ch05_02.htm

unix基础教程

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<a name="unut-ch-5-sect-2.4" /><div class="sect2"><h3 class="sect2">5.2.4. Command Forms</h3><p>
<a name="IXT-5-122921" />
<a name="IXT-5-122922" /></p>

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

<tr><td><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&amp;</tt></td><td>Execute <em class="emphasis">cmd</tt></em></tt> in background.</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">;</tt> <tt><em class="replaceable">cmd2</em></tt></td><td><p>Command sequence; execute multiple <em class="emphasis">cmd</tt></em></tt> s on the same line.</p>
</td></tr>
<tr><td><tt class="literal">(</tt><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">;</tt> <tt><em class="replaceable">cmd2</tt></em></tt><tt class="literal">)</tt></td><td><p>Subshell;
treat <em class="emphasis">cmd1</em></tt> and <em class="emphasis">cmd2</em></tt> as a command group.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">|</tt> <tt><em class="replaceable">cmd2</em></tt></td><td><p>Pipe; use output from <em class="emphasis">cmd1</em></tt> as input to <em class="emphasis">cmd2</tt></em></tt>.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">&#8216;</tt><tt><em class="replaceable">cmd2</tt></em></tt><tt class="literal">&#8216;</tt></td><td><p>Command substitution;
use <em class="emphasis">cmd2</em></tt> output as arguments to <em class="emphasis">cmd1</em></tt>.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">&amp;&amp;</tt> <tt><em class="replaceable">cmd2</em></tt></td><td><p>AND; execute <em class="emphasis">cmd1</em></tt> and then (if <em class="emphasis">cmd1</tt></em></tt> succeeds)
<em class="emphasis">cmd2</em></tt>.
This is a &#8220;short-circuit&#8221; operation;
<em class="emphasis">cmd2</em></tt> is never executed if <em class="emphasis">cmd1</em></tt> fails.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">||</tt> <tt><em class="replaceable">cmd2</em></tt></td><td><p>OR; execute either <em class="emphasis">cmd1</em></tt> or (if <em class="emphasis">cmd1</tt></em></tt> fails)
<em class="emphasis">cmd2</em></tt>.
This is a &#8220;short-circuit&#8221; operation;
<em class="emphasis">cmd2</em></tt> is never executed if <em class="emphasis">cmd1</em></tt> succeeds.</p>
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</table><p>

<a name="unut-ch-5-sect-2.4.1" /><div class="sect3"><h3 class="sect3">5.2.4.1. Examples</h3><blockquote><pre class="code">% <tt class="userinput"><b>nroff file &gt; file.out &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>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>
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<a name="unut-ch-5-sect-2.5" /><div class="sect2"><h3 class="sect2">5.2.5. Redirection Forms</h3><p>
<a name="IXT-5-122923" />
<a name="IXT-5-122924" /></p>
<table border="1" cellpadding="3"><tr><th><em class="emphasis">File Desciptor</em></tt></th><th><em class="emphasis">Name</em></tt></th><th><em class="emphasis">Common Abbreviation</em></tt></th><th><em class="emphasis">Typical Default</em></tt></th></tr>

<tr><td>0</td><td>Standard input</td><td>stdin</td><td>Keyboard</td></tr>
<tr><td>1</td><td>Standard output</td><td>stdout</td><td>Terminal</td></tr>
<tr><td>2</td><td>Standard error</td><td>stderr</td><td>Terminal</td></tr>
</table><p>
<p>The usual input source or output destination can
be changed, as seen in the following sections.</p>

