ch05_08.htm

unix基础教程

<p>
<a name="IXT-5-123074" />If <em class="emphasis">variable</em> is given, shift the words in a word list variable;
i.e., <em class="emphasis">name</em><tt class="literal">[2]</tt> becomes <em class="emphasis">name</em><tt class="literal">[1]</tt>.
With no argument,
shift the positional parameters (command-line arguments); i.e.,
<tt class="literal">$2</tt> becomes <tt class="literal">$1</tt>.
<tt class="literal">shift</tt> is typically


used in a <tt class="literal">while</tt> loop.
See additional Example under <b class="emphasis-bold"><a href="#while">while</a></b>.</p>
<h4 class="refsect2">Example</h4><blockquote><pre class="code">while ($#argv)      <i class="lineannotation">While there are arguments</i>
    if (-f $argv[1])  
       wc -l $argv[1]
    else
       echo "$argv[1] is not a regular file"
    endif
    shift           <i class="lineannotation">Get the next argument</i>
end</pre></blockquote>

</td></tr><tr><td valign="top"><a name="source">source</a></a></td><td><tt class="literal">source</tt> [<tt class="literal">-h</tt>] <em class="replaceable"><tt>script</tt></em>
<p>
<a name="IXT-5-123075" />Read and execute commands from a C shell script.  
With <tt class="literal">-h</tt>, the commands are


added to the history list but aren't executed.</p>
<h4 class="refsect2">Example</h4><blockquote><pre class="code">source ~/.cshrc</pre></blockquote>

</td></tr>

<tr><td valign="top"><a name="stop">stop</a></a></td><td><p><tt class="literal">stop</tt> [<em class="replaceable"><tt>jobIDs</tt></em>]</p>
<p>
<a name="IXT-5-123076" />Suspend the current background job or the
background job specified by <em class="emphasis">jobIDs</em>; this is
the complement of <tt class="literal">CTRL-Z</tt> or <tt class="literal">suspend</tt>.</p>
</td></tr><tr><td valign="top"><a name="suspend">suspend</a></a></td><td><tt class="literal">suspend</tt>
<p>
<a name="IXT-5-123077" />Suspend the current foreground job; similar to <tt class="literal">CTRL-Z</tt>.
Often used to stop an <tt class="literal">su</tt> command.</p>
</td></tr><tr><td valign="top"><a name="switch">switch</a></a></td><td><tt class="literal">switch</tt>
<p>
<a name="IXT-5-123078" />Process commands depending on the value of a variable.
When you need to handle more than three choices, <tt class="literal">switch</tt>
is a useful alternative to an <tt class="literal">if-then-else</tt> statement.
If the <em class="emphasis">string</em> variable matches <em class="emphasis">pattern1</em>,
the first set of <em class="emphasis">commands</em> is executed; if <em class="emphasis">string</em> 
matches <em class="emphasis">pattern2</em>, the second set of <em class="emphasis">commands</em> is executed;
and so on.  If no patterns match, execute commands under the
<tt class="literal">default</tt> case.
<em class="emphasis">string</em> can be specified using command substitution, 
variable substitution, or filename expansion.
Patterns can be specified using pattern-matching symbols <tt class="literal">*</tt>, 
<tt class="literal">?</tt>, and <tt class="literal">[</tt> <tt class="literal">]</tt>.  <tt class="literal">breaksw</tt> exits the <tt class="literal">switch</tt> after <em class="emphasis">commands</em> 
are executed.  If <tt class="literal">breaksw</tt> is omitted (which is rarely done), 
the <tt class="literal">switch</tt> continues to execute another set of commands until
it reaches a <tt class="literal">breaksw</tt> or <tt class="literal">endsw</tt>.
Here is the general syntax of <tt class="literal">switch</tt>, side-by-side with an example
that processes the first command-line argument.  
</p>


<blockquote><pre class="code">switch (<em class="replaceable"><tt>string</tt></em>)    switch ($argv[1])
  case <em class="replaceable"><tt>pattern1</tt></em>:      case -[nN]:
      <em class="replaceable"><tt>commands</tt></em>            nroff $file | lp
      breaksw             breaksw
  case <em class="replaceable"><tt>pattern2</tt></em>:      case -[Pp]:
      <em class="replaceable"><tt>commands</tt></em>            pr $file | lp
      breaksw             breaksw
  case <em class="replaceable"><tt>pattern3</tt></em>:      case -[Mm]:
      <em class="replaceable"><tt>commands</tt></em>            more $file
      breaksw             breaksw
      .                case -[Ss]:
      .                    sort $file
      .                    breaksw
  default:             default:
      <em class="replaceable"><tt>commands</tt></em>             echo "Error-no such option"
                           exit 1
      breaksw              breaksw
endsw                 endsw</pre></blockquote>
</td></tr>

