On 23/11/06, Matthew Hannigan <[EMAIL PROTECTED]> wrote:
You don't get the output in as timely a fashion;
it comes with spurts as the buffer is filled up
then written out.
What buffer? tail's manual doesn't mention anything about buffers but
my experience is that you get individual lines even then its output is
piped (i.e. even when the output is about 80 bytes long, i.e. much
less than any reasonable buffer size except a line buffer).
With grep you can force line-buffer with --line-buffering, if you
think this is the problem.
Maybe what you see is tail's sleep between retries. The default is 1
but see below.
Try this out:
while true ; do s=$RANDOM; echo $s; usleep $s; done >crap &
tail -f crap |grep 3
vs
tail -f crap | grep 3 | grep -v 8
I get output every more-or-less 1 second, which makes me suspect that
my "tail's sleep(1)" explanation is indeed the reason. Adding "-s 0"
gives me continuous output. And that's even without forcing
--line-buffering on grep. So far I thing this confirms my theory.
Another option to test this might be to "strace -tt -f ...", but so
far I wasn't convinced by your explanation so I'll leave it up to you
to experiment :^).
Another experiment - slow down the data writing into "crap" and see
whether you get individual lines of 5-6 bytes each - this again should
prove the "buffer theory" wrong, IMHO.
Cheers,
--P
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