ch04_02.htm

unix基础教程

</td><td><p>Standard error</p>
</td><td><p>stderr</p>
</td><td><p>Terminal</p>
</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-4-sect-2.5.1" /><div class="sect3"><h3 class="sect3">4.2.5.1. 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).
<a name="IXT-4-122697" /></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).
<a name="IXT-4-122698" /></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>.
<a name="IXT-4-122699" /></p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&lt;&lt;</tt> <tt><em class="replaceable">text</tt></em></tt></i></dt><dd>
<a name="IXT-4-122700" />The contents of the shell script
up to a line identical to <em class="emphasis">text</em>
become the standard input for <em class="emphasis">cmd</em>
(<em class="emphasis">text</em> can be stored in a shell variable).
This command form is sometimes called a <em class="emphasis">Here document</em>.
Input is usually typed at the keyboard or in the shell program.
Commands that typically use this syntax
include <tt class="literal">cat</tt>,
<tt class="literal">ex</tt>,
and <tt class="literal">sed</tt>.
(If <tt class="literal">&lt;&lt;-</tt>
is used, leading tabs are ignored when comparing input with
the end-of-input <em class="emphasis">text</em> marker.)
If <em class="emphasis">text</em> is quoted, the input is passed through verbatim.
Otherwise,
the contents are processed for variable and command substitutions.
The Korn shell also processes the input for arithmetic substitution.</p>
</dd>
<dt><i><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&lt;&gt;</tt> <tt><em class="replaceable">file</tt></em></tt></i></dt><dd>Korn shell only.
<a name="IXT-4-122701" />
Open <em class="emphasis">file</em></tt> for reading <em class="emphasis">and</em></tt>
writing on the standard input.  The contents are not destroyed.<a href="#FOOTNOTE-5">[5]</a></p><blockquote class="footnote"><a name="FOOTNOTE-5" /><p> [5]With <tt class="literal">&lt;</tt>, the file is opened read-only, and
writes on the file descriptor will fail.
With <tt class="literal">&lt;&gt;</tt>, the file is opened read-write;
it is up to the application to actually take advantage of this.</p>
</blockquote>
</dd>
</dl>
</div>

<a name="unut-ch-4-sect-2.5.2" /><div class="sect3"><h3 class="sect3">4.2.5.2. Redirection using file descriptors</h3>
<table border="1" cellpadding="3">

<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&amp;</tt><tt><em class="replaceable">n</tt></em></tt> </p>
</td><td><p>Send <em class="emphasis">cmd</em></tt> output to file descriptor <em class="emphasis">n</em></tt>.</p>
</td></tr>
<tr><td><p>
<a name="IXT-4-122702" />
<a name="IXT-4-122703" /><tt><em class="replaceable"><tt>cmd</em></tt> <tt><em class="replaceable">m</em></tt><tt class="literal">&gt;&amp;</tt><tt><em class="replaceable">n</tt></em></tt></p>
</td><td><p>Same, except that output that would normally go to file
descriptor <em class="emphasis">m</em></tt> is sent to file
descriptor <em class="emphasis">n</em></tt> instead.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</tt></em></tt> <tt class="literal">&gt;&amp;-</tt></p>
</td><td><p>Close standard output.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&lt;&amp;</tt><tt><em class="replaceable">n</tt></em></tt></p>
</td><td><p>Take input for <em class="emphasis">cmd</em></tt> from file descriptor <em class="emphasis">n</em></tt>.
<a name="IXT-4-122704" /></p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt><em class="replaceable">m</em></tt><tt class="literal">&lt;&amp;</tt><tt><em class="replaceable">n</tt></em></tt></p>
</td><td><p>Same, except that input that would normally come from file
descriptor <em class="emphasis">m</em></tt> comes from file
descriptor <em class="emphasis">n</em></tt> instead.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</tt></em></tt> <tt class="literal">&lt;&amp;-</tt></p>
</td><td><p>Close standard input.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&lt;&amp;</tt><tt><em class="replaceable">n</tt></em></tt><tt class="literal">-</tt></p>
</td><td><p>Move input file descriptor <em class="emphasis">n</em></tt>
instead of duplicating it.
<tt class="literal">ksh93</tt> only.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;&amp;</tt><tt><em class="replaceable">n</tt></em></tt><tt class="literal">-</tt></p>
</td><td><p>Move output file descriptor <em class="emphasis">n</em></tt>
instead of duplicating it.
<tt class="literal">ksh93</tt> only.</p>
</td></tr>
</table><p>
</div>

<a name="unut-ch-4-sect-2.5.3" /><div class="sect3"><h3 class="sect3">4.2.5.3. Multiple redirection</h3>
<table border="1" cellpadding="3">

