This tutorial should work properly on all Unix based OSes.

Watching commands:

If you need to keep an eye on a command whose output is changing, use the watch
command.

For example, to keep an eye on your load average:

$ watch ‘cat /proc/loadavg’

Every two seconds, watch runs the cat command again. Use Ctrl+c to quit. To
change the refresh rate to 10 seconds, type the following:

$ watch -n 10 ‘ls -l’

To highlight the difference between screen updates, type:

$ watch -d ‘ls -l’

Type Ctrl+c to exit the watch command.

Watching Files:

You can use the watch command to watch the size of a file. For example, to watch a large ISO file named mydownload.iso as it downloads, use the following command:

$ watch ‘ls –l mydownload.iso’

To watch the contents of a plain text file grow over time, you can use the tail com-
mand. For example, you can watch as messages are added to the /var/log/messages
file as follows:

$ tail -f /var/log/messages

Pressing Ctrl+c will exit from the tail command.