_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 user will be logged out.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>The Search Path.</b> The <span class="docEmphasis">path</span> 
variable is used by the shell to locate commands typed at the command line. The 
search is from left to right. The dot represents the current working directory. 
If the command is not found in any of the directories listed in the path, or in 
the present working directory, the shell sends the message
<span class="docEmphasis">Command not found</span> to standard error. It is 
recommended that the path be set in the <span class="docEmphasis">.login</span> 
file.<span id="ENB9-1"><a class="docLink" href="#EN9-1"><sup>[1]</sup></a></span> 
The search path is set differently in the C shell than it is in the Bourne and 
Korn shells. Each of the elements is separated by whitespace. </p>

<pre>set path = (/usr/bin /usr/ucb /bin /usr .)
echo $path
/usr/bin /usr/ucb /bin /usr .
</pre>
<p class="docText">The environment variable <span class="docEmphasis">PATH</span> 
will display as:</p>
<pre>echo $PATH
/usr/bin:/usr/ucb:/bin:.
</pre>
<p class="docText">The C shell internally updates the environment variable for
<span class="docEmphasis">PATH</span> to maintain compatibility with other 
programs (such as the Bourne or Korn shells) that may be started from this shell 
and will need to use the <span class="docEmphasis">path</span> variable.</p>
<p class="docText"><span class="docEmphStrong">The <span class="docEmphasis">
rehash</span> Command.</span> The shell builds an internal hash table consisting 
of the contents of the directories listed in the search path. (If the dot is in 
the search path, the files in the dot directory, the current working directory, 
are not put in the hash table.) For efficiency, the shell uses the hash table to 
find commands that are typed at the command line, rather than searching the path 
each time. If a new command is added to one of the directories already listed in 
the search path, the internal hash table must be recomputed. This is done by 
typing</p>
<pre>% <span class="docEmphStrong">rehash</span>
</pre>
<p class="docText">(The % is the C shell prompt.) The hash table is also 
automatically recomputed when you change your path at the prompt or start 
another shell.</p>
<p class="docText"><span class="docEmphStrong">The <span class="docEmphasis">
hashstat</span> Command.</span> The <span class="docEmphasis">hashstat</span> 
command displays performance statistics to show the effectiveness of its search 
for commands from the hash table. The statistics are in terms of &quot;hits&quot; and 
&quot;misses.&quot; If the shell finds most of its commands you use at the end of your 
path, it has to work harder than if they were at the front of the path, 
resulting in a higher number of misses than hits. In such cases, you can put the 
most heavily hit directory toward the front of the path to improve performance.</p>
<pre>% <span class="docEmphStrong">hashstat</span>
<span class="docEmphasis">2 hits, 13 misses, 13%</span>
</pre>
<p class="docText"><span class="docEmphStrong">The <span class="docEmphasis">
source</span> Command.</span> The <span class="docEmphasis">source</span> 
command is a shell built-in command, that is, part of the shell's internal code. 
It is used to execute a command or set of commands from a file. Normally, when a 
command is executed, the shell forks a child process to execute the command, so 
that any changes made will not affect the original shell, called the parent 
shell. The <span class="docEmphasis">source</span> command causes the program to 
be executed in the current shell, so that any variables set within the file will 
become part of the environment of the current shell. The
<span class="docEmphasis">source</span> command is normally used to reexecute 
the <span class="docEmphasis">.cshrc</span> or <span class="docEmphasis">.login</span> 
if either has been modified. For example, if the <span class="docEmphasis">path</span> 
is changed after logging in, type</p>
<pre>% <span class="docEmphStrong">source .login</span> or <span class="docEmphStrong">.cshrc</span>
</pre>
<p class="docText"><b>The Shell Prompts.</b> The C shell has two prompts: the
<span class="docEmphasis">primary</span> prompt, a percent sign (%), and the
<span class="docEmphasis">secondary</span> prompt, a question mark (?). The 
primary prompt is the one displayed on the terminal after you have logged in; it 
waits for you to type commands. The primary prompt can be reset. If you are 
writing scripts at the prompt that require C shell programming constructs, for 
example, decision-making or looping, the secondary prompt will appear so that 
you can continue onto the next line. It will continue to appear after each 
newline, until the construct has been properly terminated. The secondary prompt 
cannot be reset.</p>
<p class="docText"><b>The Primary Prompt.</b> When running interactively, the 
prompt waits for you to type a command and press the Enter key. If you do not 
want to use the default prompt, reset it in the .<span class="docEmphasis">cshrc</span> 
file and it will be set for this and all other C subshells. If you only want it 
set for this login session, set it at the shell prompt.</p>
<h5 id="ch09list03" class="docExampleTitle">Example 9.3 </h5>
<pre>1   % <span class="docEmphStrong">set prompt = &quot;$LOGNAME &gt; &quot;</span>
2   <span class="docEmphStrong">ellie &gt;</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 primary prompt is assigned the user's login name, 
      followed by a &gt; symbol and a space.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The new prompt is displayed.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>The Secondary Prompt.</b> The secondary prompt appears 
when you are writing on-line scripts at the prompt. Whenever shell programming 
constructs are entered, followed by a newline, the secondary prompt appears and 
continues to appear until the construct is properly terminated. Writing scripts 
correctly at the prompt takes practice. Once the command is entered and you 
press Enter, you cannot back up, and the C shell history mechanism does not save 
commands typed at the secondary prompt.</p>
<h5 id="ch09list04" class="docExampleTitle">Example 9.4 </h5>
<pre>1   % foreach pal (joe tom ann)
2   <span class="docEmphStrong">?</span> mail $pal &lt; memo
3   <span class="docEmphStrong">?</span> end
4   %
</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">This is an example of on-line scripting. Because the C 
      shell is expecting further input after the <span class="docEmphasis">
      foreach</span> loop is entered, the secondary prompt appears. The
      <span class="docEmphasis">foreach</span> loop processes each word in the 
      parenthesized list.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The first time in the loop, <span class="docEmphasis">
      joe</span> is assigned to the variable <span class="docEmphasis">pal.</span> 
      The user <span class="docEmphasis">joe</span> is sent the contents of
      <span class="docEmphasis">memo</span> in the mail. Then next time through 
      the loop, <span class="docEmphasis">tom</span> is assigned to the variable
      <span class="docEmphasis">pal,</span> and so on.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The <span class="docEmphasis">end</span> statement 
      marks the end of the loop. When all of the items in the parenthesized list 
      have been processed, the loop ends and the primary prompt is displayed.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList">The primary prompt is displayed.</span></li>
    </ol>
    </span></td>
  </tr>
</table>

