ch04_02.htm

unix基础教程

allows it to be used literally, as itself.
The following table displays characters have special meaning to the Bourne and
Korn shells.</p>
<table border="1" cellpadding="3">
<tr><th><em class="emphasis">Character</em></th><th><em class="emphasis">Meaning</em></th></tr>

<tr><td><p><tt class="literal">;</tt></p>
</td><td><p>Command separator</p>
</td></tr>
<tr><td><p><tt class="literal">&amp;</tt></p>
</td><td><p>Background execution</p>
</td></tr>
<tr><td><p><tt class="literal">( )</tt></p>
</td><td><p>Command grouping</p>
</td></tr>
<tr><td><p><tt class="literal">|</tt></p>
</td><td><p>Pipe</p>
</td></tr>
<tr><td><p><tt class="literal">&lt; &gt; &amp;</tt></p>
</td><td><p>Redirection symbols</p>
</td></tr>
<tr><td><p><tt class="literal">* ? [ ] ~ + - @ !</tt></p>
</td><td><p>Filename metacharacters</p>
</td></tr>
<tr><td><p><tt class="literal">" ' \</tt></p>
</td><td><p>Used in quoting other characters</p>
</td></tr>
<tr><td><p><tt class="literal">&#8216;</tt></p>
</td><td><p>Command substitution</p>
</td></tr>
<tr><td><p><tt class="literal">$</tt></p>
</td><td><p>Variable substitution (or command or arithmetic substitution)</p>
</td></tr>
<tr><td><p><tt class="literal">space</tt>
<tt class="literal">tab</tt>
<tt class="literal">newline</tt></p>
</td><td><p>Word separators</p>
</td></tr>
</table><p>
<p>These characters can be used for quoting:</p>
<dl><dt><tt class="literal">" "</tt></dt><dd>Everything between <tt class="literal">"</tt>
and <tt class="literal">"</tt> is
taken literally,
except for the following characters that keep their special meaning:
<a name="IXT-4-122679" /></p>

<dl><dt><tt class="literal">$</tt></dt><dd>Variable (or Korn shell command and arithmetic) substitution will occur.</p>
</dd>
<dt><tt class="literal">&#8216;</tt></dt><dd>Command substitution will occur.</p>
</dd>
<dt><tt class="literal">"</tt></dt><dd>This marks the end of the double quote.</p>
</dd>
</dl>

</dd>
<dt><tt class="literal">' '</tt></dt><dd>
<a name="IXT-4-122680" />
<a name="IXT-4-122681" />Everything between <tt class="literal">'</tt>
and <tt class="literal">'</tt>
is taken literally except
for another <tt class="literal">'</tt>.
You cannot embed another <tt class="literal">'</tt> within such a quoted string.</p>
</dd>
<dt><tt class="literal">\</tt></dt><dd>
<a name="IXT-4-122682" />The character following a <tt class="literal">\</tt> is taken literally.
Use within <tt class="literal">" "</tt>
to escape <tt class="literal">"</tt>,
<tt class="literal">$</tt>,
and <tt class="literal">&#8216;</tt>.
Often used to escape itself, spaces, or newlines.</p>
</dd>
<dt><tt class="literal">$" "</tt></dt><dd><tt class="literal">ksh93</tt> only.
Just like <tt class="literal">""</tt>, except that locale
translation is done.</p>
</dd>
<dt><tt class="literal">$' '</tt></dt><dd><tt class="literal">ksh93</tt> only.
Similar to <tt class="literal">''</tt>, but the quoted
text is processed for the following escape sequences:
</p></dd>
</dl>

<table border="1" cellpadding="3">
<tr><th><em class="emphasis">Sequence</em></th>
<th><em class="emphasis">Value</em></th><th><em class="emphasis">Sequence</em></th><th><em class="emphasis">Value</em></th></tr>

