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In a way this question is a extension of another question.

In bash you can do pretty neat stuff with file descriptors. Like

{ command > /dev/null } 2>&1 | grep filter;

to grep on stderr only.

My Question[s]:

  • How many levels of descriptors are there. ( How high can the "numbers" go) ?
  • Appart from 1 and 2 ( which are stdout and stderr ), what is the meaning and general use of those other "numbers"?
  • Is the order in which they are specified important?
    e.g. command 1>&2 3>&1 2>&3 versus command 2>&3 3>&1 1>&2
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You probably knew this already, but 1 is stdout; 0 is stdin. –  Gilles Nov 2 '10 at 18:08
    
the 0 stdin is news to me :) I learnt something +1 –  NixNinja Nov 2 '10 at 20:36
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2 Answers

up vote 2 down vote accepted
  • You may well expect them to go up to 63 on a typical Linux box.
  • There is (was?) stdlog, but it's rarely used (I believe I never saw one).
  • The order is important: check man bash REDIRECTION.
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I don't think there's anything standard about 3 going to syslog, it just happens to be the first opened by many programs that already have the standard 0, 1, 2 open. –  Gilles Nov 1 '10 at 18:21
    
@Gilles: actually I was talking about stdlog, not syslog. Too bad, it's not easy to find something viable about that. I believe it was obsoleted long ago. And I must be wrong for putting 3 as it's default number... :-P –  alex Nov 2 '10 at 10:13
    
From light googling, stdlog might have been a FORTRAN thing that never made it into Unix. I didn't find anything like a reference for this, though. –  Gilles Nov 2 '10 at 18:07
    
yeah, that seems pretty much like one of those false memories... –  alex Nov 2 '10 at 20:44
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The number you need is given by the maximum file descriptor count from the ulimit(1) command.

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