<tr><td valign="top"><a name="time">time</a></a></td><td><p><tt class="literal">time</tt> [<em class="replaceable"><tt>command</tt></em>]</p>
<p>
<a name="IXT-5-123079" />Execute a <em class="emphasis">command</em> and show how much time it uses.
With no argument, <tt class="literal">time</tt> can be used in a shell script to time it.</p>
</td></tr><tr><td valign="top"><a name="umask">umask</a></a></td><td><tt class="literal">umask</tt> [<em class="replaceable"><tt>nnn</tt></em>]
<p>
<a name="IXT-5-123080" />Display file-creation mask or set file creation mask to octal
<em class="emphasis">nnn</em>.  The file-creation mask determines which permission bits
are turned off.
See the
entry in <a href="ch02_01.htm">Chapter 2</a> for examples.</p>
</td></tr><tr><td valign="top"><a name="unalias">unalias</a></a></td><td><tt class="literal">unalias</tt> <em class="replaceable"><tt>name</tt></em>
<p>
<a name="IXT-5-123081" />Remove <em class="emphasis">name</em> from the alias list.
See <b class="emphasis-bold"><a href="#alias">alias</a></b> for more
information.</p>
</td></tr><tr><td valign="top"><a name="unhash">unhash</a></a></td><td><tt class="literal">unhash</tt>
<p>
<a name="IXT-5-123082" />Remove internal hash table.  The C shell stops using hashed values
and spends time searching the <tt class="literal">path</tt> directories to locate a
command.  See also <b class="emphasis-bold"><a href="#rehash">rehash</a></b>.</p>
</td></tr><tr><td valign="top"><a name="unlimit">unlimit</a></a></td><td><tt class="literal">unlimit</tt> [<em class="replaceable"><tt>resource</tt></em>]
<p>
<a name="IXT-5-123083" />Remove the allocation limits on <em class="emphasis">resource</em>.  If <em class="emphasis">resource</em> is not
specified, remove limits for all resources.  See <b class="emphasis-bold"><a href="#limit">limit</a></b> for more
information.</p>
</td></tr><tr><td valign="top"><a name="unset">unset</a></a></td><td><tt class="literal">unset</tt> <em class="replaceable"><tt>variables</tt></em>
<p>
<a name="IXT-5-123084" />Remove one or more <em class="emphasis">variables</em>.  Variable names may be specified
as a pattern, using filename metacharacters.  See <b class="emphasis-bold"><a href="#set">set</a></b>.</p>
</td></tr><tr><td valign="top"><a name="unsetenv">unsetenv</a></a></td><td><tt class="literal">unsetenv</tt> <em class="replaceable"><tt>variable</tt></em>
<p>
<a name="IXT-5-123085" />Remove an environment variable.  Filename


matching is <em class="emphasis">not</em> valid.
See <b class="emphasis-bold"><a href="#setenv">setenv</a></b>.</p>
</td></tr>

<tr><td valign="top"><a name="wait">wait</a></a></td><td><p><tt class="literal">wait</tt></p>
<p>
<a name="IXT-5-123086" />Pause in execution until all


background jobs complete,
or until an interrupt signal is received.</p>
</td></tr>

<tr><td valign="top"><a name="while">while</a></a></td><td><p><tt class="literal">while (</tt><em class="replaceable"><tt>expression</em><tt class="literal">)</tt></p><p>&#160;&#160;&#160;&#160;&#160;<em class="replaceable">commands</tt></em></p><p><tt class="literal">end</tt></p>
<p>
<a name="IXT-5-123087" />As long as <em class="emphasis">expression</em> is true
(evaluates to nonzero),
evaluate <em class="emphasis">commands</em> 
between <tt class="literal">while</tt> and <tt class="literal">end</tt>.
<tt class="literal">break</tt> and <tt class="literal">continue</tt> can terminate or continue the loop.
See also the Example under <b class="emphasis-bold"><a href="#shift">shift</a></b>.</p>
<h4 class="refsect2">Example</h4><blockquote><pre class="code">set user = (alice bob carol ted)
while ($argv[1] != $user[1])
   <i class="lineannotation">Cycle through each user, checking for a match</i>
   shift user
   <i class="lineannotation">If we cycled through with no match...</i>
   if ($#user == 0) then
     echo "$argv[1] is not on the list of users"
     exit 1
   endif
end</pre></blockquote>

</td></tr><tr><td valign="top"><a name="@">@</a></a></td><td><tt class="literal">@</tt> <em class="replaceable"><tt>variable</em> <tt class="literal">=</tt> <em class="replaceable">expression</em><br><tt class="literal">@</tt> <em class="replaceable">variable</em><tt class="literal">[</tt><em class="replaceable">n</em><tt class="literal">] =</tt> <em class="replaceable">expression</tt></em><br><tt class="literal">@</tt>
<p>
<a name="IXT-5-123088" />Assign the value of the arithmetic <em class="emphasis">expression</em> to <em class="emphasis">variable</em>, 
or to the <em class="emphasis">n</em>th element of <em class="emphasis">variable</em> if the index <em class="emphasis">n</em> 
is specified.  With no <em class="emphasis">variable</em> or <em class="emphasis">expression</em>
specified, print the values of all shell variables (same as <tt class="literal">set</tt>).
Expression operators as well as examples are listed in <a href="ch05_04.htm#unut-ch-5-sect-4">Section 5.4</a>.  Two special forms are also valid:</p>
<dl><dt><tt class="literal">@</tt> <tt><em class="replaceable">variable</em></tt><tt class="literal">++</tt></dt><dd>Increment <em class="emphasis">variable</em></tt> by one.</p>
</dd>
<dt><tt class="literal">@</tt> <tt><em class="replaceable">variable</em></tt><tt class="literal">--</tt></dt><dd>Decrement <em class="emphasis">variable</em></tt> by one.</p>
</dd>
</dl>
</td></tr></table><p>

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