The watch command is a system utility able to execute a command every n seconds by clearing the screen and displaying the output of the command each time the interval is over. This command is very simple to use and very effective: it can be used to check the size in bytes of a file while downloading it, or it can be used on Linux to jiggle around the mouse every ~5 minutes with the command xdotool in order to keep the computer awake.


watch has a very simple syntax: the amount in seconds to wait between executions is specified in seconds using the -n flag and the default value is 2. Also, differences between older and newer outputs are highlighted if the -d flag is used. Example: Watch the size of "ubuntu.iso" every 5 seconds
watch -n 5 du -h ubuntu.iso
Example: Watch the contents of "site.html" every 2 seconds and highlight the differencies between iterations
watch -d cat site.html