<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">2&gt;</tt><tt><em class="replaceable">file</tt></em></tt></p>
</td><td><p>Send standard error to <em class="emphasis">file</em></tt>; standard output remains the same (e.g., the screen).</p>
</td></tr>
<tr><td><p>
<a name="IXT-4-122705" /><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;</tt> <tt><em class="replaceable">file</tt></em></tt> <tt class="literal">2&gt;&amp;1</tt></p>
</td><td><p>Send both standard error and standard output to <em class="emphasis">file</em></tt>.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">&gt;</tt> <tt><em class="replaceable">f1</em></tt> <tt class="literal">2&gt;</tt><tt><em class="replaceable">f2</tt></em></tt></p>
</td><td><p>Send standard output to file <em class="emphasis">f1</em></tt>,
standard error to file <em class="emphasis">f2</em></tt>.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">| tee</tt> <tt><em class="replaceable">files</tt></em></tt></p>
</td><td><p>Send output of <em class="emphasis">cmd</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>
<tr><td><p><tt><em class="replaceable"><tt>cmd</em></tt> <tt class="literal">2&gt;&amp;1 | tee</tt> <tt><em class="replaceable">files</tt></em></tt></p>
</td><td><p>Send standard output and error output
of <em class="emphasis">cmd</em></tt> to standard output
(usually the terminal) and to <em class="emphasis">files</em></tt>.</p>
</td></tr>
</table><p>
<p>No space should appear between file descriptors and a redirection symbol;
spacing is optional in the other cases.</p>
</div>

<a name="unut-ch-4-sect-2.5.4" /><div class="sect3"><h3 class="sect3">4.2.5.4. 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>sed 's/^/XX /g' &lt;&lt; END_ARCHIVE</b></tt>
&gt; <tt class="userinput"><b>This is often how a shell archive is "wrapped",</b></tt>
&gt; <tt class="userinput"><b>bundling text for distribution.  You would normally</b></tt>
&gt; <tt class="userinput"><b>run sed from a shell program, not from the command line.</b></tt>
&gt; <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>
<p>To redirect standard output to standard error:</p>
<blockquote><pre class="code">$ <tt class="userinput"><b>echo "Usage error:  see administrator" 1&gt;&amp;2</b></tt></pre></blockquote>
<p>The following command
sends output (files found) to <tt class="literal">filelist</tt> and error
messages (inaccessible files) to file <tt class="literal">no_access</tt>:</p>
<blockquote><pre class="code">$ <tt class="userinput"><b>find / -print &gt; filelist 2&gt;no_access</b></tt></pre></blockquote>
</div>
</div>

<a name="unut-ch-4-sect-2.6" /><div class="sect2"><h3 class="sect2">4.2.6. Coprocesses</h3><p>
<a name="IXT-4-122706" />
<a name="IXT-4-122707" />Coprocesses are a feature of the Korn shell only.</p>

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

<tr><td><p><tt><em class="replaceable"><tt>cmd1</em></tt> <tt class="literal">|</tt> <tt><em class="replaceable">cmd2</tt></em></tt> <tt class="literal">|&amp;</tt></p>
</td><td><p>Coprocess; execute the pipeline in the background.
The shell sets up a two-way pipe, allowing redirection of
both standard input and standard output.</p>
</td></tr>
<tr><td><p><tt class="literal">read -p</tt> <tt><em class="replaceable"><tt>var</tt></em></tt></p>
</td><td><p>Read coprocess output into variable <em class="emphasis">var</em></tt>.</p>
</td></tr>
<tr><td><p><tt class="literal">print -p</tt> <tt><em class="replaceable"><tt>string</tt></em></tt></p>
</td><td><p>Write <em class="emphasis">string</em></tt> to the coprocess.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</tt></em></tt> <tt class="literal">&lt;&amp;p</tt></p>
</td><td><p>Take input for <em class="emphasis">cmd</em></tt> from the coprocess.</p>
</td></tr>
<tr><td><p><tt><em class="replaceable"><tt>cmd</tt></em></tt> <tt class="literal">&gt;&amp;p</tt></p>
</td><td><p>Send output of <em class="emphasis">cmd</em></tt> to the coprocess.</p>
</td></tr>
<tr><td><p><tt class="literal">exec</tt> <tt><em class="replaceable"><tt>n</tt></em></tt><tt class="literal">&lt;&amp;p</tt></p>
</td><td><p>Move input for coprocess to file descriptor <em class="emphasis">n</em></tt>.</p>
</td></tr>
<tr><td><p><tt class="literal">exec</tt> <tt><em class="replaceable"><tt>n</tt></em></tt><tt class="literal">&gt;&amp;p</tt></p>
</td><td><p>Move output from coprocess to file descriptor <em class="emphasis">n</em></tt>.</p>
</td></tr>
</table><p>
<p>Moving the coprocess input and output file descriptors to standard
file descriptors allows you to open multiple coprocesses.</p>

<a name="unut-ch-4-sect-2.6.1" /><div class="sect3"><h3 class="sect3">4.2.6.1. Examples</h3><blockquote><pre class="code">$
<tt class="userinput"><b>ed - memo |&amp;</b></tt>		        <i class="lineannotation">Start coprocess</i>
$ <tt class="userinput"><b>print -p /word/</b></tt>		<i class="lineannotation">Send ed command to coprocess</i>
$ <tt class="userinput"><b>read -p search</b></tt>		<i class="lineannotation">Read output of ed command into variable search</i>
$ <tt class="userinput"><b>print "$search"</b></tt>		<i class="lineannotation">Show the line on standard output</i>
A word to the wise.</pre></blockquote>
</div>
</div>


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