Send command to a background process

Question

I have a previously running process (process1.sh) that is running in the background with a PID of 1111 (or some other arbitrary number). How could I send something like

Answer 1

If you don't want to be limited to signals, your program must support one of the Inter Process Communication methods. See the corresponding Wikipedia article.

A simple method is to make it listen for commands on a Unix domain socket.