Re: weird bash pipe behavior

[Yan Seiner]

Greg Wooledge wrote:
> On Wed, Jan 20, 2010 at 07:34:35AM -0800, CptDpondo wrote:
>   
> > mencoder .... 2>&1 | tr '\r' '\n' | grep -v -B 1 '^Pos'
> >
> > this eventually creates the correct output, but it's buffered until
> > mencoder completes.  I have no idea why; the tr command streams
> > without buffering and I've used grep for years and never run into this
> > behavior.
> >     
>
> grep definitely has this buffering behavior.  If you're using GNU grep,
> you can give it the --line-buffered option to change this, at least
> according to whichever contributor added that one to
> http://mywiki.wooledge.org/BashFAQ/009
>
>   
That is the correct solution.  Apparently the behavior is not limited to 
grep; any process in a pipe that's longer than 2 can exhibit this.  
Something about libc behavior...

I replaced grep with awk, ran in to the same problem, and ended up using 
fflush() after scanning each line.

Interesting.  I learned something today.


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