问题
Going with a typical Apache access log, you can run:
tail -f access_log | grep "127.0.0.1"
Which will only show you the logs (as they are created) for the specified IP address.
But why does this fail when you pipe it though grep a second time, to further limit the results?
For example, a simple exclude for ".css":
tail -f access_log | grep "127.0.0.1" | grep -v ".css"
won't show any output.
回答1:
I believe the problem here is that the first grep is buffering the output which means the second grep won't see it until the buffer is flushed.
Try adding the --line-buffered option on your first grep:
tail -f access_log | grep --line-buffered "127.0.0.1" | grep -v ".css"
For more info, see "BashFAQ/009 -- What is buffering? Or, why does my command line produce no output: tail -f logfile | grep 'foo bar' | awk ..."
回答2:
This is the result of buffering, it will eventually print when enough data is available.
Use the --line-buffered option as suggested by Shawn Chin or if stdbuf is available you can get the same effect with:
tail -f access_log | stdbuf -oL grep "127.0.0.1" | grep -v ".css"
来源：https://stackoverflow.com/questions/13858912/piping-tail-output-though-grep-twice