How do you write a script that alerts you in the Terminal like mail's "You have mail"

Question

I guess I'm looking for how to make a script to run whenever you press ENTER in bash, so that it can alert you by echoing a line about some event if an event has occurred.

Answer 1

You can use the special PROMPT_COMMAND variable, look it up in man bash.

$ PROMPT_COMMAND='echo hello'
hello
$ 
hello
$ 

Set it at the end of ~/.bashrc. I suggest you write a function and call that function in the PROMPT_COMMAND.

Answer 2

There have been tools to do this for ages, starting with the venerable 'biff'. Gnubiff looks like the most capable (it does IMAP and POP3) option in ubuntu:

http://packages.ubuntu.com/natty/gnubiff