_chapter 9.htm

linux、unix初学者的必读书籍 详细讲述了shell编程方法与技巧

      <p class="docList">The alias for the <span class="docEmphasis">more</span> 
      command is set to <span class="docEmphasis">mroe.</span> This is handy if 
      you can't spell.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The alias definition is enclosed in quotes because of 
      the whitespace. The alias <span class="docEmphasis">lF</span> is a 
      nickname for the command <span class="docEmphasis">ls 朼lF.</span></span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">When <span class="docEmphasis">cd</span> is executed, 
      the alias for <span class="docEmphasis">cd</span> will cause
      <span class="docEmphasis">cd</span> to go to the directory named as an 
      argument and will then reset the prompt to the current working directory 
      followed by the string <span class="docEmphasis">&gt;.</span> The
      <span class="docEmphasis">!*</span> is used by the alias in the same way 
      it is used by the history mechanism. The backslash prevents the history 
      mechanism from evaluating the <span class="docEmphasis">!*</span> first 
      before the alias has a chance to use it. The <span class="docEmphasis">\!*</span> 
      represents the arguments from the most recent command on the history list.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>Deleting Aliases.</b> The <span class="docEmphasis">
unalias</span> command is used to delete an alias. To temporarily turn off an 
alias, the alias name is preceded by a backslash.</p>
<h5 id="ch09list22" class="docExampleTitle">Example 9.22 </h5>
<pre>1   % <span class="docEmphStrong">unalias mroe</span>
2   % <span class="docEmphStrong">\cd ..</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 <span class="docEmphasis">unalias</span> command 
      deletes the alias <span class="docEmphasis">mroe</span> from the list of 
      defined aliases.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The alias <span class="docEmphasis">cd</span> is 
      temporarily turned off for this execution of the command only.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>Alias Loop.</b> An alias loop occurs when an alias 
definition references another alias that references back to the original alias.</p>
<h5 id="ch09list23" class="docExampleTitle">Example 9.23 </h5>
<pre>1   % alias m more
2   % alias mroe m
3   % alias m mroe         <span class="docEmphasis"># Causes a loop</span>
4   % <span class="docEmphStrong">m datafile</span>
    <span class="docEmphasis">Alias loop.</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 alias is <span class="docEmphasis">m.</span> The 
      alias definition is <span class="docEmphasis">more.</span> Every time
      <span class="docEmphasis">m</span> is used, the <span class="docEmphasis">
      more</span> command is executed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The alias is <span class="docEmphasis">mroe.</span> The 
      alias definition is <span class="docEmphasis">m.</span> If
      <span class="docEmphasis">mroe</span> is typed, the alias
      <span class="docEmphasis">m</span> is invoked and the
      <span class="docEmphasis">more</span> command is executed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">This is the culprit. If alias <span class="docEmphasis">
      m</span> is used, it invokes alias <span class="docEmphasis">mroe,</span> 
      and alias <span class="docEmphasis">mroe</span> references back to
      <span class="docEmphasis">m,</span> causing an alias loop. Nothing bad 
      happens. You just get an error message.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">Alias <span class="docEmphasis">m</span> is used. It is 
      circular. <span class="docEmphasis">M</span> calls
      <span class="docEmphasis">mroe</span> and <span class="docEmphasis">mroe</span> 
      calls <span class="docEmphasis">m,</span> then <span class="docEmphasis">m</span> 
      calls <span class="docEmphasis">mroe,</span> etc., etc. Rather than 
      looping forever, the C shell catches the problem and displays an error 
      message.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<h4 class="docSection2Title" id="ch09lev2sec6">9.1.6 Job Control</h4>
<p class="docText">Job control is a powerful feature of the C shell that allows 
you to run programs, called <span class="docEmphasis">jobs,</span> in the 
background or foreground. Normally, a command typed at the command line runs in 
the foreground, and will continue until it has finished. If you have windows, 
job control may not be necessary, since you can simply open another window to 
start a new task. On the other hand, with a single terminal, job control is a 
very useful feature. For a list of job commands, see
<a class="docLink" href="#ch09table01">Table 9.1</a>.</p>

