piping in UNIX doubt

Posted by Happy Mittal on Stack Overflow See other posts from Stack Overflow or by Happy Mittal
Published on 2010-04-15T09:33:01Z Indexed on 2010/04/15 9:43 UTC
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piping

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unix

In The Unix Programming Environment by K & P, it is written that
" The programs in a pipeline actually run at the same time, not one after another.
This means that programs in a pipeline can be interactive;"

How can programs run at same time?
For ex: $ who | grep mary | wc -l
How grep mary will be executed until who is run or how wc -l will be executed until it knows results of previous programs?

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