bash - Correct behavior of pipe operator in linux

Question

As the pipe operator is unidirectional and takes output from one command and uses it as input for another. I typed

sleep 5 | echo "Hello World"

I expected the sleep process to complete first and then print "Hello World", but actually the opposite happens where Hello World is printed and then sleep command is executed.

Can some one explain why this happens?

question from:https://stackoverflow.com/questions/65889678/correct-behavior-of-pipe-operator-in-linux

Answer 1

The pipe operator runs both processes concurrently. If the second process needs to read input, it will wait for the first process to write something to the pipe.

But echo doesn't read input, so it doesn't wait.

If you instead wrote

sleep 5 | { read _; echo "Hello World"; }

the output would be delayed because read waits for something to be available in the pipe (in this case it will just read EOF because sleep doesn't write anything to stdout).