<tr><td><tt class="literal">\a</tt></td>
<td>Alert</td><td><tt class="literal">\</tt><em class="replaceable"><tt>nnn</em></td><td>Octal value <em class="emphasis">nnn</tt></em></td></tr>
<tr><td><tt class="literal">\b</tt></td>
<td>Backspace</td><td><tt class="literal">\x</tt><em class="replaceable"><tt>nn</em></td><td>Hexadecimal value <em class="emphasis">nn</tt></em></td></tr>
<tr><td><tt class="literal">\f</tt></td>
<td>Form feed</td><td><tt class="literal">\'</tt></td><td>Single quote</td></tr>
<tr><td><tt class="literal">\n</tt></td>
<td>Newline</td><td><tt class="literal">\"</tt></td><td>Double quote</td></tr>
<tr><td><tt class="literal">\r</tt></td>
<td>Carriage return</td><td><tt class="literal">\\</tt></td><td>Backslash</td></tr>
<tr><td><tt class="literal">\t</tt></td>
<td>Tab</td><td><tt class="literal">\E</tt></td><td>Escape</td></tr>
<tr><td><tt class="literal">\v</tt></td>
<td>Vertical tab</td><td></td><td>&nbsp;</td></tr>
</table><p>

<a name="unut-ch-4-sect-2.3.1" /><div class="sect3"><h3 class="sect3">4.2.3.1. Examples</h3><blockquote><pre class="code">$ <tt class="userinput"><b>echo 'Single quotes "protect" double quotes'</b></tt>
Single quotes "protect" double quotes
$ <tt class="userinput"><b>echo "Well, isn't that \"special\"?"</b></tt>
Well, isn't that "special"?
$ <tt class="userinput"><b>echo "You have `ls | wc -l` files in `pwd`"</b></tt>
You have      43 files in /home/bob
$ <tt class="userinput"><b>echo "The value of \$x is $x"</b></tt>
The value of $x is 100</pre></blockquote>
</div>
</div>

<a name="unut-ch-4-sect-2.4" /><div class="sect2"><h3 class="sect2">4.2.4. Command Forms</h3>
<a name="IXT-4-122683" />
<a name="IXT-4-122684" />
<a name="IXT-4-122685" />
<a name="IXT-4-122686" />
<a name="IXT-4-122687" />
<a name="IXT-4-122688" />
<a name="IXT-4-122689" />
<a name="IXT-4-122690" />
<a name="IXT-4-122691" />

<table border="1" cellpadding="3">

<tr><td><p><em class="replaceable"><tt>cmd</tt></em> <tt class="literal">&amp;</tt></p>
</td><td><p>Execute <em class="emphasis">cmd</em> in background.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd1</em> <tt class="literal">;</tt> <em class="replaceable">cmd2</tt></em></p>
</td><td><p>Command sequence; execute multiple <em class="emphasis">cmd</em>s on the same line.</p>
</td></tr>
<tr><td><p><tt class="literal">{</tt> <em class="replaceable"><tt>cmd1</em> <tt class="literal">;</tt> <em class="replaceable">cmd2</tt></em> <tt class="literal">; }</tt></p>
</td><td><p>Execute commands as a group in the current shell.</p>
</td></tr>
<tr><td><p><tt class="literal">(</tt><em class="replaceable"><tt>cmd1</em> <tt class="literal">;</tt> <em class="replaceable">cmd2</tt></em><tt class="literal">)</tt></p>
</td><td><p>Execute commands as a group in a subshell.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd1</em> <tt class="literal">|</tt> <em class="replaceable">cmd2</tt></em></p>
</td><td><p>Pipe; use output from <em class="emphasis">cmd1</em> as input to <em class="emphasis">cmd2</em>.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd1</em> <tt class="literal">&#8216;</tt><em class="replaceable">cmd2</tt></em><tt class="literal">&#8216;</tt></p>
</td><td><p>Command substitution; use <em class="emphasis">cmd2</em> output as arguments to <em class="emphasis">cmd1</em>.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd1</em> <tt class="literal">$(</tt><em class="replaceable">cmd2</tt></em><tt class="literal">)</tt></p>
</td><td><p>Korn shell command substitution; nesting is allowed.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd</em>   <tt class="literal">$((</tt><em class="replaceable">expression</tt></em><tt class="literal">))</tt></p>
</td><td><p>Korn shell arithmetic substitution.
Use the result of <em class="emphasis">expression</em>
as argument to <em class="emphasis">cmd</em>.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd1</em> <tt class="literal">&amp;&amp;</tt> <em class="replaceable">cmd2</tt></em></p>
</td><td><p>AND; execute <em class="emphasis">cmd1</em> and then
(if <em class="emphasis">cmd1</em> succeeds)
<em class="emphasis">cmd2</em>.
This is a &#8220;short-circuit&#8221; operation;
<em class="emphasis">cmd2</em> is never executed if <em class="emphasis">cmd1</em> fails.</p>
</td></tr>
<tr><td><p><em class="replaceable"><tt>cmd1</em> <tt class="literal">||</tt> <em class="replaceable">cmd2</tt></em></p>
</td><td><p>OR; execute either <em class="emphasis">cmd1</em> or (if <em class="emphasis">cmd1</em> fails) <em class="emphasis">cmd2</em>.
This is a &#8220;short-circuit&#8221; operation;
<em class="emphasis">cmd2</em> is never executed if <em class="emphasis">cmd1</em> succeeds.</p>
</td></tr>
</table><p>

