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📁 linux、unix初学者的必读书籍 详细讲述了shell编程方法与技巧
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<title>Appendix C</title>
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<table width="100%" border="1" bgcolor="#EBEBFF"><tr><td width="5%" align="left" valign="middle"><a href="Appendix B.htm"><img src="Larrow.gif" width="17" height="19" border="0"></a></td><td align="center" valign="middle"><a class="docLink" href="Front matter.htm">CONTENTS</a></td><td width="5%" align="right" valign="middle"><img src="Rarrow.gif" width="17" height="19" border="0"></td></tr></table>
<h2 class="docAppendixTitle">Appendix C. Steps for Using Quoting Correctly</h2>
<ul><li>&nbsp;<a class="docLink" href="#app16lev1sec1">C.1 Backslash</a></li>
<li>&nbsp;<a class="docLink" href="#app16lev1sec2">C.2 Single Quotes</a></li>
<li>&nbsp;<a class="docLink" href="#app16lev1sec3">C.3 Double Quotes</a></li>
<li>&nbsp;<a class="docLink" href="#app16lev1sec4">C.4 Combining Quotes</a></li>
<li>&nbsp;<a class="docLink" href="#app16lev1sec5">C.5 Setting the Shell Variable</a></li>
</ul>

<h3 class="docSection1Title" id="app16lev1sec1">C.1 Backslash</h3>
<span style="font-weight: bold">
<ol class="docList" type="1">
  <li><span style="font-weight: normal">
  <p class="docList">Precedes a character and escapes that character.</span></li>
  <li><span style="font-weight: normal">
  <p class="docList">Same as putting single quotes around one character.</span></li>
</ol>
<h3 class="docSection1Title" id="app16lev1sec2">C.2 Single Quotes</h3>
<span style="FONT-WEIGHT: bold">
<ol class="docList" type="1">
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Must be matched.</span></li>
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Protect all metacharacters from interpretation except the
  following:<ol class="docList" type="a">
    <li>
    <p class="docList">Itself.</li>
    <li>
    <p class="docList">Exclamation point (<span class="docEmphasis">csh</span>
    only).</li>
    <li>
    <p class="docList">Backslash.</li>
  </ol>
  </span></li>
</ol>
</span>
<h4 class="docSection2Title" id="ch16lev2sec1">Examples</h4>
<table cellSpacing="0" cellPadding="1" width="100%" border="1">
  <colgroup span="3" align="left">
  </colgroup>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphBoldItalic">C
    Shell</span> </td>
    <td class="docTableCell" vAlign="top"><span class="docEmphBoldItalic">Bourne
    Shell</span> </td>
    <td class="docTableCell" vAlign="top"><span class="docEmphBoldItalic">Korn
    Shell</span> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo '$&gt;&lt;%^&amp;*'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo '$*&amp;!&gt;&lt;?'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo '$*&amp;!&gt;&lt;?'</tt> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo 'I need $5.00\!'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo 'I need \$5.00!'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo 'I need \$5.00!'</tt> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>ho 'She cried, &quot;Help&quot;'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo 'She cried, &quot;Help&quot;'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo 'She cried, &quot;Help&quot;</tt>' </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo '\\\\'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo '\\\\'</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>print '\\\\'</tt> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>\\\\</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>\\</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>\\</tt> </td>
  </tr>
</table>
</span>
<h3 class="docSection1Title" id="app16lev1sec3">C.3 Double Quotes</h3>
<span style="FONT-WEIGHT: bold">
<ol class="docList" type="1">
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Must be matched.</span></li>
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Protect all metacharacters from interpretation except the
  following:<ol class="docList" type="a">
    <li>
    <p class="docList">Itself.</li>
    <li>
    <p class="docList">Exclamation point (<span class="docEmphasis">csh</span>
    only).</li>
    <li>
    <p class="docList">$ used for variable substitution.</li>
    <li>
    <p class="docList"><tt>' '</tt> Backquotes for command substitution.</li>
  </ol>
  </span></li>
</ol>
</span>
<h4 class="docSection2Title" id="ch16lev2sec2">Examples:</h4>
<table cellSpacing="0" cellPadding="1" width="100%" border="1">
  <colgroup span="3" align="left">
  </colgroup>
  <tr>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">C
    Shell</span> </th>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Bourne Shell</span> </th>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">Korn
    Shell</span> </th>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;Hello $LOGNAME\!&quot;</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;Hello $LOGNAME!&quot;</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>print &quot;Hello $LOGNAME!&quot;</tt> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;I don't care&quot;</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;I don't care&quot;</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>print &quot;I don't care&quot;</tt> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;The date is 'date'&quot;</tt>
    </td>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;The date is 'date'&quot;</tt>
    </td>
    <td class="docTableCell" vAlign="top"><tt>print &quot;The date is $(date)&quot;</tt>
    </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;\\\\&quot;</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>echo &quot;\\\\&quot;</tt> </td>
    <td class="docTableCell" vAlign="top"><tt>print &quot;\\\\&quot;</tt> </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><tt>\\\\</tt></td>
    <td class="docTableCell" vAlign="top"><tt>\</tt></td>
    <td class="docTableCell" vAlign="top"><tt>\</tt></td>
  </tr>
</table>
<h3 class="docSection1Title" id="app16lev1sec4">C.4 Combining Quotes</h3>
<p class="docText">The end result is to be able to embed the shell variable in
the <span class="docEmphasis">awk</span> command line and have the shell expand
the variable without interfering with <span class="docEmphasis">nawk</span>'s
field designators, $1 and $2.</p>
<h3 class="docSection1Title" id="app16lev1sec5">C.5 Setting the Shell Variable</h3>
<p class="docText"><br>
name=&quot;Jacob&nbsp;Savage&quot;&nbsp;&nbsp;&nbsp;&nbsp;(Bourne&nbsp;and&nbsp;Korn&nbsp;Shell)<br>
set&nbsp;name&nbsp;=&nbsp;&quot;Jacob&nbsp;Savage&quot;&nbsp;(&nbsp;C&nbsp;Shell&nbsp;)<br>
<br>
(The&nbsp;line&nbsp;from&nbsp;the&nbsp;datafile)<br>
Jacob&nbsp;Savage:408-298-7732:934&nbsp;La&nbsp;Barbara&nbsp;Dr.&nbsp;,&nbsp;San&nbsp;Jose,&nbsp;CA:02/27/78:500000<br>
<br>
(The&nbsp;<span class="docEmphasis">nawk</span>&nbsp;command&nbsp;line)<br>
<span class="docEmphStrong">nawk -F: '$1 ~ /^' &quot;$name&quot; '/{print $2}' datafile</span><br>
<br>
(Output)<br>
<span class="docEmphasis">408-298-7732</span></p>
<h4 class="docSection2Title" id="ch16lev2sec3">Step1:</h4>
<p class="docText">Test your knowledge of the UNIX command at the command line
before plugging in any shell variables.</p>
<p class="docText"><br>
<span class="docEmphStrong">nawk -F: '$1 ~ /^Jacob Savage/{print $2}' filename</span><br>
<br>
(Output)<br>
<span class="docEmphasis">408-298-7732</span></p>
<h4 class="docSection2Title" id="ch16lev2sec4">Step 2:</h4>
<p class="docText">Plug in the shell variable without changing anything else.
