unix - How does shell execute piped commands? -

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i want understand how shell executes piped commands ? e.g. cat | more. aware executing normal command shell fork, execute , child returns. how shell internally handle execution of piped commands ?

considering example cat | grep, shell first forks start cat, , forks once more start grep.

before calling 1 of exec* family of functions in 2 newly created processes start 2 programs, tricky part setting pipe , redirecting descriptors. pipe(2) system call used in shell process before forking return pair of descriptors both children inherit - reading end , writing end.

the reading end closed in first process (cat), , stdout redirected writing end using dup2(2) system call. similarly, writing end in second process (grep) closed , stdin redirected reading end again using dup2(2).

this way both programs unaware of pipe because work standard input/output.


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