linux bash 和 grep 错误？

Question

Doing the following:

First console

touch /tmp/test

Second console

tail -f /tmp/test |grep propo |grep -v miles

Third console

echo propo >> /tmp/test

Second console must show "propo" but it doesn't shows anything, if you run in second console instead:

tail -f /tmp/test |grep propo

And do echo propo >> /tmp/test it will show propo, but the grep -v is for miles not for propo

Why?

Test into your own environment if you want, it's pretty obvious but not working.

Answer 1

Why?

Most probably because the output of a command when piped to another command is fully buffered, not line buffered. The output could be buffered in the first pipe or by grep.

Use stdbuf -oL to force line buffering and grep --line-buffered for line buffered grep.

Answer 2

the problem is that grep does not use line buffering by default; so the output will be buffered. You could use grep --line-buffered :

tail -f /tmp/test | grep --line-buffered propo | grep -v miles