Capture output of a command, process it and ONLY then output to terminal

I would like to capture the stdout (as sent to a terminal), and perform some processing after it is output on the terminal (for the user). I want to know how to grab the output, process and make changes accordingly. (If possible) I want this to happen for every command I type on the terminal. --- Edit: After a dialogue in a temporary chat forum it was clear that - we should - both look for which commands are issued - and what output there is from the commands and - do something, if a particular command or output is detected.