Leave all quotes as they were.</p>
<p class="docText"><br>
<span class="docEmphStrong">nawk -F: '$1 ~ /^$name/{print $2}' datafile</span></p>
<p class="docText">Starting at the left-hand side of the
<span class="docEmphasis">awk</span> command leave the first quote as is; right
before the shell dollar sign in <span class="docEmphasis">$name,</span> place
another single quote. Now the first quote is matched and all text within these
two quotes is protected from shell interference. The variable is exposed. Now
put another single quote right after the <span class="docEmphasis">e</span> in
<span class="docEmphasis">$name.</span> This starts another matched set of
single quotes ending after <span class="docEmphasis">awk</span>'s closing curly
brace. Everything within this set of quotes is also protected from shell
interpretation.</p>
<center>
<p class="docText">
<img alt="graphics/apcfig01.gif" src="apcfig01.gif" border="0" width="285" height="43"></p>
</center>
<h4 class="docSection2Title" id="ch16lev2sec5">Step 3:</h4>
<p class="docText">Enclose the shell variable in a set of double quotes. This
allows the variable to be expanded but the value of the variable will be treated
as single string if it contains whitespace. The whitespace must be protected so
that the command line is parsed properly.</p>
<center>
<p class="docText">
<img alt="graphics/apcfig02.gif" src="apcfig02.gif" border="0" width="309" height="47"></p>
</center>
<p class="docText">Count the number of quotes. There should be an even number of
single quotes and an even number of double quotes.</p>
<h4 class="docSection2Title" id="ch16lev2sec6">Example:</h4>
<p class="docText"><br>
<span class="docEmphStrong">oldname=&quot;Ellie Main&quot;</span><br>
<span class="docEmphStrong">newname=&quot;Eleanor Quigley&quot;</span><br>
&nbsp;</p>
<span style="FONT-WEIGHT: bold">
<ol class="docList" type="1">
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Make sure the command works.</p>
  <p class="docText"><br>
  <span class="docEmphStrong">nawk -F: '/^Ellie Main/{$1=&quot;Eleanor Quigley&quot;;
  print $0}' datafile</span><br>
&nbsp;</span></li>
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Plug in the variables.</p>
  <p class="docText"><br>
  <span class="docEmphStrong">nawk -F: '/^$oldname/{$1=&quot;$newname&quot;; print $0}'
  datafile</span><br>
&nbsp;</span></li>
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Play the quoting game. Starting at the first single quote
  at the left, move across the line until you come to the variable
  <span class="docEmphasis">$oldname</span> and place another single quote just
  before the dollar sign. Put another single quote right after the last letter
  in the variable name.</p>
  <p class="docList">Now move to the right and place another single quote right
  before the dollar sign in <span class="docEmphasis">$newname.</span> Put
  another single quote after the last character in <span class="docEmphasis">$newname.</span></p>
  <center>
  <p class="docText">
  <img alt="graphics/apcfig03.gif" src="apcfig03.gif" border="0" width="402" height="53"></p>
  </center>
  <p></span></li>
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Count the number of single quotes. If the number of single
  quotes is an even number, each quote has a matching quote. If not, you have
  forgotten a step.</span></li>
  <li><span style="FONT-WEIGHT: normal">
  <p class="docList">Enclose each of the shell variables in double quotes. The
  double quotes are placed snugly around the shell variable.</p>
  <center>
  <p class="docText">
  <img alt="graphics/apcfig04.gif" src="apcfig04.gif" border="0" width="442" height="54"></p>
  </center>
  <p>&nbsp;</span></li>
</ol>
</span>
<table width="100%" border="1" bgcolor="#EBEBFF"><tr><td width="5%" align="left" valign="middle"><a href="Appendix B.htm"><img src="Larrow.gif" width="17" height="19" border="0"></a></td><td align="center" valign="middle"><a class="docLink" href="Front matter.htm">CONTENTS</a></td><td width="5%" align="right" valign="middle"><img src="Rarrow.gif" width="17" height="19" border="0"></td></tr></table>
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