<a name="IXT-4-122692" />
<a name="IXT-4-122693" />
<a name="unut-ch-4-sect-2.4.1" /><div class="sect3"><h3 class="sect3">4.2.4.1. Examples</h3><blockquote><pre class="code">$ <tt class="userinput"><b>nroff file &gt; file.txt &amp;</b></tt>             <i class="lineannotation">Format in the background</i>
$ <tt class="userinput"><b>cd; ls</b></tt>                              <i class="lineannotation">Execute sequentially</i>
$ <tt class="userinput"><b>(date; who; pwd) &gt; logfile</b></tt>          <i class="lineannotation">All output is redirected</i>
$ <tt class="userinput"><b>sort file | pr -3 | lp</b></tt>              <i class="lineannotation">Sort file, page output, then print</i>
$ <tt class="userinput"><b>vi `grep -l ifdef *.c`</b></tt>              <i class="lineannotation">Edit files found by grep</i>
$ <tt class="userinput"><b>egrep '(yes|no)' `cat list`</b></tt>         <i class="lineannotation">Specify a list of files to search</i>
$ <tt class="userinput"><b>egrep '(yes|no)' $(cat list)</b></tt>        <i class="lineannotation">Korn shell version of previous</i>
$ <tt class="userinput"><b>egrep '(yes|no)' $(&lt;list)</b></tt>           <i class="lineannotation">Same, but faster</i>
$ <tt class="userinput"><b>grep XX file &amp;&amp; lp file</b></tt>             <i class="lineannotation">Print file if it contains the pattern;</i>
$ <tt class="userinput"><b>grep XX file || echo "XX not found"</b></tt> <i class="lineannotation">otherwise, echo an error message</i></pre></blockquote>
</div>
</div>

<a name="unut-ch-4-sect-2.5" /><div class="sect2"><h3 class="sect2">4.2.5. Redirection Forms</h3><p>
<a name="IXT-4-122694" />
<a name="IXT-4-122695" />
<a name="IXT-4-122696" /></p>
<table border="1" cellpadding="3"><tr><th><em class="emphasis">File Descriptor</em></th><th><em class="emphasis">Name</em></th><th><em class="emphasis">Common Abbreviation</em></th><th><em class="emphasis">Typical Default</em></th></tr>

<tr><td><p>0</p>
</td><td><p>Standard input</p>
</td><td><p>stdin</p>
</td><td><p>Keyboard</p>
</td></tr>
<tr><td><p>1</p>
</td><td><p>Standard output</p>
</td><td><p>stdout</p>
</td><td><p>Terminal</p>
</td></tr>
<tr><td><p>2</p>