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Redirection - Duplicating File Descriptors

From: Gingko
Subject: Redirection - Duplicating File Descriptors
Date: Sun, 05 Aug 2001 10:01:00 GMT


I would like to ask a question about a specific feature of  'bash' :

Both the 'man' pages and the reference manual describe the following in the 
section :

Duplicating File Descriptors
The redirection operator


is used to duplicate input file descriptors. If word expands to one or more 
digits, the file
descriptor denoted by n is made to be a copy of that file descriptor. If the 
digits in word do
not specify a file descriptor open for input, a redirection error occurs. If 
word evaluates
to -, file descriptor n is closed. If n is not specified, the standard input 
(file descriptor 0)
is used.

The operator


is used similarly to duplicate output file descriptors. If n is not specified, 
the standard
output (file descriptor 1) is used. If the digits in word do not specify a file 
descriptor open
for output, a redirection error occurs. As a special case, if n is omitted, and 
word does not
expand to one or more digits, the standard output and standard error are 
redirected as described

Does somebody have a precise example of this ?

I more or less understand that on that way it is possible to redirect a file 
stream towards two
file descriptors, but whatever I try to type as "word" doesn't give consistant 
results to me.

Does it allow to, saying a trivial example, redirect the output of a 'ls' 
command to a file AND
leave it to the standard output at the same time ?

If yes, how would I type this on the command line ?

If not, is there a way to accomplish this in a pure 'bash' command form, 
without writing my own
(of course short, but...) C program for doing it ?
Is there a standard Unix/Linux command for that ?

I'm sorry if this is not the correct workgroup for this, but finding one is not 
very obvious.

Thank you very much in advance for your(s) answer(s).

Gilles Reeves.

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