Question

I had the impression sed wasn’t blocking, because when I do say:

iostat | sed 

sed processes the data as it arrives, but when I do

iostat | sed | netcat 

Then sed blocks netcat.

Am I right?

Answer 1

sed will work in buffered mode when it doesn’t print to a terminal. This means that it will try to fill its internal buffer before doing any processing and output by default.

This is done to increase throughput, because normally in a pipe you don’t care about the timing, but want as much data processed in a given time as possible.

Passing -u to sed will tell it to work unbuffered, therefore working the same way it works when output goes to a terminal.