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Old 10-01-2007, 04:00 PM   #1
WingnutOne
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Question Pipe standard error?


Is there a way to redirect standard error to the input of another command (BASH / RHE4)?
I can send it to a file, grep the file for what I need, then delete the file, but this method seems a bit sloppy.

Is there a more efficient way to do it?
(I tried redirecting stderr to stdout, then greping for what I need, but stdout occasionally includes text that fowls up my grep sequence.)

Thanks!
 
Old 10-01-2007, 07:56 PM   #2
Tinkster
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You should be able to just swap the file descriptors for stdout
and stderr if stderr is all you need to post-process...

Code:
(cmd1 | cmd2) 3>&1 1>&2 2>&3

Cheers,
Tink
 
Old 10-01-2007, 11:16 PM   #3
I_like_TUX
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Tinkster taught me a way to put everything in one line because I used to do : exec 3>&1 ... cmd1 | cmd2

Actually, it is an interesting question and also a good practice. I explored a little bit and found that if you want stdout of cmd1 to disappear and only stderr of cmd1 piped into cmd2 without being displayed, you can do the following:

I wrote a sample script for testing:
#!/bin/bash
echo "output for stdout" >&1
echo "output for stderr" >&2
exit 0

$ (./a_cmd.sh | wc -l) 3>&1 1>&2 2>&3
output for stderr
1

$ (./a_cmd.sh | wc -l) 3>&1 1>/dev/null 2>&1 1>&3
1

Tinkster, the above works but I don't know how it works. Can you explain? Thanks.

Last edited by I_like_TUX; 10-02-2007 at 02:25 AM. Reason: remove un-needed stuffs
 
Old 10-02-2007, 09:06 AM   #4
WingnutOne
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Maybe I'm more of a no0b than you realize: I know "1" is stdout and "2" is stderr, but what's the "3" mean? Stdout and stderr combined?

Like IlTux above, I can plug different commands into the formula and get different results, but I don't know what's really happening to generate the differences.

Last edited by WingnutOne; 10-02-2007 at 09:44 AM. Reason: remove pun, add content
 
Old 10-02-2007, 01:44 PM   #5
osor
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Quote:
Originally Posted by WingnutOne View Post
Maybe I'm more of a no0b than you realize: I know "1" is stdout and "2" is stderr, but what's the "3" mean? Stdout and stderr combined?
Itís just a placeholder file descriptor. See man exec.
 
  


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