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    <p>I apologize for post this as an answer instead of a comment, but my "comment" is too large...</p> <p>In the MS-DOS standard, all running programs have open these <em>Standard Handles</em>: 0-STDIN (keyboard), 1-STDOUT (screen), 2-STDERR (screen), 3-STDAUX (serial port) and 4-STDPRN (printer). Although Windows documentation clearly indicate that handles 3-9 are undefined, handle 3 have a special treatment by CMD.EXE. I have three reasons to think this:</p> <p>1- Handle 3 is connected to CON: device (keyboard for input, screen for output); handles 4-9 does not:</p> <pre><code>C&gt;ver Microsoft Windows XP [Version 5.1.2600] C&gt;echo To handle 3 &gt;&amp;3 To handle 3 C&gt;echo To handle 4 &gt;&amp;4 The handle could not be duplicated during redirection of handle 1. C&gt;set /P var=From handle 3: &lt;&amp;3 From handle 3: Value entered in keyboard C&gt;echo %var% Value entered in keyboard C&gt;set /P var=From handle 4: &lt;&amp;4 The handle could not be duplicated during redirection of handle 0. </code></pre> <p>2- The strange behavior of handle 3 stated in the present topic, that was solved in two differents ways. I discovered that if handles 0, 1 or 2 are redirected with handle 3 (and possibly handles 4-9), the redirection of handles 0, 1 or 2 becomes "permanent"; this behaviour doesn't happens if the handle 0, 1 or 2 are the <em>last</em> handle(s) in the list of redirections that include handle 3. This problem is entirely avoided if handles 0, 1 or 2 are redirected with handles 4-9 in any order, but not handle 3.</p> <p>3- The results obtained with my <a href="http://www.dostips.com/forum/viewtopic.php?f=3&amp;t=2800" rel="nofollow">TypeOfHandle.com program</a>. This program is a pure MS-DOS executable file that check the handle given in its parameter and return via errorlevel a value of 3 if the handle is connected to the CONsole device, or a value of 128 if the handle is redirected to a disk file. These are the results:</p> <pre><code>C&gt;typeofhandle 0 C&gt;echo %errorlevel% 3 C&gt;typeofhandle 0 &lt; anyFile.txt C&gt;echo %errorlevel% 128 C&gt;typeofhandle 1 C&gt;echo %errorlevel% 3 C&gt;typeofhandle 1 &gt; anyFile.txt C&gt;echo %errorlevel% 128 C&gt;typeofhandle 3 C&gt;echo %errorlevel% 0 C&gt;typeofhandle 3 &lt;&amp;3 anyFile.txt C&gt;echo %errorlevel% 0 C&gt;typeofhandle 3 &gt;&amp;3 anyFile.txt C&gt;echo %errorlevel% 0 </code></pre> <p>Handles 3-9 behaves strange in other aspect:</p> <p>The results obtained with my <a href="http://www.dostips.com/forum/viewtopic.php?f=3&amp;t=2823" rel="nofollow">SetFilePointer.com program</a>. Although handles 3-9 may be used in a Batch file to achieve input/output from/to several files at once, such mechanism only allows <em>sequential access</em> because my SetFilePointer program can't move the File Pointer of handles 3-9. SetFilePointer program correctly works on handles 0, 1 and 2; this feature allows to write a complete Relational Database application in Batch. This topic is described in detail at <a href="http://www.dostips.com/forum/viewtopic.php?f=3&amp;t=3126" rel="nofollow">this post</a></p>
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    1. POWhy doesn't my stderr redirection end after command finishes? And how do I fix it?
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    1. COThanks. I didn't quite get all that, but that's OK. This trick seems to make handle 4 behave normally. (ECHO This is 4 1>&4) 3>NUL: DOS programs run under NTVDM, who might have own rules.
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    2. COOr maybe not, if I run "FILE.BAT 3>NUL:", 4 will redirect the same place as 3.
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    3. CO@Aacini - Good evidence for Windows behavior, but the theory is not quite correct. See my updated answer for an explanation and link to test cases.
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