多个Unix管道不起作用 [英] Multiple unix pipes not working

问题描述

第一个管道可以正常工作(打印"c"):

This first pipeline works fine (printing "c"):

echo "a" | sed 's/a/b/' | sed 's/b/c/'

这个人没有达到我的期望(当我在我的fifo中输入"a"时，什么都不会打印出来):

This one does not do what I expect (nothing gets printed when I feed an "a" into my fifo ):

mkfifo fifo;
cat fifo | sed 's/a/b/' | sed 's/b/c/' 

但是，如果我从后面的管道中删除第二个"sed"命令，则确实会显示"b".我认为我对管道和重定向的理解必须过于简单. 有人可以向我解释如何解决第二种情况，以便我可以对fifo的内容运行两个连续的命令吗?

However, if I remove the second "sed" command from the latter pipeline, I do get a "b" printed. I think my understanding of pipes and redirects must be too simplistic. Can someone explain to me how to fix the 2nd case so that I can run two successive commands on the contents of the fifo?

(请注意，这不是特定于fifo的问题，netcat也会发生相同的行为.我发布了有关netcat的类似问题，但没有得到答案)

(note this isn't a problem specific to fifo, the same behavior occurs with netcat too. I posted a similar question about netcat but got no answers)

推荐答案

缓冲. sed会根据输出是否为tty来更改其缓冲.当您有两个sed时，第一个确定它的输出不是tty，因此它正在缓冲.因此，当您拥有:

Buffering. sed is changing it's buffering depending on whether the output is a tty or not. When you have two sed's, the first determines it's output is not a tty so it is buffering. So when you have:

cat fifo | sed 's/a/b'

sed不会缓冲，因为它的输出是到tty的，因此您可以看到数据，但是当您拥有:

sed is not buffering as it's output is to a tty so you see the data but when you have:

cat fifo | sed 's/a/b' | sed 's/c/d'

第一个sed正在缓冲数据.根据您所运行的特定sed，有不同的方法来禁用缓冲. GNU sed具有--unbuffered选项，而BSD sed具有-l选项可切换到行缓冲.

the first sed is buffering the data. Depending on the specific sed you are running, there are different ways to disable buffering. GNU sed has the --unbuffered option while BSD sed has the -l option to switch to line buffering.

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