<table cellSpacing="0" cellPadding="1" width="100%" border="1">
  <caption>
  <h5 id="ch09table01" class="docTableTitle">Table 9.1. Job Control Commands</h5>
  </caption>
  <colgroup span="2" align="left">
  </colgroup>
  <tr>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Command</span> </th>
    <th class="docTableHeader" vAlign="top"><span class="docEmphBoldItalic">
    Meaning</span> </th>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">jobs</span>
    </td>
    <td class="docTableCell" vAlign="top">Lists all the jobs running. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top">^Z (Control-Z) </td>
    <td class="docTableCell" vAlign="top">Stops (suspends) the job; the prompt 
    appears on the screen. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">bg</span>
    </td>
    <td class="docTableCell" vAlign="top">Starts running the stopped job in the 
    background. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">fg</span>
    </td>
    <td class="docTableCell" vAlign="top">Brings a background job to the 
    foreground. </td>
  </tr>
  <tr>
    <td class="docTableCell" vAlign="top"><span class="docEmphasis">kill</span>
    </td>
    <td class="docTableCell" vAlign="top">Sends the <span class="docEmphasis">
    kill</span> signal to a specified job. </td>
  </tr>
</table>

<p class="docText"><b>The Ampersand and Background Jobs.</b> If you expect a 
command to take a long time to complete, you can append the command with an 
ampersand and the job will execute in the background. The C shell prompt returns 
immediately and now you can type another command. Now the two commands are 
running concurrently, one in the background and one in the foreground. They both 
send their standard output to the screen. If you place a job in the background, 
it is a good idea to redirect its output either to a file or pipe it to a device 
such as a printer.</p>
<h5 id="ch09list24" class="docExampleTitle">Example 9.24 </h5>
<pre>1   % <span class="docEmphStrong">find . -name core -exec rm {} \; &amp;</span>
2   <span class="docEmphasis">[1]  543</span>
3   %
</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 <span class="docEmphasis">find</span> command runs 
      in the background. (Without the <span class="docEmphasis">杙rint</span> 
      option, the <span class="docEmphasis">find</span> command does not send 
      any output to the screen).<span id="ENB9-3"><a class="docLink" href="#EN9-3"><sup>[3]</sup></a></span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The number in square brackets indicates this is the 
      first job to be run in the background and the PID for this process is
      <span class="docEmphasis">543.</span></span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The prompt returns immediately. The shell waits for 
      user input.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>The Suspend Key Sequence and Background Jobs.</b> To 
suspend a program, the suspend key sequence, ^Z, is issued. The job is now 
suspended (stopped), the shell prompt is displayed, and the program will not 
resume until the <span class="docEmphasis">fg</span> or
<span class="docEmphasis">bg</span> commands are issued. (When using the
<span class="docEmphasis">vi</span> editor, the <span class="docEmphasis">ZZ</span> 
command writes and saves a file. Do not confuse this with ^Z, which would 
suspend the <span class="docEmphasis">vi</span> session.) If you try to log out 
when a job is suspended, the message <span class="docEmphasis">There are stopped 
jobs</span> appears on the screen.</p>
<p class="docText"><span class="docEmphStrong">The <span class="docEmphasis">
jobs</span> Command.</span> The C shell built-in command,
<span class="docEmphasis">jobs,</span> displays the programs that are currently 
active and either running or suspended in the background.
<span class="docEmphasis">Running</span> means the job is executing in the 
background. When a job is <span class="docEmphasis">stopped,</span> it is 
suspended; it is not in execution. In both cases, the terminal is free to accept 
other commands.</p>
<h5 id="ch09list25" class="docExampleTitle">Example 9.25 </h5>
<pre>(The Command Line)
1   % <span class="docEmphStrong">jobs</span>
2   <span class="docEmphasis">[1] + Stopped vi filex</span>
    <span class="docEmphasis">[2] - Running sleep 25</span>

3   % <span class="docEmphStrong">jobs -l</span>
    <span class="docEmphasis">[1] + 355  Stopped vi filex</span>
    <span class="docEmphasis">[2] - 356  Running sleep 25</span>

4   <span class="docEmphasis">[2] Done sleep 25</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 <span class="docEmphasis">jobs</span> command lists 
      the currently active jobs.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The notation <span class="docEmphasis">[1]</span> is 
      the number of the first job; the plus sign indicates that the job is not 
      the most recent job to be placed in the background; the dash indicates 
      that this is the most recent job put in the background;
      <span class="docEmphasis">Stopped</span> means that this job was suspended 
      with ^Z and is not currently active.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The <span class="docEmphasis">杔</span> option (long 
      listing) displays the number of the job as well as the PID of the job. The 
      notation <span class="docEmphasis">[2]</span> is the number of the second 
      job, in this case, the last job placed in the background. The dash 
      indicates that this is the most recent job. The <span class="docEmphasis">
      sleep</span> command is running in the background.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">After <span class="docEmphasis">sleep</span> has been 
      running for <span class="docEmphasis">25</span> seconds, the job will 
      complete and a message saying that it