compress/compress.1

   1 .\"     $NetBSD: compress.1,v 1.6 1997/09/15 10:58:37 lukem Exp $
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   7 .\" James A. Woods, derived from original work by Spencer Thomas
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  38 .\"     @(#)compress.1  8.2 (Berkeley) 4/18/94
  39 .\"
  40 .Dd April 18, 1994
  41 .Dt COMPRESS 1
  42 .Os BSD 4.3
  43 .Sh NAME
  44 .Nm compress ,
  45 .\".Nm uncompress ,
  46 .Nm uncompress
  47 .\".Nm zcat
  48 .Nd compress and expand data
  49 .Sh SYNOPSIS
  50 .Nm
  51 .Op Fl cfv
  52 .Op Fl b Ar bits
  53 .Op Ar
  54 .Nm uncompress
  55 .Op Fl cfv
  56 .Op Ar
  57 .\".Nm zcat
  58 .\".Op Ar
  59 .Sh DESCRIPTION
  60 .Nm
  61 reduces the size of the named files using adaptive Lempel-Ziv coding.
  62 Each
  63 .Ar file
  64 is renamed to the same name plus the extension
  65 .Dq .Z .
  66 As many of the modification time, access time, file flags, file mode,
  67 user ID, and group ID as allowed by permissions are retained in the
  68 new file.
  69 If compression would not reduce the size of a
  70 .Ar file ,
  71 the file is ignored.
  72 .Pp
  73 .Nm uncompress
  74 restores the compressed files to their original form, renaming the
  75 files by deleting the
  76 .Dq .Z
  77 extension.
  78 .\".Pp
  79 .\".Nm Zcat
  80 .\"is an alias for
  81 .\".Dq "uncompress -c" .
  82 .Pp
  83 If renaming the files would cause files to be overwritten and the standard
  84 input device is a terminal, the user is prompted (on the standard error
  85 output) for confirmation.
  86 If prompting is not possible or confirmation is not received, the files
  87 are not overwritten.
  88 .Pp
  89 If no files are specified, the standard input is compressed or uncompressed
  90 to the standard output.
  91 If either the input and output files are not regular files, the checks for
  92 reduction in size and file overwriting are not performed, the input file is
  93 not removed, and the attributes of the input file are not retained.
  94 .Pp
  95 The options are as follows:
  96 .Bl -tag -width Ds
  97 .It Fl b
  98 Specify the
  99 .Ar bits
 100 code limit (see below).
 101 .It Fl c
 102 Compressed or uncompressed output is written to the standard output.
 103 No files are modified.
 104 .It Fl f
 105 Force compression of
 106 .Ar file ,
 107 even if it is not actually reduced in size.
 108 Additionally, files are overwritten without prompting for confirmation.
 109 .It Fl v
 110 Print the percentage reduction of each file.
 111 .El
 112 .Pp
 113 .Nm
 114 uses a modified Lempel-Ziv algorithm.
 115 Common substrings in the file are first replaced by 9-bit codes 257 and up.
 116 When code 512 is reached, the algorithm switches to 10-bit codes and
 117 continues to use more bits until the
 118 limit specified by the
 119 .Fl b
 120 flag is reached (the default is 16).
 121 .Ar Bits
 122 must be between 9 and 16.
 123 .Pp
 124 After the
 125 .Ar bits
 126 limit is reached,
 127 .Nm
 128 periodically checks the compression ratio.
 129 If it is increasing,
 130 .Nm
 131 continues to use the existing code dictionary.
 132 However, if the compression ratio decreases,
 133 .Nm
 134 discards the table of substrings and rebuilds it from scratch.  This allows
 135 the algorithm to adapt to the next "block" of the file.
 136 .Pp
 137 The
 138 .Fl b
 139 flag is omitted for
 140 .Ar uncompress
 141 since the
 142 .Ar bits
 143 parameter specified during compression
 144 is encoded within the output, along with
 145 a magic number to ensure that neither decompression of random data nor
 146 recompression of compressed data is attempted.
 147 .Pp
 148 .ne 8
 149 The amount of compression obtained depends on the size of the
 150 input, the number of
 151 .Ar bits
 152 per code, and the distribution of common substrings.
 153 Typically, text such as source code or English is reduced by 50\-60%.
 154 Compression is generally much better than that achieved by Huffman
 155 coding (as used in the historical command pack), or adaptive Huffman
 156 coding (as used in the historical command compact), and takes less
 157 time to compute.
 158 .Pp
 159 The
 160 .Nm
 161 utility exits 0 on success, and >0 if an error occurs.
 162 .Sh SEE ALSO
 163 .Xr zcat 1
 164 .Rs
 165 .%A Welch, Terry A.
 166 .%D June, 1984
 167 .%T "A Technique for High Performance Data Compression"
 168 .%J "IEEE Computer"
 169 .%V 17:6
 170 .%P pp. 8-19
 171 .Re
 172 .Sh HISTORY
 173 The
 174 .Nm
 175 command appeared in
 176 .Bx 4.3 .