Event Detection System with Linux [closed]

How would you go about implementing an event detection system based on a Linux machine that can run a program to alert a remote client, whether it is with or without a web server and in any language? The only exceptions would be the use of special environments like express.

I am trying not to be too vague, but i’m very new to this topic. How would one go about researching this? Everything I have found has not gotten me far.