If you anytime need to execute a command once and again and again, you can use
watch and tell it to execute the Linux command in a give interval.
The syntax of the command is:
watch [option(s)] command
The default interval of execution of the given command is two (2) seconds, but you define a different interval using the option -n.
Here are two useful uses of
watch -n 5 free -m
Which will show you the use of memory each five seconds.
watch -n 30 vnstat -h
Which will show you the bandwidth used hourly, read more about vnstat.
You can also make watch to highlight any change it detect between two screens of output.
watch -n 30 -d vnstat -h
And if you want to maintain the highlight on, add =cummulative to the -d option, like this:
watch -n 30 -d=cummulative vnstat -h