228 lines
7.2 KiB
HTML
228 lines
7.2 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>Parameter expansions</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>Parameter expansions</h2>
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<p>The format for parameter expansion is as follows:</p>
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<p>${<em>expression</em>}</p>
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<p>where <em>expression</em> consists of all characters until the
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matching right brace (<strong>}</strong>). Any right brace
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characters escaped by a backslash or within a string with quotation marks, as
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well as characters in embedded arithmetic expansions, command
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substitutions, and variable expansions, are not examined in
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determining the matching right brace.</p>
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<p>The simplest form for parameter expansion is:</p>
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<p>${<em>parameter</em>}</p>
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<p>The value, if any, of <em>parameter</em> is substituted. The
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parameter name or symbol can be enclosed in braces, which are
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optional except for positional parameters with more than one digit
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or when <em>parameter</em> is followed by a character that might be
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interpreted as part of the name. If a parameter expansion occurs
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inside double quotation marks, then:</p>
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<ol>
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<li>Path name expansion is not performed on the results of the
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expansion.</li>
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<li>Field splitting is not performed on the results of the
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expansion, with the exception of <strong>@</strong> special
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parameter.</li>
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</ol>
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<p>A parameter expansion can be modified by using one of the
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following formats:</p>
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<dl>
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<dt><strong>${<em>parameter:-word</em>}</strong></dt>
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<dd>Use Default Values. If <em>parameter</em> is unset or null, the
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expansion of <em>word</em> is substituted. Otherwise, the value of
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<em>parameter</em> is substituted.</dd>
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<dt><strong>${<em>parameter:=word</em>}</strong></dt>
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<dd>Assign Default Values. If <em>parameter</em> is unset or null,
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the expansion of <em>word</em> is assigned to <em>parameter</em>.
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In all cases, the final value of <em>parameter</em> is substituted.
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Only variables, not positional parameters or special parameters,
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can be assigned in this way.</dd>
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<dt><strong>${<em>parameter:?word</em>]}</strong></dt>
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<dd>Indicate Error if Null or Unset. If <em>parameter</em> is unset
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or null, the expansion of <em>word</em> (or a message indicating it
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is unset if word is omitted) is written to standard error and a
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non-interactive shell exits with a nonzero exit status. Otherwise,
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the value of <em>parameter</em> is substituted.</dd>
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<dt><strong>${<em>parameter:+word</em>}</strong></dt>
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<dd>Use Alternate Value. If <em>parameter</em> is unset or null,
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null is substituted. Otherwise, the expansion of <em>word</em> is
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substituted.</dd>
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</dl>
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<p>In the preceding four parameter expansions, using a colon in the
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format results in a test for a parameter that is unset or null;
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removing the colon results in a test for a parameter that is only
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unset.</p>
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<dl>
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<dt><strong>${#<em>parameter</em>}</strong></dt>
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<dd>String Length. If <em>parameter</em> is <strong>@</strong> or
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<strong>*</strong>, the number of positional parameters is
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substituted. Otherwise, the length of the value of <em>
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parameter</em> is substituted.</dd>
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<dt><strong>${<em>parameter</em>%<em>word</em>}</strong></dt>
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<dd>Remove Smallest Suffix Pattern. The <em>word</em> is expanded
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to produce a <a href="pattern.htm">pattern</a>. Then the result is
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<em>parameter</em> after removing the smallest portion of the
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suffix matched by the pattern.</dd>
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<dt><strong>${<em>parameter</em>%%<em>word</em>}</strong></dt>
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<dd>Remove Largest Suffix Pattern. The <em>word</em> is expanded to
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produce a <a href="pattern.htm">pattern</a>. Then the result is <em>
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parameter</em> after removing the largest portion of the suffix
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matched by the pattern.</dd>
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<dt><strong>${<em>parameter</em>#<em>word</em>}</strong></dt>
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<dd>Remove Smallest Prefix Pattern. The <em>word</em> is expanded
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to produce a <a href="pattern.htm">pattern</a>. Then the result is
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<em>parameter</em> after removing the smallest portion of the
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prefix matched by the pattern.</dd>
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<dt><strong>${<em>parameter</em>##<em>word</em>}</strong></dt>
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<dd>Remove Largest Prefix Pattern. The <em>word</em> is expanded to
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produce a <a href="pattern.htm">pattern</a>. Then the result is <em>
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parameter</em> after removing the largest portion of the prefix
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matched by the pattern.</dd>
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<dt>
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<strong>${<em>parameter:offset</em>}</strong></dt>
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<dt><strong>${<em>parameter:offset:length</em>}</strong></dt>
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<dd>Substring Starting at Offset. The value of this expansion is the substring
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starting at the byte specified by <em>offset</em> for <em>length</em> bytes.
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If <em>length</em> is not
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specified or the value of <em>length</em> causes the expansion to exceed the
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length of <em>parameter</em>, the substring ends with the last byte of
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<em>parameter</em>. Both <em>offset</em> and <em>length</em> are
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<a href="arithexp.htm">arithmetic expressions</a> and must evaluate to a value
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that is greater than or equal to zero. The first byte of <em>parameter</em> is
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defined by an offset of zero.</dd>
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<dt><strong>${<em>parameter</em>/<em>pattern</em>/<em>string</em>}</strong></dt>
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<dt><strong>${<em>parameter</em>//<em>pattern</em>/<em>string</em>}</strong></dt>
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<dd>Substitute String for Pattern. The value of this expansion is the value
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of <em>parameter</em> with the longest match of <em>pattern</em> replaced with
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<em>string</em>. In the first form, only the first match of <em>pattern</em> is replaced.
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In the second form, all matches of <em>pattern</em> are replaced. If <em>pattern</em>
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begins with #, it must match at the beginning of <em>parameter</em>. If
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<em>pattern</em> begins with a %, it must match at the end of <em>parameter</em>.
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</dd>
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</dl>
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<p><strong>Examples</strong></p>
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<ol>
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<li>Expand the variable QSH_VERSION.
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<pre>
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echo ${QSH_VERSION}
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</pre>
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</li>
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<li>Expand the variable filename and use a default value.
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<pre>
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echo ${filename:-/tmp/default.txt}
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</pre>
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</li>
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<li>Expand the variable index and assign a default value.
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<pre>
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echo ${index:=0}
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</pre>
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</li>
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<li>Expand the variable filename and indicate an error if unset.
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<pre>
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echo ${filename:?Variable is not set}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using string length.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${#DIRLIST}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using remove smallest suffix pattern.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${DIRLIST%/*}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using remove largest suffix pattern.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${DIRLIST%%:*}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using remove smallest prefix pattern.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${DIRLIST#/usr}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using remove largest prefix pattern.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${DIRLIST##*/}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using a substring starting at offset.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${DIRLIST:5:3}
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</pre>
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</li>
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<li>Expand the variable DIRLIST using a substitute string for pattern.
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<pre>
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DIRLIST=/usr/bin:/home/mike
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echo ${DIRLIST/m?ke/joel}
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</pre>
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</li>
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</ol>
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</body>
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</html>
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