xclock is a tool that comes bundled in almost all Linux distributions, but all we use it for is to check whether your
$DISPLAY variable is set correctly. (When you configure
$DISPLAY and start xclock, an analog clock would pop up in the correct display.)
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xclock can be quite handy to get notified when a long-running task finishes. For example, I have this gitolite server which uses git pushes to update. A push may take several minutes if the number of repos is huge, so you just can't stare at the terminal till it finishes. What I do is
git push; xclock
and move to another task. A clock will pop up just when the push is completed. This is also useful when you compile an app that takes ages to build.