This topic is fairly known to most older system administrators however I doubt it is used very often anymore. If you happen to manage any servers where there are typically various people logged in and working on different items from the CLI then it can be useful to be able to send them messages. Using the write command you can send messages to users logged in on specific TTY port or by username. Below are a couple examples of how you would use the write command from a Linux shell.
Error(s) executing /usr/bin/nagios -v /usr/share/nagios/monarch/workspace/ nagios.cfg Permission Denied
Earlier in the week when moving Nagios from one server to another I ran into a bunch of random issues with Monarch which is the Nagios configuration file manager I use. After I got everything installed and thought that I was past any errors I ran a Pre Flight Test on the Nagios configuration files which returned an error. In the article below I describe in more detail the error that displayed, how I went about troubleshooting the error, and what I finally did to resolve the error.