On Thu, Nov 23, 2006 at 05:10:14PM +1100, Penedo wrote: > 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
grep's buffer, the normal stdio buffer I guess > 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 Doesn't me; I get it spurty; like 50 seconds or so between output. > --line-buffering on grep. So far I thing this confirms my theory. Weird. With --line-buffered on the first grep I get it continuous. If only the second grep is --line-buffered, then it goes spurty again. That points to the first grep buffering by default, to me. > 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. It's possible it's a difference in kernels/shells/something else, but that's certainly unexpected. I didn't even know about tail's -s option. That's what I get for spending too much time in a non-GNU wilderness. I still think there's a case for a 'less' like tool, or less itself to do some filtering, modifiable on the fly. I'm sure there's some perl module/program to do it. Matt -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
