问题
This first pipeline works fine (printing "c"):
echo "a" | sed 's/a/b/' | sed 's/b/c/'
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/'
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?
(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)
回答1:
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 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'
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.
来源：https://stackoverflow.com/questions/25112713/multiple-unix-pipes-not-working