为什么两次使用 grep 时没有显示输出? [英] Why no output is shown when using grep twice?

问题描述

基本上我想知道为什么这不输出任何内容:

Basically I'm wondering why this doesn't output anything:

tail --follow=name file.txt | grep something | grep something_else 

你可以假设它应该产生输出我已经运行了另一行来确认

You can assume that it should produce output I have run another line to confirm

cat file.txt | grep something | grep something_else

看起来你不能多次通过管道输出 tail ！?任何人都知道交易是什么，有解决方案吗?

It seems like you can't pipe the output of tail more than once!? Anyone know what the deal is and is there a solution?

为了回答到目前为止的问题，该文件肯定有应该由 grep 显示的内容.作为grep是否像这样完成的证据:

To answer the questions so far, the file definitely has contents that should be displayed by the grep. As evidence if the grep is done like so:

tail --follow=name file.txt | grep something

输出正确显示，但如果使用它:

Output shows up correctly, but if this is used instead:

tail --follow=name file.txt | grep something | grep something

没有显示输出.

如果有帮助，我正在运行 ubuntu 10.04

If at all helpful I am running ubuntu 10.04

推荐答案

在管道内部时，您可能还会遇到 grep 缓冲问题.即，您看不到

You might also run into a problem with grep buffering when inside a pipe. ie, you don't see the output from

   tail --follow=name file.txt | grep something > output.txt

因为 grep 会缓冲它自己的输出.

since grep will buffer its own output.

使用 grep 的 --line-buffered 开关来解决这个问题:

Use the --line-buffered switch for grep to work around this:

tail --follow=name file.txt | grep --line-buffered something > output.txt

如果您想尽快将后续结果放入 output.txt 文件中，这将非常有用.

This is useful if you want to get the results of the follow into the output.txt file as rapidly as possible.

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