如何获得“即时"的将"tail -f"的输出作为输入?

问题描述

我想监视一个日志文件，当新的日志消息与我定义的模式匹配(例如包含错误")时，然后向我发送电子邮件.

I want to monitor a log file, when a new log message match my defined pattern (say contain "error"), then send out an email to me.

为此，我编写了一个python脚本monitor.py，主要部分如下:

To do that, I wrote a python script monitor.py, the main part looks like:

import sys

for line in sys.stdin:
    if "error" in line:
        print line 

当我使用tail my.log | python monitor.py时效果很好，然后我切换到tail -f my.log | python monitor.py，则它不起作用，至少不是立即生效.

It works well when I use tail my.log | python monitor.py, then I switch to tail -f my.log | python monitor.py, then it doesn’t work, at least not immediately.

我已经做过一些测试，当日志中的新内容累积到8KB时，我的python脚本可以从tail中获取输出.因此，我高度怀疑这是由stdin/stdout缓冲区大小控制的.如何立即获得输出?

I have done some tests, when the new content to the log accumulate up to 8KB, then my python script can get output from tail. So I highly suspect that this is controlled by the stdin/stdout buffer size. How can I get the output immediately?

还有一个问题，当我使用tail -f my.log和tail -f my.log | grep error时，为什么它可以立即显示输出?

One more question, when I use tail -f my.log and tail -f my.log | grep error, why it could show me the output immediately?

推荐答案

如果stdout连接到TTY，则大多数Linux程序将使用行缓冲，否则将使用全缓冲.您可以使用 stdbuf 强制行缓冲.

Most Linux programs will use line buffering if stdout is connecting to a TTY and full buffering otherwise. You can use stdbuf to force line buffering.

stdbuf -oL tail -f my.log | python monitor.py

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