<a name="unut-ch-5-sect-2.5.1" /><div class="sect3"><h3 class="sect3">5.2.5.1. Simple redirection</h3><dl><dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;</tt> <tt><em class="replaceable">file</em></tt></i></dt><dd>Send output of <em class="emphasis">cmd</em></tt> to <em class="emphasis">file</tt></em></tt> (overwrite).</p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;!</tt> <tt><em class="replaceable">file</tt></em></tt></i></dt><dd>Same as above, even if <tt class="literal">noclobber</tt> is set.</p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&gt;</tt> <tt><em class="replaceable">file</em></tt></i></dt><dd>Send output of <em class="emphasis">cmd</em></tt> to <em class="emphasis">file</tt></em></tt> (append).</p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&gt;!</tt> <tt><em class="replaceable">file</em></tt></i></dt><dd>Same as above, but write to <em class="emphasis">file</tt></em></tt> even if
<tt class="literal">noclobber</tt> is set.</p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&lt;</tt> <tt><em class="replaceable">file</em></tt></i></dt><dd>Take input for <em class="emphasis">cmd</em></tt> from <em class="emphasis">file</tt></em></tt>.</p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&lt;&lt;</tt> <tt><em class="replaceable">text</em></tt></i></dt><dd>Read standard input up to a line identical to
<em class="emphasis">text</em></tt> (<em class="emphasis">text</tt></em></tt> can be stored in a shell
variable).  Input is usually typed at the terminal or in the shell
program. Commands that typically use this syntax include <tt class="literal">cat</tt>,
<tt class="literal">echo</tt>, <tt class="literal">ex</tt>, and <tt class="literal">sed</tt>.
If <em class="emphasis">text</em></tt> is quoted (using any of the shell-quoting
mechanisms), the input is passed through verbatim.</p>
</dd>
</dl>
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<a name="unut-ch-5-sect-2.5.2" /><div class="sect3"><h3 class="sect3">5.2.5.2. Multiple redirection</h3>
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<tr><td><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&amp;</tt> <tt><em class="replaceable">file</em></tt></td><td>Send both standard output and standard error to <em class="emphasis">file</tt></em></tt>.</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&amp;!</tt> <tt><em class="replaceable">file</tt></em></tt></td><td>Same as above, even if <tt class="literal">noclobber</tt> is set.</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&gt;&amp;</tt> <tt><em class="replaceable">file</tt></em></tt></td><td><p>Append standard output and standard error
to end of <em class="emphasis">file</em></tt>.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&gt;&amp;!</tt> <tt><em class="replaceable">file</em></tt></td><td><p>Same as above, but append to or create <em class="emphasis">file</tt></em></tt> even if
<tt class="literal">noclobber</tt> is set.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">|&amp;</tt> <tt><em class="replaceable">cmd2</tt></em></tt></td><td>Pipe standard error together with standard output.</td></tr>
<tr><td><tt class="literal">(</tt><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;</tt> <tt><em class="replaceable">f1</em></tt><tt class="literal">) &gt;&amp;</tt> <tt><em class="replaceable">f2</em></tt></td><td><p>Send standard output to file <em class="emphasis">f1</tt></em></tt>,
standard error to file <em class="emphasis">f2</em></tt>.</p>
</td></tr>
<tr><td><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">| tee</tt> <tt><em class="replaceable">files</em></tt></td><td><p>Send output of <em class="emphasis">cmd</tt></em></tt> to standard output (usually the terminal)
and to <em class="emphasis">files</em></tt>.  (See the Example in
<a href="ch02_01.htm">Chapter 2</a>,
under <b class="emphasis-bold">tee</b>.)</p>
</td></tr>
</table><p>
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<a name="unut-ch-5-sect-2.5.3" /><div class="sect3"><h3 class="sect3">5.2.5.3. Examples</h3><blockquote><pre class="code">% <tt class="userinput"><b>cat part1 &gt; book</b></tt>
% <tt class="userinput"><b>cat part2 part3 &gt;&gt; book</b></tt>
% <tt class="userinput"><b>mail tim &lt; report</b></tt>
% <tt class="userinput"><b>cc calc.c &gt;&amp; error_out</b></tt>
% <tt class="userinput"><b>cc newcalc.c &gt;&amp;! error_out</b></tt>
% <tt class="userinput"><b>grep Unix ch* |&amp; pr</b></tt>
% <tt class="userinput"><b>(find / -print &gt; filelist) &gt;&amp; no_access</b></tt>

% <tt class="userinput"><b>sed 's/^/XX /g' &lt;&lt; "END_ARCHIVE"</b></tt>
<tt class="userinput"><b>This is often how a shell archive is "wrapped",</b></tt>
<tt class="userinput"><b>bundling text for distribution.  You would normally</b></tt>
<tt class="userinput"><b>run sed from a shell program, not from the command line.</b></tt>


<tt class="userinput"><b>"END_ARCHIVE"</b></tt>
XX This is often how a shell archive is "wrapped",
XX bundling text for distribution.  You would normally
XX run sed from a shell program, not from the command line.</pre></blockquote>
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