_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

variable, has been changed since you logged on, you can use the
<span class="docEmphasis">dot</span> command to re-execute the
<span class="docEmphasis">.profile</span> without logging out and then logging 
back in.</p>
<h5 id="ch08list07" class="docExampleTitle">Example 8.7 </h5>
<pre>$ <span class="docEmphStrong">. .profile</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <p class="docText">The <span class="docEmphasis">dot</span> command executes 
    the initialization file, <span class="docEmphasis">.profile,</span> within 
    this shell. Local and global variables are redefined within this shell. The
    <span class="docEmphasis">dot</span> command makes it unnecessary to log out 
    and then log back in again.<span id="ENB8-1"><a class="docLink" href="#EN8-1"><sup>[1]</sup></a></span></td>
  </tr>
</table>

<h4 class="docSection2Title" id="ch08lev2sec2">8.1.2 The Command Line</h4>
<p class="docText">After logging on, the shell displays its primary prompt, a 
dollar sign, by default. The shell is your command interpreter. When the shell 
is running interactively, it reads commands from the terminal and breaks the 
command line into words. A command line consists of one or more words (tokens), 
separated by whitespace (blanks and/or tabs), and terminated with a newline, 
which is generated by pressing Enter. The first word is the command and 
subsequent words are the command's arguments. The command may be a UNIX 
executable program such as <span class="docEmphasis">ls</span> or
<span class="docEmphasis">pwd,</span> a built-in command such as
<span class="docEmphasis">cd</span> or <span class="docEmphasis">test,</span> or 
a shell script. The command may contain special characters, called
<span class="docEmphasis">metacharacters,</span> that the shell must interpret 
while parsing the command line. If a command line is long and you want to 
continue typing on the next line, the backslash character, followed by a newline, 
will allow you to continue typing on the next line. The secondary prompt will 
appear until the command line is terminated.</p>
<p class="docText"><b>The Exit Status.</b> After a command or program 
terminates, it returns an exit status to the parent process. The exit status is 
a number between 0 and 255. By convention, when a program exits, if the status 
returned is zero, the command was successful in its execution. When the exit 
status is nonzero, the command failed in some way. The shell status variable,
<span class="docEmphasis">?,</span> is set to the value of the exit status of 
the last command that was executed. Success or failure of a program is 
determined by the programmer who wrote the program.</p>
<h5 id="ch08list08" class="docExampleTitle">Example 8.8 </h5>
<pre>1   $ grep &quot;john&quot; /etc/passwd
    <span class="docEmphasis">john:MgVyBsZJavd16s:9496:40:John Doe:/home/falcon/john:/bin/sh</span>
2   $ <span class="docEmphStrong">echo $?</span>
    <span class="docEmphasis">0</span>
3   $ grep &quot;nicky&quot; /etc/passwd
4   $ <span class="docEmphStrong">echo $?</span>
    <span class="docEmphasis">1</span>
5   $ grep &quot;scott&quot; /etc/passsswd
    <span class="docEmphasis">grep: /etc/passsswd: No such file or directory</span>
6   $ <span class="docEmphStrong">echo $?</span>
    <span class="docEmphasis">2</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">grep</span> program 
      searches for the pattern <span class="docEmphasis">john</span> in the
      <span class="docEmphasis">/etc/passwd</span> file and is successful. The 
      line from <span class="docEmphasis">/etc/passwd</span> is displayed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The <span class="docEmphasis">?</span> variable is set 
      to the exit value of the <span class="docEmphasis">grep</span> command. 
      Zero indicates success.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The <span class="docEmphasis">grep</span> program 
      cannot find user <span class="docEmphasis">nicky</span> in the
      <span class="docEmphasis">/etc/passwd</span> file.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">If the <span class="docEmphasis">grep</span> program 
      cannot find the pattern, it returns an exit status of
      <span class="docEmphasis">1.</span></span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The <span class="docEmphasis">grep</span> fails because 
      the <span class="docEmphasis">/etc/passsswd</span> file cannot be opened.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">If <span class="docEmphasis">grep</span> cannot find 
      the file, it returns an exit status of <span class="docEmphasis">2.</span></span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>Multiple Commands at the Command Line.</b> A command line 
can consist of multiple commands. Each command is separated by a semicolon, and 
the command line is terminated with a newline.</p>
<h5 id="ch08list09" class="docExampleTitle">Example 8.9 </h5>
<pre>$ <span class="docEmphStrong">ls; pwd; date</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <p class="docText">The commands are executed from left to right, one after 
    the other, until the newline is reached.</td>
  </tr>
</table>

