Why doesn't my stderr redirection end after command finishes? And how do I fix it?

Question

In Windows, either in command line or a batch file, the command DIR 2>NUL: 3>&2 (you can replace DIR with anything, even if isn't a file or a command) will make all errors from then on missing unless you write 2>CON: after every command. Why is CMD even doing this? And how do you get it back to normal without starting a new CMD process? DIR 2>CON: 3>&2 will only work for that command alone.

EDIT: This will work with files as well. DIR 2>TEXT.TXT 3>&2 Any errors after that will be appended to the file.

Answer 1

Here is a test script that reproduces the problem you are seeing.

@echo off 2>nul 3>nul (   echo I want to see stream1   1>&2 echo I don't want to see this stream2   1>&3 echo I don't want to see this stream3 ) echo stream1 works fine 1>&2 echo stream2 is now "permanently" void. I don't see this. 1>&3 echo stream3 works fine 

And here is the output

I want to see stream1 stream1 works fine stream3 works fine 

stderr (stream 2) has been disabled "permanently", even for the parent CMD.EXE shell.

You can avoid the "permanent" aspect by doing your redirection in stages:

@echo off 2>nul (   3>nul (     echo I want to see stream1     1>&2 echo I don't want to see this stream2     1>&3 echo I don't want to see this stream3   ) ) echo stream1 works fine 1>&2 echo stream2 works fine 1>&3 echo stream3 works fine 

And here is the desired output:

I want to see stream1 stream1 works fine stream2 works fine stream3 works fine 

I don't really understand what is going on. But I have done some interesting experiments. Check out this thread: http://www.dostips.com/forum/viewtopic.php?f=3&t=2836&start=30

Addendum

As Erbert has discovered and shared in his comment, the fix is even easier if you just switch the order of the redirection - no need to stage it.

@echo off 3>nul 2>nul (   echo I want to see stream1   1>&2 echo I don't want to see this stream2   1>&3 echo I don't want to see this stream3 ) echo stream1 works fine 1>&2 echo stream2 works fine 1>&3 echo stream3 works fine 

Update 2012-04-03 I believe I finally understand the mechanics of Windows CMD.EXE redirection. I have a working theory that fully accounts for all the weird behavior, including why reversing the order prevents the "permanent" redirection. It also explains Aacini's observation that handle 3 appears to be connected to CON: (It isn't, it is actually undefined as per Windows documentation).

The key points are:

1 - Whenever a handle (stream) is redirected, the original definition is transferred to the first available undefined handle. Successive redirections are always carried out from left to right.

2 - When the redirection is over, the original definitions are normally restored. But if there is a chain of redirections, then the restoration is only done 1 level deep. This is the source of the "permanent" redirection.

Edit 2014-12-19: Put another way, the restoration appears to be done using a queue structure (FIFO - First In First Out), when it should have been implemented as a stack (LIFO - Last In First Out).

3 - When CMD.EXE carries out the redirection, it saves the current definition in the undefined handle first, then it redirects the first handle. If the first handle is redirected to the originally undefined handle, then it is effectively redirected to its original definition! That is why echo hello 1>&3 outputs to the console.

The full theory and test cases are available in two consecutive posts at http://www.dostips.com/forum/viewtopic.php?p=14612#p14612.