On Thu, Jan 21, 2010 at 06:11:10AM -0800, Yan Seiner wrote:
> Greg Wooledge wrote:
> >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...
It's not the pipeline; it's the command that's doing the writing which
causes the undesired behavior.
Most standard Unix commands will use large buffers when writing if
their stdout is not a terminal. Compare:
$ for i in {1..10}; do sleep 1; echo $i; done | grep .
$ for i in {1..10}; do sleep 1; echo $i; done | grep . | cat
$ for i in {1..10}; do sleep 1; echo $i; done | while read x; do echo "$x";
done | cat
The first one writes a number every second. The second one writes 10
numbers after 10 seconds. The grep command buffered its output because
stdout wasn't a terminal in the second case -- hence the --line-buffered
workaround for GNU grep mentioned previously.
The third one writes a number every second despite being a 3-command
pipeline. The while/read/echo loop doesn't buffer its output the way
grep does.
