Is there any output from sleep? Where does cat get its input from? What has to happen before the shell will display a prompt?

Enter the following command:
$ sleep 30 | cat /etc/services

There is no output from sleep (try giving the command sleep 30 by itself). The
/etc/services file provides input for cat (when cat has an argument, it does not
check standard input). The sleep command has to run to completion before
the shell will display a prompt.