A Pipeline in Unix is a chain of commands that are connected through a stream in such a way that output of one command becomes input for another command.
E.g. ls –l | grep “abc” | wc –l
In the above example we have created pipeline of three commands ls, grep and wc.
First ls –l command is executed and gives the list of files in a directory. Then grep command searches for any line with word “abc” in it. Finally wc –l command counts the number of lines that are returned by grep command.
In general a Pipeline is uni-directional. The data flows from left to right direction.