How to send a prompt to a remote machine like the “you have mail”, ie not using wall

Question

I am currently using wall to show a message on a remote machine when an activity is started and finished. But this is intrusive, as it will write across an editor they might have open etc.

What I like is the "you have unread mail message" that shows up on the next line if the user presses return on the terminal.

What is this, and how can I interact with it remotely?

Answer 1

This line is printed by the shell. Different shells implement looking for mail in slightly different ways, but unless can count on people to configure their shells to your liking (and possibly pick a different shell), you need to send them mail.

Good shells designed for interactive use have a way to run arbitrary commands before displaying a prompt, in addition to the built-in mail notification. You can have your users cat a file there, or do something more complicated. For zsh users, put this in ~/.zshrc:

precmd () {
  if [[ /etc/sysadmin-message -nt ~/.sysadmin-message-timestamp ]]; then
    cat /etc/sysadmin-message
    touch ~/.sysadmin-message-timestamp
  fi
}

For bash users, put this in ~/.bashrc:

PROMPT_COMMAND='
  if [[ /etc/sysadmin-message -nt ~/.sysadmin-message-timestamp ]]; then
    cat /etc/sysadmin-message
    touch ~/.sysadmin-message-timestamp
  fi
'

Answer 2

Wall will write across open programs, but most editors have a "redraw" command of some sort to clean up anything that got written to the screen other than by the editor. The text doesn't actually break the contents of the editor.

To do what you want you will have to be creative with the precmd() function of your users shells. You will have to write up some sort of notification queue and have the users shell configuration set to check in with that queue before each command. This involves some over-head, so be careful how to write that bit.