Re: Mixing exec redirection with explicit redirection, unexpected results

[Chet Ramey]

root@enterprise.herff-jones.com wrote:

>       When I use the 'exec' built-in to redirect standard output 
>       for a script, and then also use explicit redirection on 
>       other commands, pointing both to the same output file, 
>       I get unpredictable (but repeatable) results with the output 
>       out of order.
> 
> Repeat-By:
>       The following script:
> 
> #!/bin/bash
> 
>> /tmp/foo
> exec 1>/tmp/foo
> 
> echo a
> echo B >>/tmp/foo
> echo c
> echo D >>/tmp/foo
> echo e
> echo F >>/tmp/foo
> 
> ...should produce the following contents in /tmp/foo:
> a
> B
> c
> D
> e
> F
> 
> ...but instead it produces:
> a
> c
> e
> F
> 
> 
> 
> Fix:
>       I am at a loss for how to understand this behavior -- sorry, 
>       I can't suggest a fix. :-(

It's not a bug.  The important thing to remember is that the appends,
since they use a new file descriptor with a separate file offset, don't
affect the file offset associated with fd 1.  Each subsequent echo
to fd 1 overwrites the previous echo that used an explicit append.

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/