<h4 class="docSection2Title" id="ch09lev2sec3">9.1.3 The Command Line</h4>
<p class="docText">After logging in, the C shell displays its primary prompt, by 
default a percent sign. 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 (or 
tokens) separated by whitespace (blanks and/or tabs) and terminated with a 
newline, generated by pressing the Enter key. The first word is the command, and 
subsequent words are the command's options and/or arguments. The command may be 
a UNIX executable program such as <span class="docEmphasis">ls</span> or
<span class="docEmphasis">pwd,</span> an alias, a built-in command such as
<span class="docEmphasis">cd</span> or <span class="docEmphasis">jobs,</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 the last character in the command line is a 
backslash, followed by a newline, the line can be continued to the next line.<span id="ENB9-2"><a class="docLink" href="#EN9-2"><sup>[2]</sup></a></span></p>
<p class="docText"><b>The Exit Status.</b> After a command or program 
terminates, it returns an <span class="docEmphasis">exit status</span> 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 C shell status variable 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="ch09list05" class="docExampleTitle">Example 9.5 </h5>
<pre>1 % grep &quot;ellie&quot; /etc/passwd
    <span class="docEmphasis">ellie:GgMyBsSJavd16s:9496:40:Ellie Quigley:/home/jody/ellie</span>
2 % <span class="docEmphStrong">echo $status</span>
    <span class="docEmphasis">0</span>
3 % grep &quot;nicky&quot; /etc/passwd
4   % <span class="docEmphStrong">echo $status</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 $status</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">ellie</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 status variable is set to the exit value of the
      <span class="docEmphasis">grep</span> command; <span class="docEmphasis">0</span> 
      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">The <span class="docEmphasis">grep</span> program 
      cannot find the pattern, so 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 file <span class="docEmphasis">/etc/passsswd</span> cannot be opened.</span></li>
      <li><span style="FONT-WEIGHT: normal">
      <p class="docList"><span class="docEmphasis">Grep</span> cannot find the 
      file, so it returns an exit status of <span class="docEmphasis">2.</span></span></li>
    </ol>
    </span></td>
  </tr>
</table>

<p class="docText"><b>Command Grouping.</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="ch09list06" class="docExampleTitle">Example 9.6 </h5>
<pre>% <span class="docEmphStrong">ls; pwd; cal 2001</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 until the 
    newline is reached.</td>
  </tr>
</table>

<p class="docText">Commands may also be grouped so that all of the output is 
either piped to another command or redirected to a file. The shell executes 
commands in a subshell.</p>
<h5 id="ch09list07" class="docExampleTitle">Example 9.7 </h5>
<pre>1  % <span class="docEmphStrong">( ls ; pwd; cal 2001 ) &gt; outputfile</span>
2  % <span class="docEmphStrong">pwd; ( cd / ; pwd ) ; pwd</span>
   <span class="docEmphasis">/home/jody/ellie</span>
   <span class="docEmphasis">/</span>