Sometimes you need to remind users to restart their machines after a kernel update has occurred. If you are already using ClusterSSH to remotely manage several computers at once, using xmessage will allow you to send a message to everyone with a restart reminder. To use xmessage:

xmessage [your message goes here] -display :0 &

This creates a small popup on the user’s screen that looks like this:

The message above was created as follows:

xmessage Hello World! Today is a good day! -display :0 &