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

Question

I would like to pipe standard output of a program while keeping it on screen.

With a simple example (echo use here is just for illustration purpose) :

$ echo 'ee' | foo
ee <- the output I would like to see

I know tee could copy stdout to file but that's not what I want.
$ echo 'ee' | tee output.txt | foo

I tried
$ echo 'ee' | tee /dev/stdout | foo but it does not work since tee output to /dev/stdout is piped to foo

Answer 1

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, §10.1:

/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