEE ALSO
 165 .Xr gunzip 1 ,
 166 .Xr gzexe 1 ,
 167 .Xr gzip 1 ,
 168 .Xr zcat 1 ,
 169 .Xr zmore 1 ,
 170 .Xr znew 1
 171 .Rs
 172 .%A Welch, Terry A.
 173 .%D June, 1984
 174 .%T "A Technique for High Performance Data Compression"
 175 .%J "IEEE Computer"
 176 .%V 17:6
 177 .%P pp. 8-19
 178 .Re
 179 .Sh STANDARDS
 180 The
 181 .Nm compress
 182 and
 183 .Nm uncompress
 184 utilities conform to
 185 .St -p1003.1-2001 .
 186 .Sh HISTORY
 187 The
 188 .Nm
 189 command appeared in
 190 .Bx 4.3 .