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<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
~/tcltk$ <b>tclsh</b>
% <b>nslookup orbdesigns.com</b>
Server:         192.168.1.3
Address:        192.168.1.3#53
 
Name:   orbdesigns.com
Address: 64.81.69.163
 
% <b>set d [date]</b>
Sun Mar 25 13:51:59 PST 2001
% <b>puts stdout $d</b>
 
% <b>set d [exec date]</b>
Sun Mar 25 13:52:19 PST 2001
% <b>puts stdout $d</b>
Sun Mar 25 13:52:19 PST 2001

******

% <b>if [catch {open foo r} Chan] {
    puts stdout "Sorry, Dave...\n"
}</b>
% <b>gets $Chan</b>
One
% <b>gets $Chan</b>
Two
% <b>eof $Chan</b>
0
% <b>close $Chan</b>
%
			</code>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>Processes and file I/O with Tcl</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 14 页（共15 页）</nobr></font></td>
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<p>The <i>exec</i> command is used to explicitly execute external 
				commands. Under Linux, most external commands can be run 
				directly when Tcl is in interactive mode, as shown 
				in the example at the left.  
				Running with <i>exec</i> returns the stdout output of a 
				program not to the screen, but to Tcl, which allows the 
				data to be assigned to a variable. 
				When a program is started in the background, the immediate return 
				value is the PID for the program. <i>exec</i>'d programs 
				can take full advantage of I/O redirection and pipelines 
				in the UNIX environment.</p>
<p>Other process commands are <i>exit</i>, which terminates a running Tcl 
				script, and <i>pid</i>, which returns the PID of current (or specified) 
				process, handy for a variety of purposes. Tcl does not 
				incorporate any native process control commands, but you can 
				use the exec command in concert with PID data to accomplish 
				many tasks.</p>
<p>File manipulation uses the following commands: <i>open</i>, 
				<i>close</i>, <i>gets</i>, <i>puts</i>, <i>read</i>, <i>tell</i>, 		
				<i>seek</i>, <i>eof</i>,  and <i>flush</i>. As demonstrated at left, 
				the <i>catch</i> command is useful in error checking during file
				opening commands. When the program output needs to be printed 
				before a newline character is encountered, as in a user data prompt, 
				use <i>flush</i> to write the output buffer.</p>
<p>An additional feature (in supported environments) is the ability to 
				open pipelines in the same manner that you might a file. For 
				instance, after opening a pipeline channel with <i>set Channel 
				[open "|sort foobar" r]</i>, the output of the first <i>gets</i> 
				is going to be "Eight" (alphabetically, out of file data "One" 
				through "Ten", on 10 separate lines).</p>
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