How to pipe stdout while keeping it on screen ? (and not to a output file)

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Here is a solution that works at on any Unix / Linux implementation, assuming it cares to follow the POSIX standard. It works on some non Unix environments like cygwin too.

echo 'ee' | tee /dev/tty | foo

Reference: The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition, ยง10.1:

/dev/tty

Associated with the process group of that process, if any. It is
useful for programs or shell procedures that wish to be sure of
writing messages to or reading data from the terminal no matter how
output has been redirected. It can also be used for applications that
demand the name of a file for output, when typed output is desired and
it is tiresome to find out what terminal is currently in use. In each process, a synonym for the controlling terminal

Some environments like Google Colab have been reported not to implement /dev/tty while still having their tty command returning a usable device. Here is a workaround:

tty=$(tty)
echo 'ee' | tee $tty | foo

or with an ancient Bourne shell: 

tty=`tty`
echo 'ee' | tee $tty | foo

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