The watch command periodically calls the process we're interested in. It displays the output of this process in full screen, by default every 2 seconds, which gives us the impression of an application running continuously.

watch [options] command

• Options:
• -n: change the interval at which the process is called
• -d: highlight changes (highlights differences between successive calls)
• -d=cumulative: keep highlighted changes
• -t: disables displaying the header

Usage examples:

• watch free - monitors memory usage
• watch lsof -i - monitors all connections
• watch 'lsof -i | grep LISTEN' - monitors all established connections

In the last example, quotes were used because we're using pipe processing.