<p class="docText"><b>Grouping Commands.</b> Commands may also be grouped so 
that all of the output is either piped to another command or redirected to a 
file.</p>
<h5 id="ch08list10" class="docExampleTitle">Example 8.10 </h5>
<pre>$ <span class="docEmphStrong">( ls ; pwd; date ) &gt; outputfile</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <p class="docText">The output of each of the commands is sent to the file 
    called <span class="docEmphasis">outputfile.</span> The spaces inside the 
    parentheses are necessary.</td>
  </tr>
</table>

<p class="docText"><b>Conditional Execution of Commands.</b> With conditional 
execution, two command strings are separated by the special metacharacters, 
double ampersands (<span class="docEmphasis">&amp;&amp;</span>) and double vertical bars 
(||). The command on the right of either of these metacharacters will or will 
not be executed based on the exit condition of the command on the left.</p>
<h5 id="ch08list11" class="docExampleTitle">Example 8.11 </h5>
<pre>$ <span class="docEmphStrong">cc prgm1.c 杘 prgm1 &amp;&amp; prgm1</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <p class="docText">If the first command is successful (has a zero exit 
    status), the command after the <span class="docEmphasis">&amp;&amp;</span> is 
    executed; i.e., if the <span class="docEmphasis">cc</span> program can 
    successfully compile <span class="docEmphasis">prgm1.c,</span> the resulting 
    executable program, <span class="docEmphasis">prgm1,</span> will be 
    executed.</td>
  </tr>
</table>

<h5 id="ch08list12" class="docExampleTitle">Example 8.12 </h5>
<pre>$ <span class="docEmphStrong">cc prog.c &gt;&amp; err || mail bob &lt; err</span>
</pre>

<table cellSpacing="0" width="90%" border="1" align="center">
  <tr>
    <td>
    <h2 class="docSidebarTitle">EXPLANATION</h2>
    <p class="docText">If the first command fails (has a nonzero exit status), 
    the command after the <span class="docEmphasis">||</span> is executed; i.e., 
    if the <span class="docEmphasis">cc</span> program cannot compile
    <span class="docEmphasis">prog.c,</span> the errors are sent to a file 
    called <span class="docEmphasis">err,</span> and user
    <span class="docEmphasis">bob</span> will be mailed the
    <span class="docEmphasis">err</span> file.</td>
  </tr>
</table>

<p class="docText"><b>Commands in the Background.</b> Normally, when you execute 
a command, it runs in the foreground, and the prompt does not reappear until the 
command has completed execution. It is not always convenient to wait for the 
command to complete. By placing an ampersand (<span class="docEmphasis">&amp;</span>) 
at the end of the command line, the shell will return the shell prompt 
immediately and execute the command in the background concurrently. You do not 
have to wait to start up another command. The output from a background job will 
be sent to the screen as it processes. Therefore, if you intend to run a command 
in the background, the output of that command might be redirected to a file or 
piped to another device, such as a printer, so that the output does not 
interfere with what you are doing.</p>
<p class="docText">The <span class="docEmphasis">$!</span> variable contains the 
PID number of the last job put in the background.</p>
<h5 id="ch08list13" class="docExampleTitle">Example 8.13 </h5>
<pre>1   $ <span class="docEmphStrong">man sh | lp&amp;</span>
2   <span class="docEmphasis">[1] 1557</span>
3   $ <span class="docEmphStrong">kill -9 $!</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 output of the <span class="docEmphasis">man</span> 
      command (the manual pages for the <span class="docEmphasis">sh</span> 
      command) is piped to the printer. The ampersand at the end of the command 
      line puts the job in the background.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">There are two numbers that appear on the screen: the 
      number in square brackets indicates that this is the first job to be 
      placed in the background; the second number is the PID, or the process 
      identification number of this job.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The shell prompt appears immediately. While your 
      program is running in the background, the shell is waiting for another 
      command in the foreground.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The <span class="docEmphasis">!</span> variable 
      evaluates to the PID of the job most recently put in the background. If 
      you get it in time, you will kill this job before it goes to the print 
      queue.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<h4 class="docSection2Title" id="ch08lev2sec3">8.1.3 Metacharacters (Wildcards)</h4>
<p class="docText">Metacharacters are special characters used to represent 
something other than themselves. Shell metacharacters are called
<span class="docEmphasis">wildcards.</span>
<a class="docLink" href="#ch08table01">Table 8.1</a> lists metacharacters and 
what they do.</p>

<table cellSpacing="0" cellPadding="1" width="100%" border="1">
  <caption>
  <h5 id="ch08table01" class="docTableTitle">Table 8.1. Shell Metacharacters</h5>
  </caption>