为什么我不能通过管道多次过滤尾部的输出？ [英] Why can't I filter tail's output multiple times through pipes?

问题描述

意外的是，这种方式失败（ no output ;在 sh ， zsh ， bash p>

echofoo\\\
played\\\
bar> / tmp / t&& tail -f / tmp / t | grep播放| sed'＃p＃st＃g'

注意，两次 grep 也会失败，表示它是完全不相关的命令被使用：

 ＃echo -efoo\\\
played\\\
bar> / tmp / t&& tail -f / tmp / t | grep播放| grep发挥作用：
  

grep

 ＃echo -efoo\\\
played\\\
bar> / tmp / t&& tail -f / tmp / t | grep演奏
演奏
  

sed 单独运作：

 ＃echo -efoo\\\
played\\\
bar> / tmp / t&& tail -f / tmp / t | sed's＃pl＃st＃g'` 
 foo 
 stay 
 bar 
  

使用 cat 而不是 tail ，它可以工作：

 ＃echo -efoo\\\
played\\\
bar> / tmp / t&& cat / tmp / t | grep播放| sed's＃pl＃st＃g'
 stay 
  

使用 journalctl --follow ，它就像 tail 一样失败。

无法连接两次的原因是什么

解决方案

这是一个缓冲问题 - 第一个grep缓冲它输出到管道到另一个命令，但不是如果它打印到标准输出。有关其他信息，请参阅 http://mywiki.wooledge.org/BashFAQ/009 。

Unexpectedly, this fails (no output; tried in sh, zsh, bash):

echo "foo\nplayed\nbar" > /tmp/t && tail -f /tmp/t | grep played | sed 's#pl#st#g'

Note that two times grep also fails, indicating that it's quite irrelevant which commands are used:

# echo -e "foo\nplayed\nbar" > /tmp/t && tail -f /tmp/t | grep played | grep played

grep alone works:

# echo -e "foo\nplayed\nbar" > /tmp/t && tail -f /tmp/t | grep played
played

sed alone works:

# echo -e "foo\nplayed\nbar" > /tmp/t && tail -f /tmp/t | sed 's#pl#st#g'`
foo
stayed
bar

With cat instead of tail, it works:

# echo -e "foo\nplayed\nbar" > /tmp/t && cat /tmp/t | grep played | sed 's#pl#st#g'
stayed

With journalctl --follow, it fails just like with tail.

What's the reason for being unable to pipe twice?

解决方案

It's a buffering issue - the first grep buffers it's output when it's piping to another command but not if it's printing to stdout. See http://mywiki.wooledge.org/BashFAQ/009 for additional info.

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