117 lines
3.7 KiB
HTML
117 lines
3.7 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<html>
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<meta http-equiv="Content-Type" content=
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"text/html; charset=utf-8">
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<title>compress - Compress data</title>
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<LINK rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</HEAD>
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<body bgcolor="#FFFFFF">
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<!-- Java sync-link -->
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<SCRIPT LANGUAGE="Javascript" SRC="../rzahg/synch.js" TYPE="text/javascript"></SCRIPT>
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<h2>compress - Compress
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data</h2>
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<strong>Synopsis</strong>
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<p><strong>compress [-cfv] [-b</strong> <em>
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bits</em><strong>]</strong> [<em>file ...</em>]</p>
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<p><strong>Description</strong></p>
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<p>The <strong>compress</strong> utility reduces the size of the
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<em>files</em> using adaptive Lempel-Ziv coding. Each <em>file</em>
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is renamed to the same name plus the extension ".Z". As many of the
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modification time, access time, file flags, file mode, user ID, and
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group ID as allowed by permissions are retained in the new file. If
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compression would not reduce the size of a file, the <em>file</em>
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is ignored.</p>
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<p>If renaming <em>file</em> would cause files to be overwritten
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and the standard input device is a terminal, the user is prompted
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(on standard error) for confirmation. If prompting is not possible
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or confirmation is not received, the files are not overwritten.</p>
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<p><strong>Options</strong></p>
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<dl>
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<dt><strong>-b</strong> <em>bits</em></dt>
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<dd>Specify the bits code limit (see below for details).</dd>
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<dt><strong>-c</strong></dt>
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<dd>Compressed output is written to the standard output. No files
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are modified.</dd>
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<dt><strong>-f</strong></dt>
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<dd>Force compression of <em>file</em>, even if it is not actually
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reduced in size. Additionally, files are overwritten without
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prompting for confirmation.</dd>
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<dt><strong>-v</strong></dt>
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<dd>Print the percentage of reduction for each <em>file</em>.</dd>
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</dl>
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<p><strong>Operands</strong></p>
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<p>Each <em>file</em> is a pathname of a file to compress. If no
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<em>files</em> are specified, the standard input is compressed to
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the standard output. If either the input or output files are not
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regular files, the checks for reduction in size and file
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overwriting are not performed, the input file is not removed, and
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the attributes of the input file are not retained.</p>
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<p><strong>Extended Description</strong></p>
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<p>The <strong>compress</strong> utility uses a modified Lempel-Ziv
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algorithm. Common substrings in the file are first replaced by
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9-bit codes 257 and up. When code 512 is reached, the algorithm
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switches to 10-bit codes and continues to use more bits until the
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limit specified by the <strong>-b</strong> flag is reached (the
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default is 16). Bits must be between 9 and 16.</p>
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<p>After the bits limit is reached, <strong>compress</strong>
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periodically checks the compression ratio. If it is increasing,
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<strong>compress</strong> continues to use the existing code
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dictionary. However, if the compression ratio decreases, <strong>
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compress</strong> discards the table of substrings and rebuilds it
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from scratch. This allows the algorithm to adapt to the next
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"block" of the file.</p>
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<p>The amount of compression obtained depends on the size of the
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input, the number of bits per code, and the distribution of common
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substrings. Typically, text such as source code or English is
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reduced by 50-60%.</p>
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<p><strong>Exit Status</strong></p>
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<ul>
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<li>0 on success</li>
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<li>>0 if an error occurs.</li>
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</ul>
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<p><strong>Related information</strong></p>
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<ul>
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<li><a href="pax.htm">pax - Portable archive interchange</a></li>
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<li><a href="uncompress.htm">uncompress - Expand compressed
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data</a></li>
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<li><a href="zcat.htm">zcat - Expand and concatenate data</a></li>
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</ul>
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<!-- end ibmbody========================================================= -->
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</body>
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</html>
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