问题
I am trying to debug one scenario by checking the logs, here is my command
tail -f eclipse.log | grep 'enimation' | grep -i 'tap'
Basically what I am trying to to is that, of all of the line, I am printing lines with enimation in it, then of all of the animation, I want to see the animation with "tap" in it.
Here is the sammple data for which it is returning empty results
*******enimation error*********TapExpand
*******enimation error*********TapShrink
This is returning empty results.
While if I run this command
tail -f eclipse.log | grep -i 'enimation.*tap'
it returns correct results. Can someone please explain to me, what is the difference between the above two command and why there is a discrepancy in the results. They both seem identical to me.
回答1:
grep is buffering it's output. To tell GNU grep to spit out output line-by-line you need to use --line-buffered option in grep to make it work:
tail -f eclipse.log | grep --line-buffered 'enimation' | grep --line-buffered -i 'tap'
As per man grep:
--line-buffered
Force output to be line buffered. By default, output is line buffered when standard
output is a terminal and block buffered otherwise.
回答2:
The output of the grep in the middle is not a terminal, so it is using block buffering instead of line buffering. You have to force line buffering using --line-buffered option.
tail -f eclipse.log | grep --line-buffered 'enimation' | grep -i 'tap'
In case of other commands, which don't provide such an option, you can use stdbuf command to force line buffering, e.g.:
tail -f eclipse.log | stdbuf -o1 grep 'enimation' | grep -i 'tap'
回答3:
Your output is getting buffered. Try:
tail -f eclipse.log | grep --line-buffered 'enimation' | grep --line-buffered -i 'tap'
回答4:
Try adding the -E option to grep. A lot of reflex features don't work without it. I usually refer to it as the "yes, I do know what I'm doing" option.
来源：https://stackoverflow.com/questions/26362450/piping-of-grep-is-not-working-with-tail