_chapter 8.htm

linux、unix初学者的必读书籍 详细讲述了shell编程方法与技巧

  <colgroup span="2" align="left">
  </colgroup>
  <tr>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Metacharacter</span> </th>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Meaning</span> </th>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">\</span>
    </td>
    <td class="docTableCell" vAlign="top">Literally interprets the following 
    character. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">&amp;</span>
    </td>
    <td class="docTableCell" vAlign="top">Processes in the background. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">;</span>
    </td>
    <td class="docTableCell" vAlign="top">Separates commands. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">$</span>
    </td>
    <td class="docTableCell" vAlign="top">Substitutes variables. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">?</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches for a single character. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">[abc]</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches for one character from a set 
    of characters. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">[!abc]</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches for one character
    <span class="docEmphasis">not</span> from the set of characters. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">*</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches for zero or more characters.
    </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">(cmds)</span>
    </td>
    <td class="docTableCell" vAlign="top">Executes commands in a subshell. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">{cmds}</span>
    </td>
    <td class="docTableCell" vAlign="top">Executes commands in current shell.
    </td>
  </tr>
</table>

<h4 class="docSection2Title" id="ch08lev2sec4">8.1.4 Filename Substitution</h4>
<p class="docText">When evaluating the command line, the shell uses 
metacharacters to abbreviate filenames or pathnames that match a certain set of 
characters. The filename substitution metacharacters listed in
<a class="docLink" href="#ch08table02">Table 8.2</a> are expanded into an 
alphabetically listed set of filenames. The process of expanding the 
metacharacter into filenames is also called <span class="docEmphasis">filename 
substitution,</span> or <span class="docEmphasis">globbing.</span> If a 
metacharacter is used and there is no filename that matches it, the shell treats 
the metacharacter as a literal character.</p>

<table cellSpacing="0" cellPadding="1" width="100%" border="1">
  <caption>
  <h5 id="ch08table02" class="docTableTitle">Table 8.2. Shell Metacharacters and Filename 
  Substitution</h5>
  </caption>
  <colgroup span="2" align="left">
  </colgroup>
  <tr>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Metacharacter</span> </th>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Meaning</span> </th>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">*</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches zero or more characters. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">?</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches exactly one character. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">[abc]</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches one character in the set
    <span class="docEmphasis">a, b,</span> or <span class="docEmphasis">c.</span>
    </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">[a杬]</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches one character in the range 
    from <span class="docEmphasis">a</span> to <span class="docEmphasis">z.</span>
    </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">[! a杬]</span>
    </td>
    <td class="docTableCell" vAlign="top">Matches one character
    <span class="docEmphasis">not</span> in the range from
    <span class="docEmphasis">a</span> to <span class="docEmphasis">z.</span>
    </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">\</span>
    </td>
    <td class="docTableCell" vAlign="top">Escapes or disables the metacharacter.
    </td>
  </tr>
</table>

<p class="docText"><b>The Asterisk.</b> The asterisk is a wildcard that matches 
for zero or more of any characters in a filename.</p>
<h5 id="ch08list14" class="docExampleTitle">Example 8.14 </h5>
<pre>1   $ <span class="docEmphStrong">ls  *</span>
    <span class="docEmphasis">abc abc1 abc122 abc123 abc2 file1 file1.bak file2 file2.bak none</span>
    <span class="docEmphasis">nonsense nobody nothing nowhere one</span>
2   $ <span class="docEmphStrong">ls  *.bak</span>
    <span class="docEmphasis">file1.bak file2.bak</span>
3   $ <span class="docEmphStrong">echo a*</span>
    <span class="docEmphasis">ab abc1 abc122 abc123 abc2</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <span style="FONT-WEIGHT: bold">
    <ol class="docList" type="1">
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The asterisk expands to all of the files in the present 
      working directory. All of the files are passed as arguments to
      <span class="docEmphasis">is</span> and displayed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">All files starting with zero or more characters and 
      ending with <span class="docEmphasis">.bak</span> are matched and listed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">All files starting with <span class="docEmphasis">a,</span> 
      followed by zero or more characters, are matched and passed as arguments 
      to the <span class="docEmphasis">echo</span> command.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>The Question Mark.</b> The question mark represents a 
single character in a filename. When a filename contains one or more question 
marks, the shell performs filename substitution by replacing the question mark 
with the character it matches in the filename.</p>
<h5 id="ch08list15" class="docExampleTitle">Example 8.15 </h5>
<pre>1   $ <span class="docEmphStrong">ls</span>
    <span class="docEmphasis">abc  abc122  abc2  file1.bak  file2.bak  nonsense  nothing  one</span>
    <span class="docEmphasis">abc1  abc123  file1  file2  none  noone  nowhere</span>

2   $ <span class="docEmphStrong">ls a?c?</span>
    <span class="docEmphasis">abc1 abc2</span>

3   $ <span class="docEmphStrong">ls ??</span>	
    <span class="docEmphasis">?? not found</span>

4   $ <span class="docEmphStrong">echo  abc???</span>
    <span class="docEmphasis">abc122 abc123</span>

5   $ <span class="docEmphStrong">echo ??</span>
    <span class="docEmphasis">??</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <span style="FONT-WEIGHT: bold">
    <ol class="docList" type="1">
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The files in the current directory are listed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">Filenames starting with <span class="docEmphasis">a,</span> 
      followed by a single character, followed by <span class="docEmphasis">c</span> 
      and a single character, are matched and listed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">Filenames containing exactly two characters are listed 
      if found. Since there are no two-character files, the question marks are 
      treated as a literal filename.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">Filenames starting with <span class="docEmphasis">abc</span> 
      and followed by exactly three characters are expanded and displayed by the
      <span class="docEmphasis">echo</span> command.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">There are no files in the directory that contain 
      exactly two characters. The shell treats the question mark as a literal 
      question mark if it cannot find a match.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>The Square Brackets.</b> The brackets are used to match 
filenames containing <span class="docEmphasis">one</span> character in a set or 
range of characters.</p>
<h5 id="ch08list16" class="docExampleTitle">Example 8.16 </h5>
<pre>1   $ <span class="docEmphStrong">ls</span>
    <span class="docEmphasis">abc  abc122 abc2 file1.bak file2.bak nonsense nothing</span>
    <span class="docEmphasis">one  abc1 abc123 file1 file2 none noone nowhere</span>

2   $ <span class="docEmphStrong">ls abc[123]</span>
    <span class="docEmphasis">abc1  abc2</span>

3   $ <span class="docEmphStrong">ls abc[1