BASH redirect & grep -v help needed

[Smoot Carl-Mitchell]

On Wed, 2009-02-18 at 21:34 +0000, dave selby wrote:
> >> So any idea why xxx.out contains zsip instead of the above ?
> >
> > No idea.  What happens without the stderr redirection?
> 
> motion -c /home/dave/kmotion2/core/motion_conf/motion.conf 2>&1 | grep
> -v 'saved to'
> 
> works a treat. 'motion' starts, outputs and keeps running and I have
> to ctrl-c to stop it.
> 
> motion -c /home/dave/kmotion2/core/motion_conf/motion.conf 2>&1 | grep
> -v 'saved to' >> xxx.out
> 
> outputs nothing, but zip in xxx.out
> 
> I am wondering if the output to the BASH shell is 'instant' but the
> output to >> is only processed on 'motion' ending
> in some way ?

Are you saying "motion -c" has continuous output?  If that is the case
then you are seeing I/O buffering side effects.  Try adding
--line-buffered to the grep command.  That will flush the output buffer
after each line.
-- 
Smoot Carl-Mitchell
Computer Systems and
Network Consultant
smoot at tic.com
+1 480 922 7313
cell: +1 602 421 9005